GCC Command Options. IBM Power10
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Chapter 3: GCC Command Options 11
3 GCC Command Options
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking.
The “overall options” allow you to stop this process at an intermediate stage. For example, the ‘ -c ’ option says not to run the linker. Then the output consists of object files output by the assembler. See
Section 3.2 [Options Controlling the Kind of Output], page 33
.
Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C ++ ), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
The usual way to run GCC is to run the executable called gcc , or machine -gcc when cross-compiling, or machine -gccversion to run a specific version of GCC. When you compile C ++ programs, you should invoke GCC as g++ instead. See
, for information about the differences in behavior between gcc and g++ when compiling C ++ programs.
The gcc program accepts options and file names as operands. Many options have multiletter names; therefore multiple single-letter options may not be grouped: ‘ -dv ’ is very different from ‘ -d -v ’.
You can mix options and other arguments. For the most part, the order you use doesn’t matter. Order does matter when you use several options of the same kind; for example, if you specify ‘ -L ’ more than once, the directories are searched in the order specified. Also, the placement of the ‘ -l ’ option is significant.
Many options have long names starting with ‘ -f ’ or with ‘ -W ’—for example,
‘ -fmove-loop-invariants ’, ‘ -Wformat ’ and so on. Most of these have both positive and negative forms; the negative form of ‘ -ffoo ’ is ‘ -fno-foo ’. This manual documents only one of these two forms, whichever one is not the default.
Some options take one or more arguments typically separated either by a space or by the equals sign (‘ = ’) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the ‘ 0x ’ prefix.
Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as kB and KiB for kilobyte and kibibyte, respectively, MB and MiB for megabyte and mebibyte, GB and GiB for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes.
See
[Option Index], page 1001 , for an index to GCC’s options.
3.1 Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
12 Using the GNU Compiler Collection (GCC)
Overall Options
See
Section 3.2 [Options Controlling the Kind of Output], page 33
.
-c -S -E -o file
-dumpbase dumpbase -dumpbase-ext auxdropsuf
-dumpdir dumppfx -x language
-v -### --help [ = class [ ,...
]] --target-help --version
-pass-exit-codes -pipe -specs= file -wrapper
@ file -ffile-prefix-map= old = new
-fplugin= file -fplugin-argname = arg
-fdump-ada-spec [ -slim ] -fada-spec-parent= unit -fdump-go-spec= file
C Language Options
See
Section 3.4 [Options Controlling C Dialect], page 43 .
-ansi -std= standard -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm
-fno-builtin -fno-builtinfunction -fcond-mismatch
-ffreestanding -fgimple -fgnu-tm -fgnu89-inline -fhosted
-flax-vector-conversions -fms-extensions
-foffload= arg -foffload-options= arg
-fopenacc -fopenacc-dim= geom
-fopenmp -fopenmp-simd
-fpermitted-flt-eval-methods= standard
-fplan9-extensions -fsigned-bitfields -funsigned-bitfields
-fsigned-char -funsigned-char -fsso-struct= endianness
C ++ Language Options
See
Section 3.5 [Options Controlling C ++ Dialect], page 51 .
-fabi-version= n -fno-access-control
-faligned-new= n -fargs-in-order= n -fchar8_t -fcheck-new
-fconstexpr-depth= n -fconstexpr-cache-depth= n
-fconstexpr-loop-limit= n -fconstexpr-ops-limit= n
-fno-elide-constructors
-fno-enforce-eh-specs
-fno-gnu-keywords
-fno-implicit-templates
-fno-implicit-inline-templates
-fno-implement-inlines
-fmodule-header [ = kind ] -fmodule-only -fmodules-ts
-fmodule-implicit-inline
-fno-module-lazy
-fmodule-mapper= specification
-fmodule-version-ignore
-fms-extensions
-fnew-inheriting-ctors
-fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fpermissive
-fno-pretty-templates
-fno-rtti -fsized-deallocation
-ftemplate-backtrace-limit= n
-ftemplate-depth= n
-fno-threadsafe-statics -fuse-cxa-atexit
-fno-weak -nostdinc++
-fvisibility-inlines-hidden
-fvisibility-ms-compat
-fext-numeric-literals
-flang-info-include-translate [ = header ]
-flang-info-include-translate-not
Chapter 3: GCC Command Options 13
-flang-info-module-cmi [ = module ]
-stdlib= libstdc ++ ,libc ++
-Wabi-tag -Wcatch-value -Wcatch-value= n
-Wno-class-conversion -Wclass-memaccess
-Wcomma-subscript -Wconditionally-supported
-Wno-conversion-null -Wctad-maybe-unsupported
-Wctor-dtor-privacy -Wno-delete-incomplete
-Wdelete-non-virtual-dtor -Wno-deprecated-array-compare
-Wdeprecated-copy -Wdeprecated-copy-dtor
-Wno-deprecated-enum-enum-conversion -Wno-deprecated-enum-float-conversion
-Weffc++ -Wno-exceptions -Wextra-semi -Wno-inaccessible-base
-Wno-inherited-variadic-ctor -Wno-init-list-lifetime
-Winvalid-imported-macros
-Wno-invalid-offsetof -Wno-literal-suffix
-Wmismatched-new-delete -Wmismatched-tags
-Wmultiple-inheritance -Wnamespaces -Wnarrowing
-Wnoexcept -Wnoexcept-type -Wnon-virtual-dtor
-Wpessimizing-move -Wno-placement-new -Wplacement-new= n
-Wrange-loop-construct -Wredundant-move -Wredundant-tags
-Wreorder -Wregister
-Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates
-Wno-non-template-friend -Wold-style-cast
-Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo
-Wsized-deallocation -Wsuggest-final-methods
-Wsuggest-final-types -Wsuggest-override
-Wno-terminate -Wuseless-cast -Wno-vexing-parse
-Wvirtual-inheritance
-Wno-virtual-move-assign -Wvolatile -Wzero-as-null-pointer-constant
Objective-C and Objective-C ++ Language Options
See
Section 3.6 [Options Controlling Objective-C and Objective-C ++ Dialects], page 73 .
-fconstant-string-class= class-name
-fgnu-runtime -fnext-runtime
-fno-nil-receivers
-fobjc-abi-version= n
-fobjc-call-cxx-cdtors
-fobjc-direct-dispatch
-fobjc-exceptions
-fobjc-gc
-fobjc-nilcheck
-fobjc-std=objc1
-fno-local-ivars
-fivar-visibility= [ public|protected|private|package ]
-freplace-objc-classes
-fzero-link
-gen-decls
-Wassign-intercept -Wno-property-assign-default
-Wno-protocol -Wobjc-root-class -Wselector
-Wstrict-selector-match
-Wundeclared-selector
Diagnostic Message Formatting Options
See
Section 3.7 [Options to Control Diagnostic Messages Formatting], page 77
.
-fmessage-length= n
-fdiagnostics-plain-output
-fdiagnostics-show-location= [ once|every-line ]
-fdiagnostics-color= [ auto|never|always ]
14 Using the GNU Compiler Collection (GCC)
-fdiagnostics-urls= [ auto|never|always ]
-fdiagnostics-format= [ text|json ]
-fno-diagnostics-show-option -fno-diagnostics-show-caret
-fno-diagnostics-show-labels -fno-diagnostics-show-line-numbers
-fno-diagnostics-show-cwe
-fdiagnostics-minimum-margin-width= width
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format= [ none|separate-events|inline-events ]
-fdiagnostics-show-path-depths
-fno-show-column
-fdiagnostics-column-unit= [ display|byte ]
-fdiagnostics-column-origin= origin
-fdiagnostics-escape-format= [ unicode|bytes ]
Warning Options
See
Section 3.8 [Options to Request or Suppress Warnings], page 89
.
-fsyntax-only -fmax-errors= n -Wpedantic
-pedantic-errors
-w -Wextra -Wall -Wabi= n
-Waddress -Wno-address-of-packed-member -Waggregate-return
-Walloc-size-larger-than= byte-size -Walloc-zero
-Walloca -Walloca-larger-than= byte-size
-Wno-aggressive-loop-optimizations
-Warith-conversion
-Warray-bounds -Warray-bounds= n -Warray-compare
-Wno-attributes -Wattribute-alias= n -Wno-attribute-alias
-Wno-attribute-warning
-Wbidi-chars= [ none|unpaired|any ]
-Wbool-compare -Wbool-operation
-Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat
-Wc11-c2x-compat
-Wc++-compat -Wc++11-compat -Wc++14-compat -Wc++17-compat
-Wc++20-compat
-Wno-c++11-extensions -Wno-c++14-extensions -Wno-c++17-extensions
-Wno-c++20-extensions -Wno-c++23-extensions
-Wcast-align -Wcast-align=strict -Wcast-function-type -Wcast-qual
-Wchar-subscripts
-Wclobbered -Wcomment
-Wconversion -Wno-coverage-mismatch -Wno-cpp
-Wdangling-else -Wdangling-pointer -Wdangling-pointer= n
-Wdate-time
-Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization
-Wno-discarded-array-qualifiers -Wno-discarded-qualifiers
-Wno-div-by-zero -Wdouble-promotion
-Wduplicated-branches -Wduplicated-cond
-Wempty-body -Wno-endif-labels -Wenum-compare -Wenum-conversion
-Werror -Werror=* -Wexpansion-to-defined -Wfatal-errors
-Wfloat-conversion -Wfloat-equal -Wformat -Wformat=2
-Wno-format-contains-nul -Wno-format-extra-args
-Wformat-nonliteral -Wformat-overflow= n
-Wformat-security -Wformat-signedness -Wformat-truncation= n
-Wformat-y2k -Wframe-address
-Wframe-larger-than= byte-size -Wno-free-nonheap-object
-Wno-if-not-aligned -Wno-ignored-attributes
-Wignored-qualifiers -Wno-incompatible-pointer-types
Chapter 3: GCC Command Options 15
-Wimplicit -Wimplicit-fallthrough -Wimplicit-fallthrough= n
-Wno-implicit-function-declaration -Wno-implicit-int
-Winfinite-recursion
-Winit-self -Winline -Wno-int-conversion -Wint-in-bool-context
-Wno-int-to-pointer-cast -Wno-invalid-memory-model
-Winvalid-pch -Wjump-misses-init -Wlarger-than= byte-size
-Wlogical-not-parentheses -Wlogical-op -Wlong-long
-Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args
-Wmisleading-indentation -Wmissing-attributes -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute
-Wmissing-include-dirs -Wmissing-noreturn -Wno-missing-profile
-Wno-multichar -Wmultistatement-macros -Wnonnull -Wnonnull-compare
-Wnormalized= [ none|id|nfc|nfkc ]
-Wnull-dereference -Wno-odr
-Wopenacc-parallelism
-Wopenmp-simd
-Wno-overflow -Woverlength-strings -Wno-override-init-side-effects
-Wpacked -Wno-packed-bitfield-compat -Wpacked-not-aligned -Wpadded
-Wparentheses -Wno-pedantic-ms-format
-Wpointer-arith -Wno-pointer-compare -Wno-pointer-to-int-cast
-Wno-pragmas -Wno-prio-ctor-dtor -Wredundant-decls
-Wrestrict -Wno-return-local-addr -Wreturn-type
-Wno-scalar-storage-order -Wsequence-point
-Wshadow -Wshadow=global -Wshadow=local -Wshadow=compatible-local
-Wno-shadow-ivar
-Wno-shift-count-negative -Wno-shift-count-overflow -Wshift-negative-value
-Wno-shift-overflow -Wshift-overflow= n
-Wsign-compare -Wsign-conversion
-Wno-sizeof-array-argument
-Wsizeof-array-div
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess
-Wstack-protector -Wstack-usage= byte-size -Wstrict-aliasing
-Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow= n
-Wstring-compare
-Wno-stringop-overflow -Wno-stringop-overread
-Wno-stringop-truncation
-Wsuggest-attribute= [ pure|const|noreturn|format|malloc ]
-Wswitch -Wno-switch-bool -Wswitch-default -Wswitch-enum
-Wno-switch-outside-range -Wno-switch-unreachable -Wsync-nand
-Wsystem-headers -Wtautological-compare -Wtrampolines -Wtrigraphs
-Wtsan -Wtype-limits -Wundef
-Wuninitialized -Wunknown-pragmas
-Wunsuffixed-float-constants -Wunused
-Wunused-but-set-parameter -Wunused-but-set-variable
-Wunused-const-variable -Wunused-const-variable= n
-Wunused-function -Wunused-label -Wunused-local-typedefs
-Wunused-macros
-Wunused-parameter -Wno-unused-result
-Wunused-value -Wunused-variable
-Wno-varargs -Wvariadic-macros
-Wvector-operation-performance
-Wvla -Wvla-larger-than= byte-size -Wno-vla-larger-than
-Wvolatile-register-var -Wwrite-strings
-Wzero-length-bounds
Static Analyzer Options
-fanalyzer
-fanalyzer-call-summaries
16 Using the GNU Compiler Collection (GCC)
-fanalyzer-checker= name
-fno-analyzer-feasibility
-fanalyzer-fine-grained
-fanalyzer-state-merge
-fanalyzer-state-purge
-fanalyzer-transitivity
-fanalyzer-verbose-edges
-fanalyzer-verbose-state-changes
-fanalyzer-verbosity= level
-fdump-analyzer
-fdump-analyzer-stderr
-fdump-analyzer-callgraph
-fdump-analyzer-exploded-graph
-fdump-analyzer-exploded-nodes
-fdump-analyzer-exploded-nodes-2
-fdump-analyzer-exploded-nodes-3
-fdump-analyzer-exploded-paths
-fdump-analyzer-feasibility
-fdump-analyzer-json
-fdump-analyzer-state-purge
-fdump-analyzer-supergraph
-Wno-analyzer-double-fclose
-Wno-analyzer-double-free
-Wno-analyzer-exposure-through-output-file
-Wno-analyzer-file-leak
-Wno-analyzer-free-of-non-heap
-Wno-analyzer-malloc-leak
-Wno-analyzer-mismatching-deallocation
-Wno-analyzer-null-argument
-Wno-analyzer-null-dereference
-Wno-analyzer-possible-null-argument
-Wno-analyzer-possible-null-dereference
-Wno-analyzer-shift-count-negative
-Wno-analyzer-shift-count-overflow
-Wno-analyzer-stale-setjmp-buffer
-Wno-analyzer-tainted-allocation-size
-Wno-analyzer-tainted-array-index
-Wno-analyzer-tainted-divisor
-Wno-analyzer-tainted-offset
-Wno-analyzer-tainted-size
-Wanalyzer-too-complex
-Wno-analyzer-unsafe-call-within-signal-handler
-Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
-Wno-analyzer-use-of-uninitialized-value
-Wno-analyzer-write-to-const
-Wno-analyzer-write-to-string-literal
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations
-Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs
-Wold-style-declaration -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-sign
Debugging Options
See
Section 3.10 [Options for Debugging Your Program], page 152 .
Chapter 3: GCC Command Options 17
-g -g level -gdwarf -gdwarfversion
-gbtf -gctf -gctf level
-ggdb -grecord-gcc-switches -gno-record-gcc-switches
-gstabs -gstabs+ -gstrict-dwarf -gno-strict-dwarf
-gas-loc-support -gno-as-loc-support
-gas-locview-support -gno-as-locview-support
-gcolumn-info -gno-column-info -gdwarf32 -gdwarf64
-gstatement-frontiers -gno-statement-frontiers
-gvariable-location-views -gno-variable-location-views
-ginternal-reset-location-views -gno-internal-reset-location-views
-ginline-points -gno-inline-points
-gvms -gxcoff -gxcoff+ -gz [ = type ]
-gsplit-dwarf -gdescribe-dies -gno-describe-dies
-fdebug-prefix-map= old = new -fdebug-types-section
-fno-eliminate-unused-debug-types
-femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed [ = spec-list ]
-fno-eliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm
-fvar-tracking -fvar-tracking-assignments
Optimization Options
See
Section 3.11 [Options that Control Optimization], page 159 .
-faggressive-loop-optimizations
-falign-functions[= n [: m :[ n2 [: m2 ]]]]
-falign-jumps[= n [: m :[ n2 [: m2 ]]]]
-falign-labels[= n [: m :[ n2 [: m2 ]]]]
-falign-loops[= n [: m :[ n2 [: m2 ]]]]
-fno-allocation-dce -fallow-store-data-races
-fassociative-math -fauto-profile -fauto-profile[= path ]
-fauto-inc-dec -fbranch-probabilities
-fcaller-saves
-fcombine-stack-adjustments -fconserve-stack
-fcompare-elim -fcprop-registers -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules
-fcx-limited-range
-fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively
-fdevirtualize-at-ltrans -fdse
-fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects
-ffast-math -ffinite-math-only -ffloat-store -fexcess-precision= style
-ffinite-loops
-fforward-propagate -ffp-contract= style -ffunction-sections
-fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity
-fgcse-sm -fhoist-adjacent-loads -fif-conversion
-fif-conversion2 -findirect-inlining
-finline-functions -finline-functions-called-once -finline-limit= n
-finline-small-functions -fipa-modref -fipa-cp -fipa-cp-clone
-fipa-bit-cp -fipa-vrp -fipa-pta -fipa-profile -fipa-pure-const
-fipa-reference -fipa-reference-addressable
-fipa-stack-alignment -fipa-icf -fira-algorithm= algorithm
-flive-patching= level
-fira-region= region -fira-hoist-pressure
-fira-loop-pressure -fno-ira-share-save-slots
-fno-ira-share-spill-slots
-fisolate-erroneous-paths-dereference -fisolate-erroneous-paths-attribute
-fivopts -fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment -flive-range-shrinkage
18 Using the GNU Compiler Collection (GCC)
-floop-block -floop-interchange -floop-strip-mine
-floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -flto -flto-compression-level
-flto-partition= alg -fmerge-all-constants
-fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants -fmove-loop-stores -fno-branch-count-reg
-fno-defer-pop -fno-fp-int-builtin-inexact -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole
-fno-peephole2 -fno-printf-return-value -fno-sched-interblock
-fno-sched-spec -fno-signed-zeros
-fno-toplevel-reorder -fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-sibling-calls
-fpartial-inlining -fpeel-loops -fpredictive-commoning
-fprefetch-loop-arrays
-fprofile-correction
-fprofile-use -fprofile-use= path -fprofile-partial-training
-fprofile-values -fprofile-reorder-functions
-freciprocal-math -free -frename-registers -freorder-blocks
-freorder-blocks-algorithm= algorithm
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -freschedule-modulo-scheduled-loops
-frounding-math -fsave-optimization-record
-fsched2-use-superblocks -fsched-pressure
-fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[= n ] -fsched-stalled-insns[= n ]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic
-fschedule-fusion
-fschedule-insns -fschedule-insns2 -fsection-anchors
-fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate
-fsignaling-nans
-fsingle-precision-constant -fsplit-ivs-in-unroller -fsplit-loops
-fsplit-paths
-fsplit-wide-types -fsplit-wide-types-early -fssa-backprop -fssa-phiopt
-fstdarg-opt -fstore-merging -fstrict-aliasing -fipa-strict-aliasing
-fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts
-ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting
-ftree-loop-if-convert -ftree-loop-im
-ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize
-ftree-loop-vectorize
-ftree-parallelize-loops= n -ftree-pre -ftree-partial-pre -ftree-pta
-ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra
-ftree-switch-conversion -ftree-tail-merge
-ftree-ter -ftree-vectorize -ftree-vrp -ftrivial-auto-var-init
-funconstrained-commons -funit-at-a-time -funroll-all-loops
-funroll-loops -funsafe-math-optimizations -funswitch-loops
-fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt
-fweb -fwhole-program -fwpa -fuse-linker-plugin -fzero-call-used-regs
--param name = value -O -O0 -O1 -O2 -O3 -Os -Ofast -Og -Oz
Program Instrumentation Options
See
Section 3.12 [Program Instrumentation Options], page 232
.
Chapter 3: GCC Command Options
-p -pg -fprofile-arcs --coverage -ftest-coverage
-fprofile-abs-path
-fprofile-dir= path -fprofile-generate -fprofile-generate= path
-fprofile-info-section -fprofile-info-section= name
-fprofile-note= path -fprofile-prefix-path= path
-fprofile-update= method -fprofile-filter-files= regex
-fprofile-exclude-files= regex
-fprofile-reproducible= [ multithreaded|parallel-runs|serial ]
-fsanitize= style -fsanitize-recover -fsanitize-recover= style
-fasan-shadow-offset= number -fsanitize-sections= s1 , s2 ,...
-fsanitize-undefined-trap-on-error -fbounds-check
-fcf-protection= [ full|branch|return|none|check ]
-fharden-compares -fharden-conditional-branches
-fstack-protector -fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstack-check
-fstack-limit-register= reg -fstack-limit-symbol= sym
-fno-stack-limit -fsplit-stack
-fvtable-verify= [ std|preinit|none ]
-fvtv-counts -fvtv-debug
-finstrument-functions
-finstrument-functions-exclude-function-list= sym , sym ,...
-finstrument-functions-exclude-file-list= file , file ,...
-fprofile-prefix-map= old = new
Preprocessor Options
See
Section 3.13 [Options Controlling the Preprocessor], page 248
.
-A question = answer
-Aquestion [ = answer ]
-C -CC -D macro [ = defn ]
-dD -dI -dM -dN -dU
-fdebug-cpp -fdirectives-only -fdollars-in-identifiers
-fexec-charset= charset -fextended-identifiers
-finput-charset= charset -flarge-source-files
-fmacro-prefix-map= old = new -fmax-include-depth= depth
-fno-canonical-system-headers -fpch-deps -fpch-preprocess
-fpreprocessed -ftabstop= width -ftrack-macro-expansion
-fwide-exec-charset= charset -fworking-directory
-H -imacros file -include file
-M -MD -MF -MG -MM -MMD -MP -MQ -MT -Mno-modules
-no-integrated-cpp -P -pthread -remap
-traditional -traditional-cpp -trigraphs
-U macro -undef
-Wp, option -Xpreprocessor option
Assembler Options
See
Section 3.14 [Passing Options to the Assembler], page 256
.
-Wa, option -Xassembler option
Linker Options
See
Section 3.15 [Options for Linking], page 256
.
object-file-name -fuse-ld= linker -l library
-nostartfiles -nodefaultlibs -nolibc -nostdlib
-e entry --entry= entry
-pie -pthread -r -rdynamic
-s -static -static-pie -static-libgcc -static-libstdc++
-static-libasan -static-libtsan -static-liblsan -static-libubsan
19
20 Using the GNU Compiler Collection (GCC)
-shared -shared-libgcc -symbolic
-T script -Wl, option -Xlinker option
-u symbol -z keyword
Directory Options
See
Section 3.16 [Options for Directory Search], page 262 .
-B prefix -I dir -I-
-idirafter dir
-imacros file -imultilib dir
-iplugindir= dir -iprefix file
-iquote dir -isysroot dir -isystem dir
-iwithprefix dir -iwithprefixbefore dir
-L dir -no-canonical-prefixes --no-sysroot-suffix
-nostdinc -nostdinc++ --sysroot= dir
Code Generation Options
See
Section 3.17 [Options for Code Generation Conventions], page 265
.
-fcall-savedreg -fcall-usedreg
-ffixedreg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables
-fno-gnu-unique
-finhibit-size-directive -fcommon -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-plt
-fno-jump-tables -fno-bit-tests
-frecord-gcc-switches
-freg-struct-return -fshort-enums -fshort-wchar
-fverbose-asm -fpack-struct[= n ]
-fleading-underscore -ftls-model= model
-fstack-reuse= reuse_level
-ftrampolines -ftrapv -fwrapv
-fvisibility= [ default|internal|hidden|protected ]
-fstrict-volatile-bitfields -fsync-libcalls
Developer Options
See
Section 3.18 [GCC Developer Options], page 275
.
-d letters -dumpspecs -dumpmachine -dumpversion
-dumpfullversion -fcallgraph-info [ =su,da ] -fchecking -fchecking= n -fdbg-cntlist
-fdbg-cnt= counter-value-list
-fdisable-ipapass_name
-fdisable-rtlpass_name
-fdisable-rtlpass-name = range-list
-fdisable-treepass_name
-fdisable-treepass-name = range-list
-fdump-debug -fdump-earlydebug
-fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links
-fdump-final-insns [ = file ]
-fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline
-fdump-lang-all
-fdump-langswitch
-fdump-langswitch options
-fdump-langswitch options = filename
-fdump-passes
-fdump-rtlpass -fdump-rtlpass = filename
-fdump-statistics
-fdump-tree-all
-fdump-treeswitch
Chapter 3: GCC Command Options 21
-fdump-treeswitch options
-fdump-treeswitch options = filename
-fcompare-debug [ = opts ] -fcompare-debug-second
-fenablekind pass
-fenablekind pass = range-list
-fira-verbose= n
-flto-report -flto-report-wpa -fmem-report-wpa
-fmem-report -fpre-ipa-mem-report -fpost-ipa-mem-report
-fopt-info -fopt-infooptions [ = file ]
-fprofile-report
-frandom-seed= string -fsched-verbose= n
-fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose
-fstats -fstack-usage -ftime-report -ftime-report-details
-fvar-tracking-assignments-toggle -gtoggle
-print-file-name= library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-multi-os-directory
-print-prog-name= program -print-search-dirs -Q
-print-sysroot -print-sysroot-headers-suffix
-save-temps -save-temps=cwd -save-temps=obj -time [ = file ]
Machine-Dependent Options
See
Section 3.19 [Machine-Dependent Options], page 292 .
AArch64 Options
-mabi= name -mbig-endian -mlittle-endian
-mgeneral-regs-only
-mcmodel=tiny -mcmodel=small -mcmodel=large
-mstrict-align -mno-strict-align
-momit-leaf-frame-pointer
-mtls-dialect=desc -mtls-dialect=traditional
-mtls-size= size
-mfix-cortex-a53-835769 -mfix-cortex-a53-843419
-mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div
-mpc-relative-literal-loads
-msign-return-address= scope
-mbranch-protection= none | standard | pac-ret [+ leaf + b-key ]| bti
-mharden-sls= opts
-march= name -mcpu= name -mtune= name
-moverride= string -mverbose-cost-dump
-mstack-protector-guard= guard -mstack-protector-guard-reg= sysreg
-mstack-protector-guard-offset= offset -mtrack-speculation
-moutline-atomics
Adapteva Epiphany Options
-mhalf-reg-file -mprefer-short-insn-regs
-mbranch-cost= num -mcmove -mnops= num -msoft-cmpsf
-msplit-lohi -mpost-inc -mpost-modify -mstack-offset= num
-mround-nearest -mlong-calls -mshort-calls -msmall16
-mfp-mode= mode -mvect-double -max-vect-align= num
-msplit-vecmove-early -m1regreg
AMD GCN Options
-march= gpu -mtune= gpu -mstack-size= bytes
ARC Options
-mbarrel-shifter -mjli-always
-mcpu= cpu -mA6 -mARC600 -mA7 -mARC700
-mdpfp -mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr
-mea -mno-mpy -mmul32x16 -mmul64 -matomic
22 Using the GNU Compiler Collection (GCC)
-mnorm -mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap
-mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc -mswape
-mtelephony -mxy -misize -mannotate-align -marclinux -marclinux_prof
-mlong-calls -mmedium-calls -msdata -mirq-ctrl-saved
-mrgf-banked-regs -mlpc-width= width -G num
-mvolatile-cache -mtp-regno= regno
-malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc
-mcase-vector-pcrel -mcompact-casesi -mno-cond-exec -mearly-cbranchsi
-mexpand-adddi -mindexed-loads -mlra -mlra-priority-none
-mlra-priority-compact -mlra-priority-noncompact -mmillicode
-mmixed-code -mq-class -mRcq -mRcw -msize-level= level
-mtune= cpu -mmultcost= num -mcode-density-frame
-munalign-prob-threshold= probability -mmpy-option= multo
-mdiv-rem -mcode-density -mll64 -mfpu= fpu -mrf16 -mbranch-index
ARM Options
-mapcs-frame -mno-apcs-frame
-mabi= name
-mapcs-stack-check -mno-apcs-stack-check
-mapcs-reentrant -mno-apcs-reentrant
-mgeneral-regs-only
-msched-prolog -mno-sched-prolog
-mlittle-endian -mbig-endian
-mbe8 -mbe32
-mfloat-abi= name
-mfp16-format= name -mthumb-interwork -mno-thumb-interwork
-mcpu= name -march= name -mfpu= name
-mtune= name -mprint-tune-info
-mstructure-size-boundary= n
-mabort-on-noreturn
-mlong-calls -mno-long-calls
-msingle-pic-base -mno-single-pic-base
-mpic-register= reg
-mnop-fun-dllimport
-mpoke-function-name
-mthumb -marm -mflip-thumb
-mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
-mtp= name -mtls-dialect= dialect
-mword-relocations
-mfix-cortex-m3-ldrd
-munaligned-access
-mneon-for-64bits
-mslow-flash-data
-masm-syntax-unified
-mrestrict-it
-mverbose-cost-dump
-mpure-code
-mcmse
-mfix-cmse-cve-2021-35465
-mfdpic
AVR Options
-mmcu= mcu -mabsdata -maccumulate-args
-mbranch-cost= cost
-mcall-prologues -mgas-isr-prologues -mint8
-mdouble= bits -mlong-double= bits
-mn_flash= size -mno-interrupts
-mmain-is-OS_task -mrelax -mrmw -mstrict-X -mtiny-stack
Chapter 3: GCC Command Options 23
-mfract-convert-truncate
-mshort-calls -nodevicelib -nodevicespecs
-Waddr-space-convert -Wmisspelled-isr
Blackfin Options
-mcpu= cpu [ sirevision ]
-msim -momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly
-mlow-64k -mno-low64k -mstack-check-l1 -mid-shared-library
-mno-id-shared-library -mshared-library-id= n
-mleaf-id-shared-library -mno-leaf-id-shared-library
-msep-data -mno-sep-data -mlong-calls -mno-long-calls
-mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram
-micplb
C6X Options
-mbig-endian -mlittle-endian -march= cpu
-msim -msdata= sdata-type
CRIS Options
-mcpu= cpu -march= cpu -mtune= cpu
-mmax-stack-frame= n -melinux-stacksize= n
-metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects
-mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt
-melf -maout -melinux -mlinux -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
CR16 Options
-mmac
-mcr16cplus -mcr16c
-msim -mint32 -mbit-ops -mdata-model= model
C-SKY Options
-march= arch -mcpu= cpu
-mbig-endian -EB -mlittle-endian -EL
-mhard-float -msoft-float -mfpu= fpu -mdouble-float -mfdivdu
-mfloat-abi= name
-melrw -mistack -mmp -mcp -mcache -msecurity -mtrust
-mdsp -medsp -mvdsp
-mdiv -msmart -mhigh-registers -manchor
-mpushpop -mmultiple-stld -mconstpool -mstack-size -mccrt
-mbranch-cost= n -mcse-cc -msched-prolog -msim
Darwin Options
-all_load -allowable_client -arch -arch_errors_fatal
-arch_only -bind_at_load -bundle -bundle_loader
-client_name -compatibility_version -current_version
-dead_strip
-dependency-file -dylib_file -dylinker_install_name
-dynamic -dynamiclib -exported_symbols_list
-filelist -flat_namespace -force_cpusubtype_ALL
-force_flat_namespace -headerpad_max_install_names
-iframework
-image_base -init -install_name -keep_private_externs
-multi_module -multiply_defined -multiply_defined_unused
-noall_load -no_dead_strip_inits_and_terms
-nofixprebinding -nomultidefs -noprebind -noseglinkedit
-pagezero_size -prebind -prebind_all_twolevel_modules
-private_bundle -read_only_relocs -sectalign
24 Using the GNU Compiler Collection (GCC)
-sectobjectsymbols -whyload -seg1addr
-sectcreate -sectobjectsymbols -sectorder
-segaddr -segs_read_only_addr -segs_read_write_addr
-seg_addr_table -seg_addr_table_filename -seglinkedit
-segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches
-whatsloaded -F -gused -gfull -mmacosx-version-min= version
-mkernel -mone-byte-bool
DEC Alpha Options
-mno-fp-regs -msoft-float
-mieee -mieee-with-inexact -mieee-conformant
-mfp-trap-mode= mode -mfp-rounding-mode= mode
-mtrap-precision= mode -mbuild-constants
-mcpu= cpu-type -mtune= cpu-type
-mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee
-mexplicit-relocs -msmall-data -mlarge-data
-msmall-text -mlarge-text
-mmemory-latency= time eBPF Options
-mbig-endian -mlittle-endian -mkernel= version -mframe-limit= bytes -mxbpf mco-re -mno-co-re -mjmpext -mjmp32 -malu32 -mcpu= version
FR30 Options
-msmall-model -mno-lsim
FT32 Options
-msim -mlra -mnodiv -mft32b -mcompress -mnopm
FRV Options
-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64
-mhard-float -msoft-float
-malloc-cc -mfixed-cc -mdword -mno-dword
-mdouble -mno-double
-mmedia -mno-media -mmuladd -mno-muladd
-mfdpic -minline-plt -mgprel-ro -multilib-library-pic
-mlinked-fp -mlong-calls -malign-labels
-mlibrary-pic -macc-4 -macc-8
-mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar
-mscc -mno-scc -mcond-exec -mno-cond-exec
-mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats
-mTLS -mtls
-mcpu= cpu
GNU/Linux Options
-mglibc -muclibc -mmusl -mbionic -mandroid
-tno-android-cc -tno-android-ld
H8/300 Options
-mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300
HPPA Options
-march= architecture-type
-mcaller-copies -mdisable-fpregs -mdisable-indexing
Chapter 3: GCC Command Options 25
-mfast-indirect-calls -mgas -mgnu-ld -mhp-ld
-mfixed-range= register-range
-mjump-in-delay -mlinker-opt -mlong-calls
-mlong-load-store -mno-disable-fpregs
-mno-disable-indexing -mno-fast-indirect-calls -mno-gas
-mno-jump-in-delay -mno-long-load-store
-mno-portable-runtime -mno-soft-float
-mno-space-regs -msoft-float -mpa-risc-1-0
-mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime
-mschedule= cpu-type -mspace-regs -msio -mwsio
-munix= unix-std -nolibdld -static -threads
IA-64 Options
-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mregister-names -msdata -mno-sdata
-mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput
-mno-inline-float-divide
-minline-int-divide-min-latency
-minline-int-divide-max-throughput
-mno-inline-int-divide
-minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-inline-sqrt
-mdwarf2-asm -mearly-stop-bits
-mfixed-range= register-range -mtls-size= tls-size
-mtune= cpu-type -milp32 -mlp64
-msched-br-data-spec -msched-ar-data-spec -msched-control-spec
-msched-br-in-data-spec -msched-ar-in-data-spec -msched-in-control-spec
-msched-spec-ldc -msched-spec-control-ldc
-msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle -msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns= max-insns
LM32 Options
-mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled
-msign-extend-enabled -muser-enabled
M32R/D Options
-m32r2 -m32rx -m32r
-mdebug
-malign-loops -mno-align-loops
-missue-rate= number
-mbranch-cost= number
-mmodel= code-size-model-type
-msdata= sdata-type
-mno-flush-func -mflush-func= name
-mno-flush-trap -mflush-trap= number
-G num
M32C Options
-mcpu= cpu -msim -memregs= number
M680x0 Options
-march= arch -mcpu= cpu -mtune= tune
-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040
-m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407
-mcfv4e -mbitfield -mno-bitfield -mc68000 -mc68020
-mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort
26 Using the GNU Compiler Collection (GCC)
-mno-short -mhard-float -m68881 -msoft-float -mpcrel
-malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library
-mxgot -mno-xgot -mlong-jump-table-offsets
MCore Options
-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates
-mno-relax-immediates -mwide-bitfields -mno-wide-bitfields
-m4byte-functions -mno-4byte-functions -mcallgraph-data
-mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim
-mlittle-endian -mbig-endian -m210 -m340 -mstack-increment
MeP Options
-mabsdiff -mall-opts -maverage -mbased= n -mbitops
-mc= n -mclip -mconfig= name -mcop -mcop32 -mcop64 -mivc2
-mdc -mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax
-mmult -mno-opts -mrepeat -ms -msatur -msdram -msim -msimnovec -mtf
-mtiny= n
MicroBlaze Options
-msoft-float -mhard-float -msmall-divides -mcpu= cpu
-mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift
-mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss
-mxl-multiply-high -mxl-float-convert -mxl-float-sqrt
-mbig-endian -mlittle-endian -mxl-reorder -mxl-modeapp-model
-mpic-data-is-text-relative
MIPS Options
-EL -EB -march= arch -mtune= arch
-mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5
-mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6
-mips16 -mno-mips16 -mflip-mips16
-minterlink-compressed -mno-interlink-compressed
-minterlink-mips16 -mno-interlink-mips16
-mabi= abi -mabicalls -mno-abicalls
-mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot
-mgp32 -mgp64 -mfp32 -mfpxx -mfp64 -mhard-float -msoft-float
-mno-float -msingle-float -mdouble-float
-modd-spreg -mno-odd-spreg
-mabs= mode -mnan= encoding
-mdsp -mno-dsp -mdspr2 -mno-dspr2
-mmcu -mmno-mcu
-meva -mno-eva
-mvirt -mno-virt
-mxpa -mno-xpa
-mcrc -mno-crc
-mginv -mno-ginv
-mmicromips -mno-micromips
-mmsa -mno-msa
-mloongson-mmi -mno-loongson-mmi
-mloongson-ext -mno-loongson-ext
-mloongson-ext2 -mno-loongson-ext2
-mfpu= fpu-type
-msmartmips -mno-smartmips
-mpaired-single -mno-paired-single -mdmx -mno-mdmx
-mips3d -mno-mips3d -mmt -mno-mt -mllsc -mno-llsc
-mlong64 -mlong32 -msym32 -mno-sym32
-G num -mlocal-sdata -mno-local-sdata
-mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt
-membedded-data -mno-embedded-data
Chapter 3: GCC Command Options
-muninit-const-in-rodata -mno-uninit-const-in-rodata
-mcode-readable= setting
-msplit-addresses -mno-split-addresses
-mexplicit-relocs -mno-explicit-relocs
-mcheck-zero-division -mno-check-zero-division
-mdivide-traps -mdivide-breaks
-mload-store-pairs -mno-load-store-pairs
-munaligned-access -mno-unaligned-access
-mmemcpy -mno-memcpy -mlong-calls -mno-long-calls
-mmad -mno-mad -mimadd -mno-imadd -mfused-madd -mno-fused-madd -nocpp
-mfix-24k -mno-fix-24k
-mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400
-mfix-r5900 -mno-fix-r5900
-mfix-r10000 -mno-fix-r10000 -mfix-rm7000 -mno-fix-rm7000
-mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1
-mflush-func= func -mno-flush-func
-mbranch-cost= num -mbranch-likely -mno-branch-likely
-mcompact-branches= policy
-mfp-exceptions -mno-fp-exceptions
-mvr4130-align -mno-vr4130-align -msynci -mno-synci
-mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4
-mrelax-pic-calls -mno-relax-pic-calls -mmcount-ra-address
-mframe-header-opt -mno-frame-header-opt
MMIX Options
-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu
-mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols
-melf -mbranch-predict -mno-branch-predict -mbase-addresses
-mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options
-mmult-bug -mno-mult-bug
-mno-am33 -mam33 -mam33-2 -mam34
-mtune= cpu-type
-mreturn-pointer-on-d0
-mno-crt0 -mrelax -mliw -msetlb
Moxie Options
-meb -mel -mmul.x -mno-crt0
MSP430 Options
-msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax
-mwarn-mcu
-mcode-region= -mdata-region=
-msilicon-errata= -msilicon-errata-warn=
-mhwmult= -minrt -mtiny-printf -mmax-inline-shift=
NDS32 Options
-mbig-endian -mlittle-endian
-mreduced-regs -mfull-regs
-mcmov -mno-cmov
-mext-perf -mno-ext-perf
-mext-perf2 -mno-ext-perf2
-mext-string -mno-ext-string
-mv3push -mno-v3push
-m16bit -mno-16bit
-misr-vector-size= num
-mcache-block-size= num
-march= arch
27
28 Using the GNU Compiler Collection (GCC)
-mcmodel= code-model
-mctor-dtor -mrelax
Nios II Options
-G num -mgpopt= option -mgpopt -mno-gpopt
-mgprel-sec= regexp -mr0rel-sec= regexp
-mel -meb
-mno-bypass-cache -mbypass-cache
-mno-cache-volatile -mcache-volatile
-mno-fast-sw-div -mfast-sw-div
-mhw-mul -mno-hw-mul -mhw-mulx -mno-hw-mulx -mno-hw-div -mhw-div
-mcustominsn = N -mno-custominsn
-mcustom-fpu-cfg= name
-mhal -msmallc -msys-crt0= name -msys-lib= name
-march= arch -mbmx -mno-bmx -mcdx -mno-cdx
Nvidia PTX Options
-m64 -mmainkernel -moptimize
OpenRISC Options
-mboard= name -mnewlib -mhard-mul -mhard-div
-msoft-mul -msoft-div
-msoft-float -mhard-float -mdouble-float -munordered-float
-mcmov -mror -mrori -msext -msfimm -mshftimm
-mcmodel= code-model
PDP-11 Options
-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10
-mint32 -mno-int16 -mint16 -mno-int32
-msplit -munix-asm -mdec-asm -mgnu-asm -mlra picoChip Options
-mae= ae_type -mvliw-lookahead= N
-msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
PRU Options
-mmcu= mcu -minrt -mno-relax -mloop
-mabi= variant
RISC-V Options
-mbranch-cost= N-instruction
-mplt -mno-plt
-mabi= ABI-string
-mfdiv -mno-fdiv
-mdiv -mno-div
-march= ISA-string
-mtune= processor-string
-mpreferred-stack-boundary= num
-msmall-data-limit= N-bytes
-msave-restore -mno-save-restore
-mshorten-memrefs -mno-shorten-memrefs
-mstrict-align -mno-strict-align
-mcmodel=medlow -mcmodel=medany
-mexplicit-relocs -mno-explicit-relocs
-mrelax -mno-relax
-mriscv-attribute -mmo-riscv-attribute
-malign-data= type
Chapter 3: GCC Command Options
-mbig-endian -mlittle-endian
-mstack-protector-guard= guard -mstack-protector-guard-reg= reg
-mstack-protector-guard-offset= offset
RL78 Options
-msim -mmul=none -mmul=g13 -mmul=g14 -mallregs
-mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14
-m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts
RS/6000 and PowerPC Options
-mcpu= cpu-type
-mtune= cpu-type
-mcmodel= code-model
-mpowerpc64
-maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt
-mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd
-mfprnd -mno-fprnd
-mcmpb -mno-cmpb -mhard-dfp -mno-hard-dfp
-mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc
-m64 -m32 -mxl-compat -mno-xl-compat -mpe
-malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mno-multiple
-mupdate -mno-update
-mavoid-indexed-addresses -mno-avoid-indexed-addresses
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib
-mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian
-mdynamic-no-pic -mswdiv -msingle-pic-base
-mprioritize-restricted-insns= priority
-msched-costly-dep= dependence_type
-minsert-sched-nops= scheme
-mcall-aixdesc -mcall-eabi -mcall-freebsd
-mcall-linux -mcall-netbsd -mcall-openbsd
-mcall-sysv -mcall-sysv-eabi -mcall-sysv-noeabi
-mtraceback= traceback_type
-maix-struct-return -msvr4-struct-return
-mabi= abi-type -msecure-plt -mbss-plt
-mlongcall -mno-longcall -mpltseq -mno-pltseq
-mblock-move-inline-limit= num
-mblock-compare-inline-limit= num
-mblock-compare-inline-loop-limit= num
-mno-block-ops-unaligned-vsx
-mstring-compare-inline-limit= num
-misel -mno-isel
-mvrsave -mno-vrsave
-mmulhw -mno-mulhw
-mdlmzb -mno-dlmzb
-mprototype -mno-prototype
-msim -mmvme -mads -myellowknife -memb -msdata
-msdata= opt -mreadonly-in-sdata -mvxworks -G num
-mrecip -mrecip= opt -mno-recip -mrecip-precision
-mno-recip-precision
-mveclibabi= type -mfriz -mno-friz
-mpointers-to-nested-functions -mno-pointers-to-nested-functions
-msave-toc-indirect -mno-save-toc-indirect
-mpower8-fusion -mno-mpower8-fusion -mpower8-vector -mno-power8-vector
29
30 Using the GNU Compiler Collection (GCC)
-mcrypto -mno-crypto -mhtm -mno-htm
-mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic
-mcompat-align-parm -mno-compat-align-parm
-mfloat128 -mno-float128 -mfloat128-hardware -mno-float128-hardware
-mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard= guard -mstack-protector-guard-reg= reg
-mstack-protector-guard-offset= offset -mprefixed -mno-prefixed
-mpcrel -mno-pcrel -mmma -mno-mmma -mrop-protect -mno-rop-protect
-mprivileged -mno-privileged
RX Options
-m64bit-doubles -m32bit-doubles -fpu -nofpu
-mcpu=
-mbig-endian-data -mlittle-endian-data
-msmall-data
-msim -mno-sim
-mas100-syntax -mno-as100-syntax
-mrelax
-mmax-constant-size=
-mint-register=
-mpid
-mallow-string-insns -mno-allow-string-insns
-mjsr
-mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts
S/390 and zSeries Options
-mtune= cpu-type -march= cpu-type
-mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128
-mbackchain -mno-backchain -mpacked-stack -mno-packed-stack
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle
-m64 -m31 -mdebug -mno-debug -mesa -mzarch
-mhtm -mvx -mzvector
-mtpf-trace -mno-tpf-trace -mtpf-trace-skip -mno-tpf-trace-skip
-mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard
-mhotpatch= halfwords , halfwords
Score Options
-meb -mel
-mnhwloop
-muls
-mmac
-mscore5 -mscore5u -mscore7 -mscore7d
SH Options
-m1 -m2 -m2e
-m2a-nofpu -m2a-single-only -m2a-single -m2a
-m3 -m3e
-m4-nofpu -m4-single-only -m4-single -m4
-m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al
-mb -ml -mdalign -mrelax
-mbigtable -mfmovd -mrenesas -mno-renesas -mnomacsave
-mieee -mno-ieee -mbitops -misize -minline-ic_invalidate -mpadstruct
-mprefergot -musermode -multcost= number -mdiv= strategy
-mdivsi3_libfunc= name -mfixed-range= register-range
-maccumulate-outgoing-args
-matomic-model= atomic-model
Chapter 3: GCC Command Options
-mbranch-cost= num -mzdcbranch -mno-zdcbranch
-mcbranch-force-delay-slot
-mfused-madd -mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas
Solaris 2 Options
-mclear-hwcap -mno-clear-hwcap -mimpure-text -mno-impure-text
-pthreads
SPARC Options
-mcpu= cpu-type
-mtune= cpu-type
-mcmodel= code-model
-mmemory-model= mem-model
-m32 -m64 -mapp-regs -mno-app-regs
-mfaster-structs -mno-faster-structs -mflat -mno-flat
-mfpu -mno-fpu -mhard-float -msoft-float
-mhard-quad-float -msoft-quad-float
-mstack-bias -mno-stack-bias
-mstd-struct-return -mno-std-struct-return
-munaligned-doubles -mno-unaligned-doubles
-muser-mode -mno-user-mode
-mv8plus -mno-v8plus -mvis -mno-vis
-mvis2 -mno-vis2 -mvis3 -mno-vis3
-mvis4 -mno-vis4 -mvis4b -mno-vis4b
-mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld -mno-fsmuld
-mpopc -mno-popc -msubxc -mno-subxc
-mfix-at697f -mfix-ut699 -mfix-ut700 -mfix-gr712rc
-mlra -mno-lra
System V Options
-Qy -Qn -YP, paths -Ym, dir
TILE-Gx Options
-mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian
-mcmodel= code-model
TILEPro Options
-mcpu= cpu -m32
V850 Options
-mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace
-mtda= n -msda= n -mzda= n
-mapp-regs -mno-app-regs
-mdisable-callt -mno-disable-callt
-mv850e2v3 -mv850e2 -mv850e1 -mv850es
-mv850e -mv850 -mv850e3v5
-mloop
-mrelax
-mlong-jumps
-msoft-float
-mhard-float
-mgcc-abi
-mrh850-abi
-mbig-switch
VAX Options
-mg -mgnu -munix -mlra
Visium Options
31
32 Using the GNU Compiler Collection (GCC)
-mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float
-mcpu= cpu-type -mtune= cpu-type -msv-mode -muser-mode
VMS Options
-mvms-return-codes -mdebug-main= prefix -mmalloc64
-mpointer-size= size
VxWorks Options
-mrtp -non-static -Bstatic -Bdynamic
-Xbind-lazy -Xbind-now x86 Options
-mtune= cpu-type -march= cpu-type
-mtune-ctrl= feature-list -mdump-tune-features -mno-default
-mfpmath= unit
-masm= dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -m80387 -mhard-float -msoft-float
-mno-wide-multiply -mrtd -malign-double
-mpreferred-stack-boundary= num
-mincoming-stack-boundary= num
-mcld -mcx16 -msahf -mmovbe -mcrc32 -mmwait
-mrecip -mrecip= opt
-mvzeroupper -mprefer-avx128 -mprefer-vector-width= opt
-mmove-max= bits -mstore-max= bits
-mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx
-mavx2 -mavx512f -mavx512pf -mavx512er -mavx512cd -mavx512vl
-mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes
-mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd
-mptwrite -mprefetchwt1 -mclflushopt -mclwb -mxsavec -mxsaves
-msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop
-madx -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mhle -mlwp
-mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes -mwaitpkg
-mshstk -mmanual-endbr -mforce-indirect-call -mavx512vbmi2 -mavx512bf16 menqcmd
-mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq
-mavx5124fmaps -mavx512vnni -mavx5124vnniw -mprfchw -mrdpid
-mrdseed -msgx -mavx512vp2intersect -mserialize -mtsxldtrk
-mamx-tile -mamx-int8 -mamx-bf16 -muintr -mhreset -mavxvnni
-mavx512fp16
-mcldemote -mms-bitfields -mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy= alg
-mkl -mwidekl
-mmemcpy-strategy= strategy -mmemset-strategy= strategy
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128
-mregparm= num -msseregparm
-mveclibabi= type -mvect8-ret-in-mem
-mpc32 -mpc64 -mpc80 -mstackrealign
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel= code-model -mabi= name -maddress-mode= mode
-m32 -m64 -mx32 -m16 -miamcu -mlarge-data-threshold= num
-msse2avx -mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv
-minstrument-return= type -mfentry-name= name -mfentry-section= name
-mavx256-split-unaligned-load -mavx256-split-unaligned-store
-malign-data= type -mstack-protector-guard= guard
-mstack-protector-guard-reg= reg
-mstack-protector-guard-offset= offset
-mstack-protector-guard-symbol= symbol
-mgeneral-regs-only -mcall-ms2sysv-xlogues -mrelax-cmpxchg-loop
Chapter 3: GCC Command Options 33
-mindirect-branch= choice -mfunction-return= choice
-mindirect-branch-register -mharden-sls= choice
-mindirect-branch-cs-prefix -mneeded x86 Windows Options
-mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread
-municode -mwin32 -mwindows -fno-set-stack-executable
Xstormy16 Options
-msim
Xtensa Options
-mconst16 -mno-const16
-mfused-madd -mno-fused-madd
-mforce-no-pic
-mserialize-volatile -mno-serialize-volatile
-mtext-section-literals -mno-text-section-literals
-mauto-litpools -mno-auto-litpools
-mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
-mabi= abi-type zSeries Options See S/390 and zSeries Options.
3.2 Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done: file .c
file .m
C source code that must be preprocessed.
file .i
C source code that should not be preprocessed.
file .ii
C ++ source code that should not be preprocessed.
Objective-C source code. Note that you must link with the ‘ libobjc ’ library to make an Objective-C program work.
file .mi
Objective-C source code that should not be preprocessed.
file .mm
file .M
Objective-C ++ source code. Note that you must link with the ‘ libobjc ’ library to make an Objective-C ++ program work. Note that ‘ .M
’ refers to a literal capital M.
file .mii
Objective-C ++ source code that should not be preprocessed.
file .h
C, C ++ , Objective-C or Objective-C ++ header file to be turned into a precompiled header (default), or C, C ++ header file to be turned into an Ada spec (via the ‘ -fdump-ada-spec ’ switch).
34 Using the GNU Compiler Collection (GCC) file .cc
file .cp
file .cxx
file .cpp
file .CPP
file .c++ file .C
C ++ source code that must be preprocessed. Note that in ‘ .cxx
’, the last two letters must both be literally ‘ x ’. Likewise, ‘ .C
’ refers to a literal capital C.
file .mm
file .M
Objective-C ++ source code that must be preprocessed.
file .mii
Objective-C ++ source code that should not be preprocessed.
file .hh
file .H
file .hp
file .hxx
file .hpp
file .HPP
file .h++ file .tcc
C ++ header file to be turned into a precompiled header or Ada spec.
file .f
file .for
file .ftn
Fixed form Fortran source code that should not be preprocessed.
file .F
file .FOR
file .fpp
file .FPP
file .FTN
Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
file .f90
file .f95
file .f03
file .f08
Free form Fortran source code that should not be preprocessed.
file .F90
file .F95
file .F03
file .F08
Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
file .go
Go source code.
file .d
D source code.
file .di
D interface file.
file .dd
D documentation code (Ddoc).
Chapter 3: GCC Command Options 35 file .ads
Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration).
Such files are also called specs .
file .adb
Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies .
file .s
Assembler code.
file .S
file .sx
Assembler code that must be preprocessed.
other An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the ‘ -x ’ option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next ‘ -x ’ option. Possible values for language are: c c-header cpp-output c++ c++-header c++-system-header c++-user-header c++-cpp-output objective-c objective-c-header objective-c-cpp-output objective-c++ objective-c++-header objective-c++-cpp-output assembler assembler-with-cpp ada d f77 f77-cpp-input f95 f95-cpp-input go
-x none Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as they are if ‘ -x ’ has not been used at all).
If you only want some of the stages of compilation, you can use ‘ -x ’ (or filename suffixes) to tell gcc where to start, and one of the options ‘ -c ’, ‘ -S ’, or ‘ -E ’ to say where gcc is to stop. Note that some combinations (for example, ‘ -x cpp-output -E ’) instruct gcc to do nothing at all.
-c
-S
Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix
‘ .c
’, ‘ .i
’, ‘ .s
’, etc., with ‘ .o
’.
Unrecognized input files, not requiring compilation or assembly, are ignored.
Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix ‘ .c
’, ‘ .i
’, etc., with ‘ .s
’.
Input files that don’t require compilation are ignored.
36 Using the GNU Compiler Collection (GCC)
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.
Input files that don’t require preprocessing are ignored.
-o file Place the primary output in file file . This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.
If ‘ -o ’ is not specified, the default is to put an executable file in ‘ a.out
’, the object file for ‘ source .
suffix ’ in ‘ source .o
’, its assembler file in ‘ source .s
’, a precompiled header file in ‘ source .
suffix .gch
’, and all preprocessed C source on standard output.
Though ‘ -o ’ names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. If both share the same base name, disregarding the suffix, the result of the combination is that base name, otherwise, they are concatenated, separated by a dash.
gcc -c foo.c ...
will use ‘ foo.o
’ as the primary output, and place aux outputs and dumps next to it, e.g., aux file ‘ foo.dwo
’ for ‘ -gsplit-dwarf ’, and dump file
‘ foo.c.???r.final
’ for ‘ -fdump-rtl-final ’.
If a non-linker output file is explicitly specified, aux and dump files by default take the same base name: gcc -c foo.c -o dir/foobar.o ...
will name aux outputs ‘ dir/foobar.* ’ and dump outputs ‘ dir/foobar.c.* ’.
A linker output will instead prefix aux and dump outputs: gcc foo.c bar.c -o dir/foobar ...
will generally name aux outputs ‘ dir/foobar-foo.* ’ and ‘ dir/foobar-bar.* ’, and dump outputs ‘ dir/foobar-foo.c.* ’ and ‘ dir/foobar-bar.c.* ’.
The one exception to the above is when the executable shares the base name with the single input: gcc foo.c -o dir/foo ...
in which case aux outputs are named ‘ dir/foo.* ’ and dump outputs named
‘ dir/foo.c.* ’.
The location and the names of auxiliary and dump outputs can be adjusted by the options ‘ -dumpbase ’, ‘ -dumpbase-ext ’, ‘ -dumpdir ’, ‘ -save-temps=cwd ’, and ‘ -save-temps=obj ’.
Chapter 3: GCC Command Options 37
-dumpbase dumpbase
This option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs: gcc -save-temps -S foo.c
saves the (no longer) temporary preprocessed file in ‘ foo.i
’, and then compiles to the (implied) output file ‘ foo.s
’, whereas: gcc -save-temps -dumpbase save-foo -c foo.c
preprocesses to in ‘ save-foo.i
’, compiles to ‘ save-foo.s
’ (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file
‘ foo.o
’.
Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by ‘ -fdump-* ’ options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g.
-savetemps , -gsplit-dwarf , -fcallgraph-info ) without it.
Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given ‘ -dumpbase basename.suf
’ by also specifying ‘ -dumpbase-ext
.suf
’.
If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. ‘ -dumpdir ’ or ‘ -save-temps=* ’) is ignored, and instead of appending to it, dumpbase fully overrides it: gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...
creates auxiliary and dump outputs named ‘ alt/foo.* ’, disregarding ‘ dir/ ’ in
‘ -o ’, the ‘ ./ ’ prefix implied by ‘ -save-temps=cwd ’, and ‘ pfx’ in ‘ -dumpdir ’.
When ‘ -dumpbase ’ is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx , as specified under ‘ -dumpdir ’. Then, each input file is compiled using the combined dumppfx , and default values for dumpbase and auxdropsuf are computed for each input file: gcc foo.c bar.c -c -dumpbase main ...
creates ‘ foo.o
’ and ‘ bar.o
’ as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named ‘ main-foo.* ’ and ‘ main-bar.* ’.
An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values : gcc -c foo.c -o dir/foobar.o -dumpbase ’’ ...
will name aux outputs ‘ dir/foo.* ’ and dump outputs ‘ dir/foo.c.* ’. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.
The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:
38 Using the GNU Compiler Collection (GCC) gcc foo.c bar.c -o dir/foobar -dumpbase ’’ -flto ...
The compilation of the source files will name auxiliary outputs ‘ dir/foo.* ’ and ‘ dir/bar.* ’, and dump outputs ‘ dir/foo.c.* ’ and ‘ dir/bar.c.* ’. LTO recompilation during linking will use ‘ dir/foobar.
’ as the prefix for dumps and auxiliary files.
-dumpbase-ext auxdropsuf
When forming the name of an auxiliary (but not a dump) output file, drop trailing auxdropsuf from dumpbase before appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase , i.e., the suffix of the input file when ‘ -dumpbase ’ is not present in the command line, or dumpbase is combined with dumppfx .
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...
creates ‘ dir/foo.o
’ as the main output, and generates auxiliary outputs in
‘ dir/x-foo.* ’, taking the location of the primary output, and dropping the ‘ .c
’ suffix from the dumpbase . Dump outputs retain the suffix: ‘ dir/x-foo.c.* ’.
This option is disregarded if it does not match the suffix of a specified dumpbase , except as an alternative to the executable suffix when appending the linker output base name to dumppfx , as specified below: gcc foo.c bar.c -o main.out -dumpbase-ext .out ...
creates ‘ main.out
’ as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named ‘ main-foo.* ’ and ‘ main-bar.* ’ and dump outputs named ‘ main-foo.c.* ’ and ‘ main-bar.c.* ’.
-dumpdir dumppfx
When forming the name of an auxiliary or dump output file, use dumppfx as a prefix: gcc -dumpdir pfx- -c foo.c ...
creates ‘ foo.o
’ as the primary output, and auxiliary outputs named
‘ pfx-foo.* ’, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: ‘ pfx-foo.c.* ’.
If dumppfx is to be used as a directory name, it must end with a directory separator: gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...
creates ‘ obj/bar.o
’ as the primary output, and auxiliary outputs named
‘ dir/bar.* ’, combining the given dumppfx with the default dumpbase derived from the primary output name.
Dump outputs also take the input name suffix: ‘ dir/bar.c.* ’.
It defaults to the location of the output file, unless the output file is a special file like /dev/null . Options ‘ -save-temps=cwd ’ and ‘ -save-temps=obj ’ override this default, just like an explicit ‘ -dumpdir ’ option.
In case multiple such options are given, the last one prevails: gcc -dumpdir pfx- -c foo.c -save-temps=obj ...
outputs ‘ foo.o
’, with auxiliary outputs named ‘ foo.* ’ because
‘ -save-temps=* ’ overrides the dumppfx given by the earlier ‘ -dumpdir ’
Chapter 3: GCC Command Options 39 option. It does not matter that ‘ =obj ’ is the default for ‘ -save-temps ’, nor that the output directory is implicitly the current directory. Dump outputs are named ‘ foo.c.* ’.
When compiling from multiple input files, if ‘ -dumpbase ’ is specified, dumpbase , minus a auxdropsuf suffix, and a dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx , so that each of the multiple compilations gets differently-named aux and dump outputs.
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...
outputs auxiliary dumps to ‘ dir/pfx-main-foo.* ’ and ‘ dir/pfx-main-bar.* ’, appending dumpbase - to dumppfx . Dump outputs retain the input file suffix:
‘ dir/pfx-main-foo.c.* ’ and ‘ dir/pfx-main-bar.c.* ’, respectively. Contrast with the single-input compilation: gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...
that, applying ‘ -dumpbase ’ to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in
‘ dir/pfx-main.* ’.
When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes the dumpbase - transformation above
(e.g. the compilation of ‘ foo.c
’ and ‘ bar.c
’ above, but without ‘ -c ’). If neither ‘ -dumpdir ’ nor ‘ -dumpbase ’ are given, the linker output base name, minus auxdropsuf , if specified, or the executable suffix otherwise, plus a dash is appended to the default dumppfx instead. Note, however, that unlike earlier cases of linking: gcc foo.c bar.c -dumpdir dir/pfx- -o main ...
does not append the output name ‘ main ’ to dumppfx , because ‘ -dumpdir ’ is explicitly specified. The goal is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an explicitlyspecified ‘ -dumpbase ’ would be combined with it, in order to avoid simply discarding a meaningful option.
When compiling and then linking from a single input file, the linker output base name will only be appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in single-input linking cases above, but not with an explicit ‘ -dumpdir ’ that inhibits the combination, even if overridden by ‘ -save-temps=* ’: gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...
Auxiliary outputs are named ‘ foo.* ’, and dump outputs ‘ foo.c.* ’, in the current working directory as ultimately requested by ‘ -save-temps=cwd ’.
Summing it all up for an intuitive though slightly imprecise data flow: the primary output name is broken into a directory part and a basename part; dumppfx is set to the former, unless overridden by ‘ -dumpdir ’ or
‘ -save-temps=* ’, and dumpbase is set to the latter, unless overriden by
‘ -dumpbase ’. If there are multiple inputs or linking, this dumpbase may be combined with dumppfx and taken from each input file.
Auxiliary output names for each input are formed by combining dumppfx , dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained.
40 Using the GNU Compiler Collection (GCC)
-v
-###
--help
When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase , as given or as derived from the linker output name but not from inputs, even in cases in which this combination would not otherwise be used as such, is passed down with a trailing period replacing the compiler-added dash, if any, as a ‘ -dumpdir ’ option to lto-wrapper ; being involved in linking, this program does not normally get any ‘ -dumpbase ’ and ‘ -dumpbase-ext ’, and it ignores them.
When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx , ensures it contains a directory component so that it overrides any ‘ -dumpdir ’, and passes that as ‘ -dumpbase ’ to sub-compilers.
Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
Like ‘ -v ’ except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or ./-_ . This is useful for shell scripts to capture the driver-generated command lines.
Print (on the standard output) a description of the command-line options understood by gcc . If the ‘ -v ’ option is also specified then ‘ --help ’ is also passed on to the various processes invoked by gcc , so that they can display the commandline options they accept. If the ‘ -Wextra ’ option has also been specified (prior to the ‘ --help ’ option), then command-line options that have no documentation associated with them are also displayed.
--target-help
Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed.
--help={ class | [ ^ ] qualifier } [ ,...
]
Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:
‘ optimizers ’
Display all of the optimization options supported by the compiler.
‘ warnings ’
Display all of the options controlling warning messages produced by the compiler.
‘ target ’ Display target-specific options. Unlike the ‘ --target-help ’ option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended ‘ --help= ’ syntax.
‘ params ’ Display the values recognized by the ‘ --param ’ option.
language Display the options supported for language , where language is the name of one of the languages supported in this version of GCC. If
Chapter 3: GCC Command Options 41 an option is supported by all languages, one needs to select ‘ common ’ class.
‘ common ’ Display the options that are common to all languages.
These are the supported qualifiers:
‘ undocumented ’
Display only those options that are undocumented.
‘ joined ’ Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: ‘ --help=target ’.
‘ separate ’
Display options taking an argument that appears as a separate word following the original option, such as: ‘ -o output-file ’.
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ‘ ^ ’ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to ‘ --help= ’ should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target . For example, to display all the target-specific optimization options, use:
--help=target,optimizers
The ‘ --help= ’ option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If ‘ --help ’ is also specified anywhere on the command line then this takes precedence over any ‘ --help= ’ option.
If the ‘ -Q ’ option appears on the command line before the ‘ --help= ’ option, then the descriptive text displayed by ‘ --help= ’ is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the ‘ --help= ’ option is used).
Here is a truncated example from the ARM port of gcc :
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi=
-mabort-on-noreturn
-mapcs
2
[disabled]
[disabled]
The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at ‘ -O2 ’ by using:
-Q -O2 --help=optimizers
42 Using the GNU Compiler Collection (GCC)
Alternatively you can discover which binary optimizations are enabled by ‘ -O3 ’ by using: gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify ‘ -pass-exit-codes ’, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C ++ , and Fortran front ends return
4 if an internal compiler error is encountered.
-pipe Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.
-specs= file
Process file after the compiler reads in the standard ‘ specs ’ file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1 , cc1plus , as , ld , etc. More than one ‘ -specs= file ’ can be specified on the command line, and they are processed in order, from left to right. See
Section 3.20 [Spec Files], page 487
, for information about the format of the file .
-wrapper Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under ‘ gdb --args ’, thus the invocation of cc1 is ‘ gdb --args cc1 ...
’.
-ffile-prefix-map= old = new
When compiling files residing in directory ‘ old ’, record any references to them in the result of the compilation as if the files resided in directory
‘ new ’ instead.
Specifying this option is equivalent to specifying all the individual ‘ -f*-prefix-map ’ options. This can be used to make reproducible builds that are location independent.
See also ‘ -fmacro-prefix-map ’,
‘ -fdebug-prefix-map ’ and ‘ -fprofile-prefix-map ’.
-fplugin= name .so
Load the plugin code in file name .so, assumed to be a shared object to be dlopen’d by the compiler.
The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (See
‘ -fplugin-argname key = value ’ below).
Each plugin should define the callback functions specified in the Plugins API.
-fplugin-argname key = value
Define an argument called key with a value of value for the plugin called name .
Chapter 3: GCC Command Options 43
-fdump-ada-spec [ -slim ]
For C and C ++ source and include files, generate corresponding Ada specs. See
Section “Generating Ada Bindings for C and C ++ headers” in GNAT User’s
Guide , which provides detailed documentation on this feature.
-fada-spec-parent= unit
In conjunction with ‘ -fdump-ada-spec [ -slim ]’ above, generate Ada specs as child units of parent unit .
-fdump-go-spec= file
For input files in any language, generate corresponding Go declarations in file .
This generates Go const , type , var , and func declarations which may be a useful way to start writing a Go interface to code written in some other language.
@ file Read command-line options from file . The options read are inserted in place of the original @ file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed.
Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. The file may itself contain additional
@ file options; any such options will be processed recursively.
3.3 Compiling C
++
Programs
C ++ source files conventionally use one of the suffixes ‘ .C
’, ‘ .cc
’, ‘ .cpp
’, ‘ .CPP
’, ‘ .c++ ’,
‘ .cp
’, or ‘ .cxx
’; C ++ header files often use ‘ .hh
’, ‘ .hpp
’, ‘ .H
’, or (for shared template code)
‘ .tcc
’; and preprocessed C ++ files use the suffix ‘ .ii
’. GCC recognizes files with these names and compiles them as C ++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc ).
However, the use of gcc does not add the C ++ library.
g++ is a program that calls GCC and automatically specifies linking against the C ++ library. It treats ‘ .c
’, ‘ .h
’ and ‘ .i
’ files as C ++ source files instead of C source files unless ‘ -x ’ is used. This program is also useful when precompiling a C header file with a ‘ .h
’ extension for use in C ++ compilations. On many systems, g++ is also installed with the name c++ .
When you compile C ++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C ++ programs. See
Section 3.4 [Options Controlling C Dialect], page 43
, for explanations of options for languages related to C. See
Section 3.5 [Options Controlling C ++ Dialect], page 51
, for explanations of options that are meaningful only for C ++ programs.
3.4 Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C ++ ,
Objective-C and Objective-C ++ ) that the compiler accepts:
-ansi In C mode, this is equivalent to ‘ -std=c90 ’. In C ++ mode, it is equivalent to
‘ -std=c++98 ’.
44 Using the GNU Compiler Collection (GCC)
-std=
This turns off certain features of GCC that are incompatible with ISO C90
(when compiling C code), or of standard C ++ (when compiling C ++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C ++ style ‘ // ’ comments as well as the inline keyword.
The alternate keywords __asm__ , __extension__ , __inline__ and __typeof_
_ continue to work despite ‘ -ansi ’. You would not want to use them in an ISO
C program, of course, but it is useful to put them in header files that might be included in compilations done with ‘ -ansi ’. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without ‘ -ansi ’.
The ‘ -ansi ’ option does not cause non-ISO programs to be rejected gratuitously. For that, ‘ -Wpedantic ’ is required in addition to ‘ -ansi ’. See
Section 3.8 [Warning Options], page 89 .
The macro __STRICT_ANSI__ is predefined when the ‘ -ansi ’ option is used.
Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn’t call for; this is to avoid interfering with any programs that might use these names for other things.
Functions that are normally built in but do not have semantics defined by ISO
C (such as alloca and ffs ) are not built-in functions when ‘ -ansi ’ is used. See
Section 6.59 [Other built-in functions provided by GCC], page 722 , for details
of the functions affected.
Determine the language standard. See
Chapter 2 [Language Standards Supported by GCC], page 5
, for details of these standard versions. This option is currently only supported when compiling C or C ++ .
The compiler can accept several base standards, such as ‘ c90 ’ or ‘ c++98 ’, and
GNU dialects of those standards, such as ‘ gnu90 ’ or ‘ gnu++98 ’. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example,
‘ -std=c90 ’ turns off certain features of GCC that are incompatible with ISO
C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by ‘ -Wpedantic ’ to identify which features are GNU extensions given that version of the standard.
For example ‘ -std=gnu90 -Wpedantic ’ warns about C ++ style ‘ // ’ comments, while ‘ -std=gnu99 -Wpedantic ’ does not.
A value for this option must be provided; possible values are
Chapter 3: GCC Command Options 45
‘ c90 ’
‘ c89 ’
‘ iso9899:1990 ’
Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as ‘ -ansi ’ for C code.
‘ iso9899:199409 ’
ISO C90 as modified in amendment 1.
‘ c99 ’
‘ c9x ’
‘ iso9899:1999 ’
‘ iso9899:199x ’
ISO C99.
This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See https://gcc.gnu.org/c99status.html
for more information.
The names ‘ c9x ’ and ‘ iso9899:199x ’ are deprecated.
‘ c11 ’
‘ c1x ’
‘ iso9899:2011 ’
ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from
Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name ‘ c1x ’ is deprecated.
‘ gnu90 ’
‘ gnu89 ’
‘ gnu99 ’
‘ gnu9x ’
‘ gnu11 ’
‘ gnu1x ’
‘ gnu17 ’
‘ gnu18 ’
‘ c17 ’
‘ c18 ’
‘ iso9899:2017 ’
‘ iso9899:2018 ’
ISO C17, the 2017 revision of the ISO C standard (published in
2018). This standard is same as C11 except for corrections of defects (all of which are also applied with ‘ -std=c11 ’) and a new value of __STDC_VERSION__ , and so is supported to the same extent as
C11.
‘ c2x ’ The next version of the ISO C standard, still under development.
The support for this version is experimental and incomplete.
GNU dialect of ISO C90 (including some C99 features).
GNU dialect of ISO C99. The name ‘
GNU dialect of ISO C11. The name ‘ gnu9x gnu1x
’ is deprecated.
’ is deprecated.
GNU dialect of ISO C17. This is the default for C code.
46 Using the GNU Compiler Collection (GCC)
‘ gnu2x ’ The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete.
‘
‘ c++98 ’ c++03 ’ The 1998 ISO C ++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as ‘ -ansi ’ for C ++ code.
‘ gnu++98 ’
‘ gnu++03 ’ GNU dialect of ‘ -std=c++98 ’.
‘ c++11 ’
‘ c++0x ’ The 2011 ISO C is deprecated.
++ standard plus amendments. The name ‘ c++0x ’
‘ gnu++11 ’
‘ gnu++0x ’ GNU dialect of ‘ -std=c++11 ’. The name ‘ gnu++0x ’ is deprecated.
‘ c++14 ’
‘ c++1y ’ The 2014 ISO C is deprecated.
++ standard plus amendments. The name ‘ c++1y ’
‘ gnu++14 ’
‘ gnu++1y ’ GNU dialect of ‘ -std=c++14 ’. The name ‘ gnu++1y ’ is deprecated.
‘ c++17 ’
‘ c++1z ’ The 2017 ISO C is deprecated.
++ standard plus amendments. The name ‘ c++1z ’
‘ gnu++17 ’
‘ gnu++1z ’ GNU dialect of ‘ -std=c++17 ’. This is the default for C ++ code.
The name ‘ gnu++1z ’ is deprecated.
‘ c++20 ’
‘ c++2a ’ The 2020 ISO C ++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases.
The name ‘ c++2a ’ is deprecated.
‘ gnu++20 ’
‘ gnu++2a ’ GNU dialect of ‘ -std=c++20 ’. Support is experimental, and could change in incompatible ways in future releases. The name ‘ gnu++2a ’ is deprecated.
‘
‘ c++2b ’ c++23 ’ The next revision of the ISO C ++ standard, planned for 2023. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
‘ gnu++2b ’
‘ gnu++23 ’ GNU dialect of ‘ -std=c++2b ’. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.
Chapter 3: GCC Command Options 47
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (‘ I ’, ‘ N ’ for new or ‘ O ’ for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (‘ C ’ or ‘ F ’, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C ++ .
-fno-asm Do not recognize asm , inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__ , __inline__ and
__typeof__ instead. In C, ‘ -ansi ’ implies ‘ -fno-asm ’.
In C ++ , inline is a standard keyword and is not affected by this switch. You may want to use the ‘ -fno-gnu-keywords ’ flag instead, which disables typeof but not asm and inline . In C99 mode (‘ -std=c99 ’ or ‘ -std=gnu99 ’), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.
-fno-builtin
-fno-builtinfunction
Don’t recognize built-in functions that do not begin with ‘ __builtin_ ’ as prefix.
See
Section 6.59 [Other built-in functions provided by GCC], page 722
, for details of the functions affected, including those which are not built-in functions when ‘ -ansi ’ or ‘ -std ’ options for strict ISO C conformance are used because they do not have an ISO standard meaning.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops.
The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with ‘ -Wformat ’ for bad calls to printf when printf is built in and strlen is known not to modify global memory.
With the ‘ -fno-builtinfunction ’ option only the built-in function function is disabled.
function must not begin with ‘ __builtin_ ’. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding ‘ -fbuiltinfunction ’ option; if you wish to enable built-in functions selectively when using ‘ -fno-builtin ’ or ‘ -ffreestanding ’, you may define macros such as:
48 Using the GNU Compiler Collection (GCC)
#define abs(n)
#define strcpy(d, s)
__builtin_abs ((n))
__builtin_strcpy ((d), (s))
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C ++ .
-ffreestanding
Assert that compilation targets a freestanding environment.
This implies
‘ -fno-builtin ’.
A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main .
The most obvious example is an OS kernel.
This is equivalent to
‘ -fno-hosted ’.
See
Chapter 2 [Language Standards Supported by GCC], page 5 , for details of
freestanding and hosted environments.
-fgimple
Enable parsing of function definitions marked with __GIMPLE . This is an experimental feature that allows unit testing of GIMPLE passes.
-fgnu-tm When the option ‘ -fgnu-tm ’ is specified, the compiler generates code for the
Linux variant of Intel’s current Transactional Memory ABI specification document (Revision 1.1, May 6 2009).
This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.
For more information on GCC’s support for transactional memory, See Section
“The GNU Transactional Memory Library” in GNU Transactional Memory
Library .
Note that the transactional memory feature is not supported with non-call exceptions (‘ -fnon-call-exceptions ’).
-fgnu89-inline
The option ‘ -fgnu89-inline ’ tells GCC to use the traditional GNU semantics for inline functions when in C99 mode. See
lent to adding the gnu_inline function attribute to all inline functions (see
Section 6.33 [Function Attributes], page 548
).
The option ‘ -fno-gnu89-inline ’ explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior).
This option is not supported in ‘ -std=c90 ’ or ‘ -std=gnu90 ’ mode.
The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions. See
Section “Common Predefined Macros” in The C Preprocessor .
-fhosted
Assert that compilation targets a hosted environment.
This implies
‘ -fbuiltin ’. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int . Examples are nearly everything except a kernel. This is equivalent to ‘ -fno-freestanding ’.
Chapter 3: GCC Command Options 49
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C ++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW; struct ABC {
};
UOW UOW;
Some cases of unnamed fields in structures and unions are only accepted with this option.
See
Section 6.63 [Unnamed struct/union fields within structs/unions], page 875 , for details.
Note that this option is off for all targets except for x86 targets using ms-abi.
-foffload=disable
-foffload=default
-foffload= target-list
Specify for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent to ‘ -foffload=default ’, is to generate code for all supported offload targets. The ‘ -foffload=disable ’ form generates code only for the host fallback, while ‘ -foffload= target-list ’ generates code only for the specified comma-separated list of offload targets.
Offload targets are specified in GCC’s internal target-triplet format. You can run the compiler with ‘ -v ’ to show the list of configured offload targets under
OFFLOAD_TARGET_NAMES .
-foffload-options= options
-foffload-options= target-triplet-list = options
With ‘ -foffload-options= options ’, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the ‘ -foffload-options= targetlist = options ’ form. The target-list is a comma-separated list in the same format as for the ‘ -foffload= ’ option.
Typical command lines are
-foffload-options=-lgfortran -foffload-options=-lm
-foffload-options="-lgfortran -lm" -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906 -foffload-options=-lm
-fopenacc
Enable handling of OpenACC directives #pragma acc in C/C ++ and !$acc in Fortran.
When ‘ -fopenacc ’ is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6
https://www.openacc.org
. This option implies ‘ -pthread ’, and thus is only supported on targets that have support for ‘ -pthread ’.
50 Using the GNU Compiler Collection (GCC)
-fopenacc-dim= geom
Specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of ’:’-separated sizes, in order
’gang’, ’worker’ and, ’vector’. A size can be omitted, to use a target-specific default value.
-fopenmp Enable handling of OpenMP directives #pragma omp in C/C ++ and !$omp in Fortran.
When ‘ -fopenmp ’ is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5
https://www.openmp.org
. This option implies ‘ -pthread ’, and thus is only supported on targets that have support for ‘ -pthread ’. ‘ -fopenmp ’ implies
‘ -fopenmp-simd ’.
-fopenmp-simd
Enable handling of OpenMP’s SIMD directives with #pragma omp in C/C ++ and !$omp in Fortran. Other OpenMP directives are ignored.
-fpermitted-flt-eval-methods= style
ISO/IEC TS 18661-3 defines new permissible values for FLT_EVAL_METHOD that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of FLT_EVAL_
METHOD . As such, code conforming to C11 may not have been written expecting the possibility of the new values.
‘ -fpermitted-flt-eval-methods ’ specifies whether the compiler should allow only the values of FLT_EVAL_METHOD specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.
style is either c11 or ts-18661-3 as appropriate.
The default when in a standards compliant mode (‘ -std=c11 ’ or similar) is
‘ -fpermitted-flt-eval-methods=c11 ’. The default when in a GNU dialect
(‘ -std=gnu11 ’ or similar) is ‘ -fpermitted-flt-eval-methods=ts-18661-3 ’.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables ‘ -fms-extensions ’, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef.
See
Section 6.63 [Unnamed struct/union fields within structs/unions], page 875
, for details. This is only supported for C, not C ++ .
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned . By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types.
Chapter 3: GCC Command Options 51
-fsigned-char
Let the type char be signed, like signed char .
Note that this is equivalent to ‘ -fno-unsigned-char ’, which is the negative form of ‘ -funsigned-char ’. Likewise, the option ‘ -fno-signed-char ’ is equivalent to ‘ -funsigned-char ’.
-funsigned-char
Let the type char be unsigned, like unsigned char .
Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default.
Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default.
The type char is always a distinct type from each of signed char or unsigned char , even though its behavior is always just like one of those two.
-fsso-struct= endianness
Set the default scalar storage order of structures and unions to the specified endianness. The accepted values are ‘ big-endian ’, ‘ little-endian ’ and ‘ native ’ for the native endianness of the target (the default). This option is not supported for C ++ .
Warning: the ‘ -fsso-struct ’ switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target.
3.5 Options Controlling C
++
Dialect
This section describes the command-line options that are only meaningful for C ++ programs.
You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file ‘ firstClass.C
’ like this: g++ -g -fstrict-enums -O -c firstClass.C
In this example, only ‘ -fstrict-enums ’ is an option meant only for C ++ programs; you can use the other options with any language supported by GCC.
Some options for compiling C programs, such as ‘ -std ’, are also relevant for C ++ programs. See
Section 3.4 [Options Controlling C Dialect], page 43 .
Here is a list of options that are only for compiling C ++ programs:
-fabi-version= n
Use version n of the C ++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the C ++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G ++ as ABI bugs are fixed.
Version 1 is the version of the C ++ ABI that first appeared in G ++ 3.2.
Version 2 is the version of the C ++ ABI that first appeared in G ++ 3.4, and was the default through G ++ 4.9.
52 Using the GNU Compiler Collection (GCC)
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G ++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G ++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G ++ 4.7, corrects the promotion behavior of C ++ 11 scoped enums and the mangling of template argument packs, const/static cast, prefix ++ and –, and a class scope function used as a template argument.
Version 7, which first appeared in G ++ 4.8, that treats nullptr t as a builtin type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G ++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G ++ 5.2, corrects the alignment of nullptr_t .
Version 10, which first appeared in G ++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. ‘ stdcall ’).
Version 11, which first appeared in G ++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies ‘ -fnew-inheriting-ctors ’.
Version 12, which first appeared in G ++ 8, corrects the calling conventions for empty classes on the x86 64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor.
Version 13, which first appeared in G ++ 8.2, fixes the accidental change in version 12.
Version 14, which first appeared in G ++ 10, corrects the mangling of the nullptr expression.
Version 15, which first appeared in G ++ 11, changes the mangling of __alignof_
_ to be distinct from that of alignof , and dependent operator names.
See also ‘ -Wabi ’.
-fabi-compat-version= n
On targets that support strong aliases, G ++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.
With ‘ -fabi-version=0 ’ (the default), this defaults to 11 (GCC 7 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use ‘ -fabi-compat-version=2 ’.
If this option is not provided but ‘ -Wabi= n ’ is, that version is used for compatibility aliases. If this option is provided along with ‘ -Wabi ’ (without the version), the version from this option is used for the warning.
Chapter 3: GCC Command Options 53
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
-faligned-new
Enable support for C ++ 17 new of types that require more alignment than void* ::operator new(std::size_t) provides. A numeric argument such as
-faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t) .
This flag is enabled by default for ‘ -std=c++17 ’.
-fchar8_t
-fno-char8_t
Enable support for char8_t as adopted for C ++ 20. This includes the addition of a new char8_t fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new __cpp_char8_t and __cpp_lib_char8_t feature test macros.
This option enables functions to be overloaded for ordinary and UTF-8 strings: int f(const char *); // #1 int f(const char8_t *); // #2 int v1 = f("text"); // Calls #1 int v2 = f(u8"text"); // Calls #2 and introduces new signatures for user-defined literals: int operator""_udl1(char8_t); int v3 = u8’x’_udl1; int operator""_udl2(const char8_t*, std::size_t); int v4 = u8"text"_udl2; template<typename T, T...> int operator""_udl3(); int v5 = u8"text"_udl3;
The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C ++ 11 and later standards. For example, the following code is well-formed under ISO C ++ 11, but is ill-formed when ‘ -fchar8_t ’ is specified.
char ca[] = u8"xx"; // error: char-array initialized from wide
// string const char *cp = u8"xx";// error: invalid conversion from
// ‘const char8_t*’ to ‘const char*’ int f(const char*); auto v = f(u8"xx"); // error: invalid conversion from
// ‘const char8_t*’ to ‘const char*’ std::string s{u8"xx"}; // error: no matching function for call to
// ‘std::basic_string<char>::basic_string()’ using namespace std::literals; s = u8"xx"s; // error: conversion from
//
//
‘basic_string<char8_t>’ to non-scalar type ‘basic_string<char>’ requested
-fcheck-new
Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. This check is normally unnecessary because
54 Using the GNU Compiler Collection (GCC) the C ++ standard specifies that operator new only returns 0 if it is declared throw() , in which case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc .
See also ‘ new (nothrow) ’.
-fconcepts
-fconcepts-ts
Below ‘ -std=c++20 ’, ‘ -fconcepts ’ enables support for the C ++ Extensions for
Concepts Technical Specification, ISO 19217 (2015).
With ‘ -std=c++20 ’ and above, Concepts are part of the language standard, so
‘ -fconcepts ’ defaults to on. But the standard specification of Concepts differs significantly from the TS, so some constructs that were allowed in the TS but didn’t make it into the standard can still be enabled by ‘ -fconcepts-ts ’.
-fconstexpr-depth= n
Set the maximum nested evaluation depth for C ++ 11 constexpr functions to n . A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.
-fconstexpr-cache-depth= n
Set the maximum level of nested evaluation depth for C ++ 11 constexpr functions that will be cached to n . This is a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you.
-fconstexpr-fp-except
Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C ++ compilers have historically not followed this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined.
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except
-fconstexpr-loop-limit= n
Set the maximum number of iterations for a loop in C ++ 14 constexpr functions to n .
A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1 << 18).
-fconstexpr-ops-limit= n
Set the maximum number of operations during a single constexpr evaluation.
Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1 << 25).
Chapter 3: GCC Command Options 55
-fcoroutines
Enable support for the C ++ coroutines extension (experimental).
-fno-elide-constructors
The C ++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G ++ to call the copy constructor in all cases.
This option also causes G ++ to call trivial member functions which otherwise would be expanded inline.
In C ++ 17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.
-fno-enforce-eh-specs
Don’t generate code to check for violation of exception specifications at run time. This option violates the C ++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG . This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C ++ 11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn’t actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization
(either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the ‘ -fno-extern-tls-init ’ option.
On targets that support symbol aliases, the default is ‘ -fextern-tls-init ’.
On targets that do not support symbol aliases, the default is
‘ -fno-extern-tls-init ’.
-fno-gnu-keywords
Do not recognize typeof as a keyword, so that code can use this word as an identifier. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C ++ dialects: ‘ -ansi ’, ‘ -std=c++98 ’, ‘ -std=c++11 ’, etc.
-fimplicit-constexpr
Make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function. This option can be used in C ++ 14 mode or later. This can result in initialization changing from dynamic to static and other optimizations.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated implicitly (i.e.
by use); only emit code for explicit instantiations. If you use this option, you
56 Using the GNU Compiler Collection (GCC) must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. See
[Template Instantiation], page 884
, for more information.
-fno-implicit-inline-templates
Don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by
#pragma implementation . This causes linker errors if these functions are not inlined everywhere they are called.
-fmodules-ts
-fno-modules-ts
Enable support for C ++ 20 modules (see
Section 3.23 [C ++ Modules], page 501
).
The ‘ -fno-modules-ts ’ is usually not needed, as that is the default. Even though this is a C ++ 20 feature, it is not currently implicitly enabled by selecting that standard version.
-fmodule-header
-fmodule-header=user
-fmodule-header=system
Compile a header file to create an importable header unit.
-fmodule-implicit-inline
Member functions defined in their class definitions are not implicitly inline for modular code. This is different to traditional C ++ behavior, for good reasons.
However, it may result in a difficulty during code porting. This option makes such function definitions implicitly inline. It does however generate an ABI incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain implicitly inline, regardless.)
-fno-module-lazy
Disable lazy module importing and module mapper creation.
-fmodule-mapper= [ hostname ] : port [ ?
ident ]
-fmodule-mapper=| program [ ?
ident ] args...
-fmodule-mapper== socket [ ?
ident ]
-fmodule-mapper=<> [ inout ][ ?
ident ]
-fmodule-mapper=< in > out [ ?
ident ]
-fmodule-mapper= file [ ?
ident ]
An oracle to query for module name to filename mappings. If unspecified the
CXX_MODULE_MAPPER environment variable is used, and if that is unset, an inprocess default is provided.
-fmodule-only
Only emit the Compiled Module Interface, inhibiting any object file.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.
Chapter 3: GCC Command Options 57
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C ++ 11 constructor inheritance. This is part of C ++ 17 but also considered to be a Defect Report against
C ++ 11 and C ++ 14. This flag is enabled by default unless ‘ -fabi-version=10 ’ or lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for ‘ -std=c++17 ’.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO
C. These include ffs , alloca , _exit , index , bzero , conjf , and other related functions.
-fnothrow-opt
Treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected .
-fno-operator-names
Do not treat the operator name keywords and , bitand , bitor , compl , not , or and xor as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue.
Currently, the only such diagnostic issued by G ++ is the one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using ‘ -fpermissive ’ allows some nonconforming code to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g.
void f(T) [with
T = int] rather than void f(int) ) so that it’s clear which template is involved.
When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use ‘ -fno-pretty-templates ’ to disable them.
58 Using the GNU Compiler Collection (GCC)
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C ++ run-time type identification features ( dynamic_cast and typeid ). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G ++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes.
Mixing code compiled with ‘ -frtti ’ with that compiled with ‘ -fno-rtti ’ may not work. For example, programs may fail to link if a class compiled with
‘ -fno-rtti ’ is used as a base for a class compiled with ‘ -frtti ’.
-fsized-deallocation
Enable the built-in global declarations void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept; as introduced in C ++ 14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under ‘ -std=c++14 ’ and above. The flag
‘ -Wsized-deallocation ’ warns about places that might want to add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C ++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type.
-fstrong-eval-order
Evaluate member access, array subscripting, and shift expressions in left-toright order, and evaluate assignment in right-to-left order, as adopted for C ++ 17.
Enabled by default with ‘ -std=c++17 ’. ‘ -fstrong-eval-order=some ’ enables just the ordering of member access and shift expressions, and is the default without ‘ -std=c++17 ’.
-ftemplate-backtrace-limit= n
Set the maximum number of template instantiation notes for a single warning or error to n . The default value is 10.
-ftemplate-depth= n
Set the maximum instantiation depth for template classes to n . A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C ++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C ++ 11). The default value is 900, as the compiler can run out of stack space before hitting
1024 in some situations.
Chapter 3: GCC Command Options 59
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C ++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn’t need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the __cxa_ atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your
C library supports __cxa_atexit .
-fno-use-cxa-get-exception-ptr
Don’t use the __cxa_get_exception_ptr runtime routine.
This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with __ attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility.
Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC’s C ++ linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC’s linkage model:
1. It sets the default visibility to hidden , like ‘ -fvisibility=hidden ’.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.
60 Using the GNU Compiler Collection (GCC)
In new code it is better to use ‘ -fvisibility=hidden ’ and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal.
When this flag is given, it is a violation of the ODR to define types with the same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker.
By default, G ++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G ++ .
-fext-numeric-literals (C ++ and Objective-C ++ only)
Accept imaginary, fixed-point, or machine-defined literal number suffixes as
GNU extensions.
When this option is turned off these suffixes are treated as C ++ 11 user-defined literal numeric suffixes. This is on by default for all pre-C ++ 11 dialects and all GNU dialects: ‘ -std=c++98 ’, ‘ -std=gnu++98 ’,
‘ -std=gnu++11 ’, ‘ -std=gnu++14 ’. This option is off by default for ISO C ++ 11 onwards (‘ -std=c++11 ’, ...).
-nostdinc++
Do not search for header files in the standard directories specific to C ++ , but do still search the other standard directories. (This option is used when building the C ++ library.)
-flang-info-include-translate
-flang-info-include-translate-not
-flang-info-include-translate= header
Inform of include translation events. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the form "user" or <system> it will be resolved to a specific user or system header using the include path.
-flang-info-module-cmi
-flang-info-module-cmi= module
Inform of Compiled Module Interface pathnames. The first will note all read
CMI pathnames. The module form will not reading a specific module’s CMI.
module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in <> or "" ).
-stdlib= libstdc ++ ,libc ++
When G ++ is configured to support this option, it allows specification of alternate C ++ runtime libraries. Two options are available: libstdc ++ (the default, native C ++ runtime for G ++ ) and libc ++ which is the C ++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The
Chapter 3: GCC Command Options 61 option switches G ++ to use the headers from the specified library and to emit
-lstdc++ or -lc++ respectively, when a C ++ runtime is required for linking.
In addition, these warning options have meanings only for C ++ programs:
-Wabi-tag (C ++ and Objective-C ++ only)
Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See
Section 7.7 [C ++ Attributes], page 887
for more information about ABI tags.
-Wcomma-subscript (C ++ and Objective-C ++ only)
Warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C ++ 20 and is going to be removed in C ++ 23. However, a comma expression wrapped in ( ) is not deprecated. Example: void f(int *a, int b, int c) { a[b,c]; // deprecated in C++20, invalid in C++23 a[(b,c)]; // OK
}
In C ++ 23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G ++ will accept the formerly valid syntax for code that is not valid in C ++ 23 but used to be valid but deprecated in C ++ 20 with a pedantic warning that can be disabled with ‘ -Wno-comma-subscript ’.
Enabled by default with ‘ -std=c++20 ’ unless ‘ -Wno-deprecated ’, and with
‘ -std=c++23 ’ regardless of ‘ -Wno-deprecated ’.
-Wctad-maybe-unsupported (C ++ and Objective-C ++ only)
Warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically " opt-in " ; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern: struct allow_ctad_t; // any name works template <typename T> struct S {
};
S(T) { }
S(allow_ctad_t) -> S<void>; // guide with incomplete parameter type will never be considered
-Wctor-dtor-privacy (C ++ and Objective-C ++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there’s at least one private member function that isn’t a constructor or destructor.
-Wdelete-non-virtual-dtor (C ++ and Objective-C ++ only)
Warn when delete is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by ‘ -Wall ’.
62 Using the GNU Compiler Collection (GCC)
-Wdeprecated-copy (C ++ and Objective-C ++ only)
Warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C ++ 11 and up.
This warning is enabled by
‘ -Wextra ’. With ‘ -Wdeprecated-copy-dtor ’, also deprecate if the class has a user-provided destructor.
-Wno-deprecated-enum-enum-conversion (C ++ and Objective-C ++ only)
Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C ++ 20. For example: enum E1 { e }; enum E2 { f }; int k = f - e;
‘ -Wdeprecated-enum-enum-conversion ’ is enabled by default with
‘ -std=c++20 ’.
In pre-C ++ 20 dialects, this warning can be enabled by
‘ -Wenum-conversion ’.
-Wno-deprecated-enum-float-conversion (C ++ and Objective-C ++ only)
Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C ++ 20. For example: enum E1 { e }; enum E2 { f }; bool b = e <= 3.7;
‘ -Wdeprecated-enum-float-conversion ’ is enabled by default with
‘ -std=c++20 ’.
In pre-C ++ 20 dialects, this warning can be enabled by
‘ -Wenum-conversion ’.
-Wno-init-list-lifetime (C ++ and Objective-C ++ only)
Do not warn about uses of std::initializer_list that are likely to result in dangling pointers. Since the underlying array for an initializer_list is handled like a normal C ++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array’s lifetime. For example:
• If a function returns a temporary initializer_list , or a local initializer_list variable, the array’s lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
• If a new-expression creates an initializer_list , the array only lives until the end of the enclosing full-expression, so the initializer_list in the heap has a dangling pointer.
• When an initializer_list variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the initializer_list variable has a dangling pointer.
// li’s initial underlying array lives as long as li std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array li = { 4, 5 };
Chapter 3: GCC Command Options 63
// now the temporary is gone and li has a dangling pointer int i = li.begin()[0] // undefined behavior
• When a list constructor stores the begin pointer from the initializer_ list argument, this doesn’t extend the lifetime of the array, so if a class variable is constructed from a temporary initializer_list , the pointer is left dangling by the end of the variable declaration statement.
-Winvalid-imported-macros
Verify all imported macro definitions are valid at the end of compilation. This is not enabled by default, as it requires additional processing to determine. It may be useful when preparing sets of header-units to ensure consistent macros.
-Wno-literal-suffix (C ++ and Objective-C ++ only)
Do not warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from <inttypes.h> . For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h> int main() { int64_t i64 = 123; printf("My int64: %" PRId64"\n", i64);
}
In this case, PRId64 is treated as a separate preprocessing token.
This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn’t begin with an underscore.
Literal suffix identifiers that don’t begin with an underscore are reserved for future standardization.
These warnings are enabled by default.
-Wno-narrowing (C ++ and Objective-C ++ only)
For C ++ 11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but ‘ -Wno-narrowing ’ suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts.
With ‘ -Wnarrowing ’ in C ++ 98, warn when a narrowing conversion prohibited by C ++ 11 occurs within ‘ { } ’, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in ‘ -Wall ’ and ‘ -Wc++11-compat ’.
-Wnoexcept (C ++ and Objective-C ++ only)
Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e.
throw() or noexcept ) but is known by the compiler to never throw an exception.
64 Using the GNU Compiler Collection (GCC)
-Wnoexcept-type (C ++ and Objective-C ++ only)
Warn if the C ++ 17 feature making noexcept part of a function type changes the mangled name of a symbol relative to C ++ 14. Enabled by ‘ -Wabi ’ and
‘ -Wc++17-compat ’.
As an example: template <class T> void f(T t) { t(); }; void g() noexcept; void h() { f(g); }
In C ++ 14, f calls f<void(*)()> , but in C ++ 17 it calls f<void(*)()noexcept> .
-Wclass-memaccess (C ++ and Objective-C ++ only)
Warn when the destination of a call to a raw memory function such as memset or memcpy is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to memset below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.
std::string str = "abc"; memset (&str, 0, sizeof str);
The ‘ -Wclass-memaccess ’ option is enabled by ‘ -Wall ’. Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning.
-Wnon-virtual-dtor (C ++ and Objective-C ++ only)
Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if ‘ -Weffc++ ’ is specified.
-Wregister (C ++ and Objective-C ++ only)
Warn on uses of the register storage class specifier, except when it is part of the GNU
Section 6.47.5 [Explicit Register Variables], page 701
extension. The use of the register keyword as storage class specifier has been deprecated in
C ++ 11 and removed in C ++ 17. Enabled by default with ‘ -std=c++17 ’.
-Wreorder (C ++ and Objective-C ++ only)
Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance: struct A { int i; int j;
};
A(): j (0), i (1) { }
The compiler rearranges the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by ‘ -Wall ’.
Chapter 3: GCC Command Options 65
-Wno-pessimizing-move (C ++ and Objective-C ++ only)
This warning warns when a call to std::move prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}
But in this example, the std::move call prevents copy elision.
This warning is enabled by ‘ -Wall ’.
-Wno-redundant-move (C ++ and Objective-C ++ only)
This warning warns about redundant calls to std::move ; that is, when a move operation would have been performed even without the std::move call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn’t applicable. Consider: struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}
Here, the std::move call is redundant. Because G ++ implements Core Issue
1579, another example is: struct T { // convertible to U
...
}; struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}
In this example, copy elision isn’t applicable because the type of the expression being returned and the function return type differ, yet G ++ treats the return value as if it were designated by an rvalue.
This warning is enabled by ‘ -Wextra ’.
66 Using the GNU Compiler Collection (GCC)
-Wrange-loop-construct (C ++ and Objective-C ++ only)
This warning warns when a C ++ range-based for-loop is creating an unnecessary copy.
This can happen when the range declaration is not a reference, but probably should be. For example: struct S { char arr[128]; }; void fn () {
S arr[5]; for (const auto x : arr) { ... }
}
It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes.
This warning also warns when a loop variable in a range-based for-loop is initialized with a value of a different type resulting in a copy. For example: void fn() { int arr[10]; for (const double &x : arr) { ... }
}
In the example above, in every iteration of the loop a temporary value of type double is created and destroyed, to which the reference const double & is bound.
This warning is enabled by ‘ -Wall ’.
-Wredundant-tags (C ++ and Objective-C ++ only)
Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example: struct foo; struct foo *p; // warn that keyword struct can be eliminated
On the other hand, in this example there is no warning: struct foo; void foo (); // "hides" struct foo void bar (struct foo&); // no warning, keyword struct is necessary
-Wno-subobject-linkage (C ++ and Objective-C ++ only)
Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type
B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn’t give this warning for types defined in the main .C
file, as those are unlikely to have multiple definitions. ‘ -Wsubobject-linkage ’ is enabled by default.
-Weffc++ (C ++ and Objective-C ++ only)
Warn about violations of the following style guidelines from Scott Meyers’ Effective C ++ series of books:
• Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
Chapter 3: GCC Command Options 67
• Prefer initialization to assignment in constructors.
• Have operator= return a reference to *this .
• Don’t try to return a reference when you must return an object.
• Distinguish between prefix and postfix forms of increment and decrement operators.
• Never overload && , || , or , .
This option also enables ‘ -Wnon-virtual-dtor ’, which is also one of the effective C ++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use ‘ grep -v ’ to filter out those warnings.
-Wno-exceptions (C ++ and Objective-C ++ only)
Disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers.
-Wstrict-null-sentinel (C ++ and Objective-C ++ only)
Warn about the use of an uncasted NULL as sentinel. When compiling only with
GCC this is a valid sentinel, as NULL is defined to __null . Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.
-Wno-non-template-friend (C ++ and Objective-C ++ only)
Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as ‘ friend int foo(int) ’, where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default.
-Wold-style-cast (C ++ and Objective-C ++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C ++ program. The new-style casts ( dynamic_cast , static_cast , reinterpret_ cast , and const_cast ) are less vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual (C ++ and Objective-C ++ only)
Warn when a function declaration hides virtual functions from a base class. For example, in: struct A { virtual void f();
}; struct B: public A {
}; void f(int); the A class version of f is hidden in B , and code like:
B* b; b->f(); fails to compile.
68 Using the GNU Compiler Collection (GCC)
-Wno-pmf-conversions (C ++ and Objective-C ++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C ++ and Objective-C ++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G ++ tried to preserve unsignedness, but the standard mandates the current behavior.
-Wtemplates (C ++ and Objective-C ++ only)
Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the
STL. One may also instantiate or specialize templates.
-Wmismatched-new-delete (C ++ and Objective-C ++ only)
Warn for mismatches between calls to operator new or operator delete and the corresponding call to the allocation or deallocation function. This includes invocations of C ++ operator delete with pointers returned from either mismatched forms of operator new , or from other functions that allocate objects for which the operator delete isn’t a suitable deallocator, as well as calls to other deallocation functions with pointers returned from operator new for which the deallocation function isn’t suitable.
For example, the delete expression in the function below is diagnosed because it doesn’t match the array form of the new expression the pointer argument was returned from. Similarly, the call to free is also diagnosed.
void f ()
{ int *a = new int[n]; delete a; // warning: mismatch in array forms of expressions
} char *p = new char[n]; free (p); // warning: mismatch between new and free
The related option ‘ -Wmismatched-dealloc ’ diagnoses mismatches involving allocation and deallocation functions other than operator new and operator delete .
‘ -Wmismatched-new-delete ’ is included in ‘ -Wall ’.
-Wmismatched-tags (C ++ and Objective-C ++ only)
Warn for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided.
For example, the declaration of struct Object in the argument list of draw triggers the warning. To avoid it, either remove the redundant class-key struct or replace it with class to match its definition.
class Object { public: virtual ~Object () = 0;
Chapter 3: GCC Command Options 69
}; void draw (struct Object*);
It is not wrong to declare a class with the class-key struct as the example above shows. The ‘ -Wmismatched-tags ’ option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windowsbased compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with ‘ -Wredundant-tags ’.
-Wmultiple-inheritance (C ++ and Objective-C ++ only)
Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule.
The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-trivial C ++ 11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.
-Wnamespaces
Warn when a namespace definition is opened.
Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL.
One may also use using directives and qualified names.
-Wno-terminate (C ++ and Objective-C ++ only)
Disable the warning about a throw-expression that will immediately result in a call to terminate .
-Wno-vexing-parse (C ++ and Objective-C ++ only)
Warn about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a variable definition, but the C ++ language requires it to be interpreted as a function declaration. For instance: void f(double a) { int i(); // extern int i (void); int n(int(a)); // extern int n (int);
}
Another example: struct S { S(int); }; void f(double a) {
70 Using the GNU Compiler Collection (GCC)
S x(int(a)); // extern struct S x (int);
S y(int()); // extern struct S y (int (*) (void));
S z(); // extern struct S z (void);
}
The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead.
This warning is enabled by default.
-Wno-class-conversion (C ++ and Objective-C ++ only)
Do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called.
-Wvolatile (C ++ and Objective-C ++ only)
Warn about deprecated uses of the volatile qualifier. This includes postfix and prefix ++ and -expressions of volatile -qualified types, using simple assignments where the left operand is a volatile -qualified non-class type for their value, compound assignments where the left operand is a volatile -qualified non-class type, volatile -qualified function return type, volatile -qualified parameter type, and structured bindings of a volatile -qualified type. This usage was deprecated in C ++ 20.
Enabled by default with ‘ -std=c++20 ’.
-Wzero-as-null-pointer-constant (C ++ and Objective-C ++ only)
Warn when a literal ‘ 0 ’ is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C ++ 11.
-Waligned-new
Warn about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by ‘ -Wall ’.
Normally this only warns about global allocation functions, but
‘ -Waligned-new=all ’ also warns about class member allocation functions.
-Wno-placement-new
-Wplacement-new= n
Warn about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large.
char buf [64]; new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of ‘ -Wplacement-new ’. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code.
For example, the following new expression is not diagnosed at this level even though it has undefined behavior according to the C ++ standard because it writes past the end of the one-element array.
Chapter 3: GCC Command Options 71 struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]); new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]); new (s->a)int [32]();
-Wcatch-value
-Wcatch-value= n (C ++ and Objective-C ++ only)
Warn about catch handlers that do not catch via reference.
With
‘ -Wcatch-value=1 ’ (or ‘ -Wcatch-value ’ for short) warn about polymorphic class types that are caught by value. With ‘ -Wcatch-value=2 ’ warn about all class types that are caught by value. With ‘ -Wcatch-value=3 ’ warn about all types that are not caught by reference. ‘ -Wcatch-value ’ is enabled by ‘ -Wall ’.
-Wconditionally-supported (C ++ and Objective-C ++ only)
Warn for conditionally-supported (C ++ 11 [intro.defs]) constructs.
-Wno-delete-incomplete (C ++ and Objective-C ++ only)
Do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default.
-Wextra-semi (C ++ , Objective-C ++ only)
Warn about redundant semicolons after in-class function definitions.
-Wno-inaccessible-base (C ++ , Objective-C ++ only)
This option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the ‘ -Wextra ’ option.
struct A { int a; }; struct B : A { }; struct C : B, A { };
-Wno-inherited-variadic-ctor
Suppress warnings about use of C ++ 11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited.
-Wno-invalid-offsetof (C ++ and Objective-C ++ only)
Suppress warnings from applying the offsetof macro to a non-POD type.
According to the 2014 ISO C ++ standard, applying offsetof to a non-standardlayout type is undefined. In existing C ++ implementations, however, offsetof
72 Using the GNU Compiler Collection (GCC) typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.
The restrictions on offsetof may be relaxed in a future version of the C ++ standard.
-Wsized-deallocation (C ++ and Objective-C ++ only)
Warn about a definition of an unsized deallocation function void operator delete (void *) noexcept; void operator delete[] (void *) noexcept; without a definition of the corresponding sized deallocation function void operator delete (void *, std::size_t) noexcept; void operator delete[] (void *, std::size_t) noexcept; or vice versa. Enabled by ‘ -Wextra ’ along with ‘ -fsized-deallocation ’.
-Wsuggest-final-types
Warn about types with virtual methods where code quality would be improved if the type were declared with the C ++ 11 final specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with linktime optimization, where the information about the class hierarchy graph is more complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be improved if the method were declared with the C ++ 11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of ‘ -Wsuggest-final-types ’ and then rebuild with new annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked with the override keyword.
-Wuse-after-free
-Wuse-after-free= n
Warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels but may yield different results with optimization than without.
-Wuse-after-free=1
At level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call to realloc , regardless of whether or not the call resulted in an actual reallocatio of memory. This includes doublefree calls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.
Chapter 3: GCC Command Options 73
-Wuse-after-free=2
At level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call to free in the following function is diagnosed at this level: struct A { int refcount; void *data; }; void release (struct A *p)
{ int refcount = --p->refcount; free (p); if (refcount == 0) free (p->data); // warning: p may be used after free
}
-Wuse-after-free=3
At level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined but equality tests sometimes appear after calls to realloc as an attempt to determine whether the call resulted in relocating the object to a different address. They are diagnosed at a separate level to aid legacy code gradually transition to safe alternatives.
For example, the equality test in the function below is diagnosed at this level: void adjust_pointers (int**, int); void grow (int **p, int n)
{ int **q = (int**)realloc (p, n *= 2); if (q == p) return; adjust_pointers ((int**)q, n);
}
To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation.
‘ -Wuse-after-free=2 ’ is included in ‘ -Wall ’.
-Wuseless-cast (C ++ and Objective-C ++ only)
Warn when an expression is casted to its own type.
-Wno-conversion-null (C ++ and Objective-C ++ only)
Do not warn for conversions between NULL and non-pointer types.
‘ -Wconversion-null ’ is enabled by default.
3.6 Options Controlling Objective-C and Objective-C
++
Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C ++ languages themselves. See
Chapter 2 [Language Standards Supported by GCC], page 5 , for references.)
74 Using the GNU Compiler Collection (GCC)
This section describes the command-line options that are only meaningful for Objective-
C and Objective-C ++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file ‘ some_class.m
’ like this: gcc -g -fgnu-runtime -O -c some_class.m
In this example, ‘ -fgnu-runtime ’ is an option meant only for Objective-C and Objective-
C ++ programs; you can use the other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., ‘ -Wtraditional ’). Similarly,
Objective-C ++ compilations may use C ++ -specific options (e.g., ‘ -Wabi ’).
Here is a list of options that are only for compiling Objective-C and Objective-C ++ programs:
-fconstant-string-class= class-name
Use class-name as the name of the class to instantiate for each literal string specified with the syntax @"..." . The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below). The ‘ -fconstant-cfstrings ’ option, if also present, overrides the ‘ -fconstant-string-class ’ setting and cause @"..." literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime.
This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for
NeXT-based systems, including Darwin and Mac OS X. The macro __NEXT_
RUNTIME__ is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ( [receiver message:arg] ) in this translation unit ensure that the receiver is not nil . This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version= n
Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-
C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit)
ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C ++ object with a non-trivial default constructor. If so, synthesize a special - (id)
.cxx_construct
instance method which runs non-trivial default constructors on any such instance variables, in order, and then return self . Similarly, check if any instance variable is a C ++ object with a non-trivial destructor, and if
Chapter 3: GCC Command Options 75 so, synthesize a special - (void) .cxx_destruct
method which runs all such default destructors, in reverse order.
The - (id) .cxx_construct
and - (void) .cxx_destruct
methods thusly generated only operate on instance variables declared in the current
Objective-C class, and not those inherited from superclasses.
It is the responsibility of the Objective-C runtime to invoke all such methods in an object’s inheritance hierarchy.
The - (id) .cxx_construct
methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct
methods are invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct
and - (void) .cxx_destruct
methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C ++ . This option is required to use the Objective-C keywords @try , @throw , @catch , @finally and @synchronized . This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C ++ programs.
This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using ‘ -fno-objc-nilcheck ’. Class methods and super calls are never checked for nil in this way no matter what this flag is set to.
Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language recognized by
GCC 4.0. This only affects the Objective-C additions to the C/C ++ language; it does not affect conformance to C/C ++ standards, which is controlled by the separate C/C ++ dialect option flags. When this option is used with the
Objective-C or Objective-C ++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC.
76 Using the GNU Compiler Collection (GCC)
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently,
Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves runtime performance. Specifying the ‘ -fzero-link ’ flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. This is useful in
Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they’re declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the ‘ -fno-local-ivars ’ flag disables this behavior thus avoiding variable shadowing issues.
-fivar-visibility= [ public|protected|private|package ]
Set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named
‘ sourcename .decl
’.
-Wassign-intercept (Objective-C and Objective-C ++ only)
Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-property-assign-default (Objective-C and Objective-C ++ only)
Do not warn if a property for an Objective-C object has no assign semantics specified.
-Wno-protocol (Objective-C and Objective-C ++ only)
If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class.
The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the ‘ -Wno-protocol ’ option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them.
Chapter 3: GCC Command Options 77
-Wobjc-root-class (Objective-C and Objective-C ++ only)
Warn if a class interface lacks a superclass.
Most classes will inherit from
NSObject (or Object ) for example. When declaring classes intended to be root classes, the warning can be suppressed by marking their interfaces with
__attribute__((objc_root_class)) .
-Wselector (Objective-C and Objective-C ++ only)
Warn if multiple methods of different types for the same selector are found during compilation.
The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the ‘ -fsyntax-only ’ option is being used.
-Wstrict-selector-match (Objective-C and Objective-C ++ only)
Warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of type id or Class . When this flag is off (which is the default behavior), the compiler omits such warnings if any differences found are confined to types that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C ++ only)
Warn if a @selector(...) expression referring to an undeclared selector is found. A selector is considered undeclared if no method with that name has been declared before the @selector(...) expression, either explicitly in an
@interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks as soon as a @selector(...) expression is found, while ‘ -Wselector ’ only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
3.7 Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device’s aspect (e.g. its width, . . . ). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options.
-fmessage-length= n
Try to format error messages so that they fit on lines of about n characters. If n is zero, then no line-wrapping is done; each error message appears on a single line. This is the default for all front ends.
78 Using the GNU Compiler Collection (GCC)
Note - this option also affects the display of the ‘ #error ’ and ‘ #warning ’ preprocessor directives, and the ‘ deprecated ’ function/type/variable attribute.
It does not however affect the ‘ pragma GCC warning ’ and ‘ pragma GCC error ’ pragmas.
-fdiagnostics-plain-output
This option requests that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer that it remain more stable over time.
‘ -fdiagnostics-plain-output ’ is currently equivalent to the following options:
-fno-diagnostics-show-caret
-fno-diagnostics-show-line-numbers
-fdiagnostics-color=never
-fdiagnostics-urls=never
-fdiagnostics-path-format=separate-events
In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once ; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won’t be emitted (as prefix) again, over and over, in subsequent continuation lines. This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode.
Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line.
-fdiagnostics-color[= WHEN ]
-fno-diagnostics-color
Use color in diagnostics.
WHEN is ‘ never ’, ‘ always ’, or ‘ auto ’. The default depends on how the compiler has been configured, it can be any of the above
WHEN options or also ‘ never ’ if GCC_COLORS environment variable isn’t present in the environment, and ‘ auto ’ otherwise. ‘ auto ’ makes GCC use color only when the standard error is a terminal, and when not executing in an emacs shell.
The forms ‘ -fdiagnostics-color ’ and ‘ -fno-diagnostics-color ’ are aliases for ‘ -fdiagnostics-color=always ’ and ‘ -fdiagnostics-color=never ’, respectively.
The colors are defined by the environment variable GCC_COLORS . Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator.
(See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include ‘ 1 ’ for bold, ‘ 4 ’ for underline, ‘ 5 ’ for blink, ‘ 7 ’ for inverse, ‘ 39 ’ for default foreground color, ‘ 30 ’ to ‘ 37 ’ for foreground colors, ‘ 90 ’
Chapter 3: GCC Command Options 79 to ‘ 97 ’ for 16-color mode foreground colors, ‘ 38;5;0 ’ to ‘ 38;5;255 ’ for 88-color and 256-color modes foreground colors, ‘ 49 ’ for default background color, ‘ 40 ’ to ‘ 47 ’ for background colors, ‘ 100 ’ to ‘ 107 ’ for 16-color mode background colors, and ‘ 48;5;0 ’ to ‘ 48;5;255 ’ for 88-color and 256-color modes background colors.
The default GCC_COLORS is error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\ quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\ diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\ type-diff=01;32 where ‘ 01;31 ’ is bold red, ‘ 01;35 ’ is bold magenta, ‘ 01;36 ’ is bold cyan, ‘ 32 ’ is green, ‘ 34 ’ is blue, ‘ 01 ’ is bold, and ‘ 31 ’ is red. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.
error= SGR substring for error: markers.
warning= SGR substring for warning: markers.
note= SGR substring for note: markers.
path= SGR substring for colorizing paths of control-flow events as printed via ‘ -fdiagnostics-path-format= ’, such as the identifiers of individual events and lines indicating interprocedural calls and returns.
range1= SGR substring for first additional range.
range2= SGR substring for second additional range.
locus= quote=
SGR substring for location information, ‘ file:line ’ or
‘ file:line:column ’ etc.
SGR substring for information printed within quotes.
fixit-insert=
SGR substring for fix-it hints suggesting text to be inserted or replaced.
fixit-delete=
SGR substring for fix-it hints suggesting text to be deleted.
diff-filename=
SGR substring for filename headers within generated patches.
diff-hunk=
SGR substring for the starts of hunks within generated patches.
diff-delete=
SGR substring for deleted lines within generated patches.
diff-insert=
SGR substring for inserted lines within generated patches.
type-diff=
SGR substring for highlighting mismatching types within template arguments in the C ++ frontend.
80 Using the GNU Compiler Collection (GCC)
-fdiagnostics-urls[= WHEN ]
Use escape sequences to embed URLs in diagnostics.
For example, when
‘ -fdiagnostics-show-option ’ emits text showing the command-line option controlling a diagnostic, embed a URL for documentation of that option.
WHEN is ‘ never ’, ‘ always ’, or ‘ auto ’. ‘ auto ’ makes GCC use URL escape sequences only when the standard error is a terminal, and when not executing in an emacs shell or any graphical terminal which is known to be incompatible with this feature, see below.
The default depends on how the compiler has been configured. It can be any of the above WHEN options.
GCC can also be configured (via the ‘ --with-diagnostics-urls=auto-if-env ’ configure-time option) so that the default is affected by environment variables.
Under such a configuration, GCC defaults to using ‘ auto ’ if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or ‘ never ’ if neither are.
However, even with ‘ -fdiagnostics-urls=always ’ the behavior is dependent on those environment variables: If GCC_URLS is set to empty or ‘ no ’, do not embed URLs in diagnostics. If set to ‘ st ’, URLs use ST escape sequences. If set to ‘ bel ’, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS is not set, use TERM_URLS as a fallback.
Note: ST is an ANSI escape sequence, string terminator ‘ ESC \ ’, BEL is an
ASCII character, CTRL-G that usually sounds like a beep.
At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs or at least had bugs in some versions that are still in use, where the URL escapes are likely to misbehave, i.e. print garbage on the screen. That list is currently xfce4-terminal, certain known to be buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with the ‘ -fdiagnostics-urls=always ’.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the ‘ -fno-diagnostics-show-option ’ flag suppresses that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source line and a caret
‘ ^ ’ indicating the column. This option suppresses this information. The source line is truncated to n characters, if the ‘ -fmessage-length=n ’ option is given.
When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.
-fno-diagnostics-show-labels
By default, when printing source code (via ‘ -fdiagnostics-show-caret ’), diagnostics can label ranges of source code with pertinent information, such as the types of expressions: printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
Chapter 3: GCC Command Options 81
| char *
| long int
This option suppresses the printing of these labels (in the example above, the vertical bars and the “char *” and “long int” text).
-fno-diagnostics-show-cwe
Diagnostic messages can optionally have an associated CWE identifier. GCC itself only provides such metadata for some of the ‘ -fanalyzer ’ diagnostics.
GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.
-fno-diagnostics-show-line-numbers
By default, when printing source code (via ‘ -fdiagnostics-show-caret ’), a left margin is printed, showing line numbers. This option suppresses this left margin.
-fdiagnostics-minimum-margin-width= width
This option controls the minimum width of the left margin printed by
‘ -fdiagnostics-show-line-numbers ’. It defaults to 6.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for consumption by
IDEs. For each fix-it, a line will be printed after the relevant diagnostic, starting with the string “fix-it:”. For example: fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the above example, bytes 3 through
20 of line 45 of “test.c” are to be replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789 gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^ gtk_widget_show_all
The filename and replacement string escape backslash as “ \\" , tab as “ \ t”, newline as “ \ n”, double quotes as “ \" ”, non-printable characters as octal (e.g.
vertical tab as “ \ 013”).
An empty replacement string indicates that the given range is to be removed.
An empty range (e.g. “45:3-45:3”) indicates that the string is to be inserted at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @ void show_cb(GtkDialog *dlg)
{
gtk_widget_showall(dlg);
82 Using the GNU Compiler Collection (GCC)
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same rules as for diagnostics
(see ‘ -fdiagnostics-color ’).
-fdiagnostics-show-template-tree
In the C ++ frontend, when printing diagnostics showing mismatching template types, such as: could not convert ’std::map<int, std::vector<double> >()’ from ’map<[...],vector<double>>’ to ’map<[...],vector<float>> the ‘ -fdiagnostics-show-template-tree ’ flag enables printing a tree-like structure showing the common and differing parts of the types, such as: map<
[...], vector<
[double != float]>>
The parts that differ are highlighted with color (“double” and “float” in this case).
-fno-elide-type
By default when the C ++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as “[...]” to simplify the error message. For example: could not convert ’std::map<int, std::vector<double> >()’ from ’map<[...],vector<double>>’ to ’map<[...],vector<float>>
Specifying the ‘ -fno-elide-type ’ flag suppresses that behavior. This flag also affects the output of the ‘ -fdiagnostics-show-template-tree ’ flag.
-fdiagnostics-path-format= KIND
Specify how to print paths of control-flow events for diagnostics that have such a path associated with them.
KIND is ‘ none ’, ‘ separate-events ’, or ‘ inline-events ’, the default.
‘ none ’ means to not print diagnostic paths.
‘ separate-events ’ means to print a separate “note” diagnostic for each event within the diagnostic. For example: test.c:29:5: error: passing NULL as argument 1 to ’PyList_Append’ which requires a non-NULL parameter test.c:25:10: note: (1) when ’PyList_New’ fails, returning NULL test.c:27:3: note: (2) when ’i < count’ test.c:29:5: note: (3) when calling ’PyList_Append’, passing NULL from (1) as argument 1
‘ inline-events ’ means to print the events “inline” within the source code.
This view attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source.
For example, the same events as above might be printed as:
’test’: events 1-3
|
| 25 | list = PyList_New(0);
Chapter 3: GCC Command Options 83
|
|
|
|
|
|
^~~~~~~~~~~~~
|
(1) when ’PyList_New’ fails, returning NULL
| 26 |
| 27 | for (i = 0; i < count; i++) {
| | ~~~
|
|
| 28 |
| |
| (2) when ’i < count’ item = PyLong_FromLong(random());
|
|
| 29 |
| | gument 1
|
|
|
PyList_Append(list, item);
~~~~~~~~~~~~~~~~~~~~~~~~~
|
(3) when calling ’PyList_Append’, passing NULL from (1) as ar-
Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped.
For example:
’test’: events 1-2
|
| 133 | {
|
|
| ^
| |
| | (1) entering ’test’
| 134 | boxed_int *obj = make_boxed_int (i);
|
|
|
|
|
|
~~~~~~~~~~~~~~~~~~
|
(2) calling ’make_boxed_int’
|
+--> ’make_boxed_int’: events 3-4
|
| 120 | {
| | ^
| | |
|
|
| | (3) entering ’make_boxed_int’
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
(4) calling ’wrapped_malloc’ |
|
|
+--> ’wrapped_malloc’: events 5-6
|
|
|
7 | {
| ^
|
|
|
|
|
|
|
| |
| (5) entering ’wrapped_malloc’
8 | return malloc (size);
|
|
|
~~~~~~~~~~~~~
|
(6) calling ’malloc’
<-------------+
|
’test’: event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
84 Using the GNU Compiler Collection (GCC)
|
(etc)
|
|
| |
| (7) calling ’free_boxed_int’
-fdiagnostics-show-path-depths
This option provides additional information when printing control-flow paths associated with a diagnostic.
If this is option is provided then the stack depth will be printed for each run of events within ‘ -fdiagnostics-path-format=separate-events ’.
This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu .
-fdiagnostics-column-unit= UNIT
Select the units for the column number. This affects traditional diagnostics
(in the absence of ‘ -fno-show-column ’), as well as JSON format diagnostics if requested.
The default UNIT , ‘ display ’, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character, in the case of tab characters, or it may be smaller, in the case of multibyte characters. For example, the character “GREEK SMALL
LETTER PI (U + 03C0)” occupies one display column, and its UTF-8 encoding requires two bytes; the character “SLIGHTLY SMILING FACE (U + 1F642)” occupies two display columns, and its UTF-8 encoding requires four bytes.
Setting UNIT to ‘ byte ’ changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0.
-fdiagnostics-column-origin= ORIGIN
Select the origin for column numbers, i.e. the column number assigned to the first column. The default value of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of 0; any non-negative value may be specified.
-fdiagnostics-escape-format= FORMAT
When GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly. However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with Unicode normalization. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines.
This option controls how such bytes should be escaped.
The default FORMAT , ‘ unicode ’ displays Unicode characters that are not printable ASCII in the form ‘ <U+XXXX> ’, and bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as hexadecimal in the form ‘ <XX> ’.
Chapter 3: GCC Command Options 85
For example, a source line containing the string ‘ before ’ followed by the Unicode character U + 03C0 (“GREEK SMALL LETTER PI”, with UTF-8 encoding
0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by the string ‘ after ’ will be printed for such a diagnostic as: before<U+03C0><BF>after
Setting FORMAT to ‘ bytes ’ will display all non-printable-ASCII bytes in the form ‘ <XX> ’, thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed: before<CF><80><BF>after
-fdiagnostics-format= FORMAT
Select a different format for printing diagnostics.
FORMAT is ‘ text ’ or ‘ json ’.
The default is ‘ text ’.
The ‘ json ’ format consists of a top-level JSON array containing JSON objects representing the diagnostics.
The JSON is emitted as one line, without formatting; the examples below have been formatted for clarity.
Diagnostics can have child diagnostics. For example, this error and note: misleading-indentation.c:15:3: warning: this ’if’ clause does not guard... [-Wmisleading-indentation]
15 | if (flag)
| ^~ misleading-indentation.c:17:5: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ’if’
17 |
| y = 2;
^ might be printed in JSON form (after formatting) like this:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"display-column": 3,
"byte-column": 3,
"column": 3,
"file": "misleading-indentation.c",
"line": 15
},
"finish": {
"display-column": 4,
"byte-column": 4,
"column": 4,
"file": "misleading-indentation.c",
"line": 15
}
}
],
"message": "this \u2018if\u2019 clause does not guard...",
"option": "-Wmisleading-indentation",
"option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-
Wmisleading-indentation",
"children": [
86 Using the GNU Compiler Collection (GCC)
{
"kind": "note",
"locations": [
{
"display-column": 5,
"caret": {
"byte-column": 5,
"column": 5,
"file": "misleading-indentation.c",
"line": 17
}
}
],
"escape-source": false,
"message": "...this statement, but the latter is ..."
}
]
"escape-source": false,
"column-origin": 1,
}
] where the note is a child of the warning .
A diagnostic has a kind .
If this is warning , then there is an option key describing the command-line option controlling the warning.
A diagnostic can contain zero or more locations. Each location has an optional label string and up to three positions within it: a caret position and optional start and finish positions. A position is described by a file name, a line number, and three numbers indicating a column position:
• display-column counts display columns, accounting for tabs and multibyte characters.
• byte-column counts raw bytes.
• column is equal to one of the previous two, as dictated by the
‘ -fdiagnostics-column-unit ’ option.
All three columns are relative to the origin specified by ‘ -fdiagnostics-column-origin ’, which is typically equal to 1 but may be set, for instance, to 0 for compatibility with other utilities that number columns from 0. The column origin is recorded in the JSON output in the column-origin tag. In the remaining examples below, the extra column number outputs have been omitted for brevity.
For example, this error: bad-binary-ops.c:64:23: error: invalid operands to binary + (have ’S’ {aka
’struct s’} and ’T’ {aka ’struct t’})
64 | return callee_4a () + callee_4b ();
| ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
|
|
|
|
|
|
T {aka struct t}
S {aka struct s} has three locations. Its primary location is at the “ + ” token at column 23.
It has two secondary locations, describing the left and right-hand sides of the expression, which have labels. It might be printed in JSON form as:
{
Chapter 3: GCC Command Options 87
"children": [],
"kind": "error",
"locations": [
{
"caret": {
"column": 23, "file": "bad-binary-ops.c", "line": 64
}
},
{
"caret": {
"column": 10, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 21, "file": "bad-binary-ops.c", "line": 64
},
"label": "S {aka struct s}"
},
{
"caret": {
"column": 25, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 36, "file": "bad-binary-ops.c", "line": 64
},
"label": "T {aka struct t}"
}
],
"escape-source": false,
"message": "invalid operands to binary + ..."
}
If a diagnostic contains fix-it hints, it has a fixits array, consisting of halfopen intervals, similar to the output of ‘ -fdiagnostics-parseable-fixits ’.
For example, this diagnostic with a replacement fix-it hint: demo.c:8:15: error: ’struct s’ has no member named ’colour’; did you mean ’color’?
8 | return ptr->colour;
| ^~~~~~
| color might be printed in JSON form as:
{
"children": [],
"fixits": [
{
"next": {
"column": 21,
"file": "demo.c",
"line": 8
},
"start": {
"column": 15,
"file": "demo.c",
"line": 8
},
"string": "color"
}
],
"kind": "error",
88 Using the GNU Compiler Collection (GCC)
"locations": [
{
"caret": {
"column": 15,
"file": "demo.c",
"line": 8
},
"finish": {
"column": 20,
"file": "demo.c",
"line": 8
}
}
],
"escape-source": false,
"message": "\u2018struct s\u2019 has no member named ..."
} where the fix-it hint suggests replacing the text from start up to but not including next with string ’s value.
Deletions are expressed via an empty value for string , insertions by having start equal next .
If the diagnostic has a path of control-flow events associated with it, it has a path array of objects representing the events.
Each event object has a description string, a location object, along with a function string and a depth number for representing interprocedural paths. The function represents the current function at that event, and the depth represents the stack depth relative to some baseline: the higher, the more frames are within the stack.
For example, the intraprocedural example shown for ‘ -fdiagnostics-path-format= ’ might have this JSON for its path:
"path": [
{
"depth": 0,
"description": "when ’PyList_New’ fails, returning NULL",
"function": "test",
"location": {
"column": 10,
"file": "test.c",
"line": 25
}
},
{
"depth": 0,
"description": "when ’i < count’",
"function": "test",
"location": {
"column": 3,
"file": "test.c",
"line": 27
}
},
{
"depth": 0,
"description": "when calling ’PyList_Append’, passing NULL from (1) as argument 1",
"function": "test",
"location": {
Chapter 3: GCC Command Options 89
"column": 5,
"file": "test.c",
"line": 29
}
}
]
Diagnostics have a boolean attribute escape-source , hinting whether non-
ASCII bytes should be escaped when printing the pertinent lines of source code
( true for diagnostics involving source encoding issues).
3.8 Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don’t do anything beyond that.
-fmax-errors= n
Limits the maximum number of error messages to n , at which point GCC bails out rather than attempting to continue processing the source code. If n is 0
(the default), there is no limit on the number of error messages produced. If
‘ -Wfatal-errors ’ is also specified, then ‘ -Wfatal-errors ’ takes precedence over this option.
-w Inhibit all warning messages.
-Werror Make all warnings into errors.
-Werror= Make the specified warning into an error.
The specifier for a warning is appended; for example ‘ -Werror=switch ’ turns the warnings controlled by
‘ -Wswitch ’ into errors. This switch takes a negative form, to be used to negate
‘ -Werror ’ for specific warnings; for example ‘ -Wno-error=switch ’ makes
‘ -Wswitch ’ warnings not be errors, even when ‘ -Werror ’ is in effect.
The warning message for each controllable warning includes the option that controls the warning.
That option can then be used with ‘ -Werror= ’ and
‘ -Wno-error= ’ as described above. (Printing of the option in the warning message can be disabled using the ‘ -fno-diagnostics-show-option ’ flag.)
Note that specifying ‘ -Werror= ’ foo automatically implies ‘ -W ’ foo . However,
‘ -Wno-error= ’ foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.
You can request many specific warnings with options beginning with ‘ -W ’, for example
‘ -Wimplicit ’ to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning ‘ -Wno’ to turn off warnings; for example,
‘ -Wno-implicit ’. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to
Section 3.5 [C ++ Dialect Options],
90 Using the GNU Compiler Collection (GCC) page 51
and
Section 3.6 [Objective-C and Objective-C ++ Dialect Options], page 73
. Additional warnings can be produced by enabling the static analyzer; See
Some options, such as ‘ -Wall ’ and ‘ -Wextra ’, turn on other options, such as ‘ -Wunused ’, which may turn on further options, such as ‘ -Wunused-value ’. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command-line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas (see
[Diagnostic Pragmas], page 871 ) take effect as if they appeared at the end of the command-
line.
When an unrecognized warning option is requested (e.g., ‘ -Wunknown-warning ’),
GCC emits a diagnostic stating that the option is not recognized.
However, if the
‘ -Wno’ form is used, the behavior is slightly different: no diagnostic is produced for
‘ -Wno-unknown-warning ’ unless other diagnostics are being produced.
This allows the use of new ‘ -Wno’ options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.
The effectiveness of some warnings depends on optimizations also being enabled. For example ‘ -Wsuggest-final-types ’ is more effective with link-time optimization and
‘ -Wmaybe-uninitialized ’ does not warn at all unless optimization is enabled.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C ++ ; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C ++ . For ISO C, follows the version of the ISO C standard specified by any ‘ -std ’ option used.
Valid ISO C and ISO C ++ programs should compile properly with or without this option (though a rare few require ‘ -ansi ’ or a ‘ -std ’ option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C ++ features are supported as well. With this option, they are rejected.
‘ -Wpedantic ’ does not cause warning messages for use of the alternate keywords whose names begin and end with ‘ __ ’. This alternate format can also be used to disable warnings for non-ISO ‘ __intN ’ types, i.e. ‘ __intN__ ’. Pedantic warnings are also disabled in the expression that follows __extension__ . However, only system header files should use these escape routes; application programs should avoid them. See
Section 6.48 [Alternate Keywords], page 704 .
Some users try to use ‘ -Wpedantic ’ to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from ‘ -Wpedantic ’. We don’t have plans to support such a feature in the near future.
Chapter 3: GCC Command Options 91
Where the standard specified with ‘ -std ’ represents a GNU extended dialect of C, such as ‘ gnu90 ’ or ‘ gnu99 ’, there is a corresponding base standard , the version of ISO C on which the GNU extended dialect is based. Warnings from
‘ -Wpedantic ’ are given where they are required by the base standard.
(It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)
-pedantic-errors
Give an error whenever the base standard (see ‘ -Wpedantic ’) requires a diagnostic, in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to ‘ -Werror=pedantic ’, since there are errors enabled by this option and not enabled by the latter and vice versa.
-Wall This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros.
This also enables some language-specific warnings described in
Section 3.5 [C ++ Dialect Options], page 51
and
[Objective-C and Objective-C ++ Dialect Options], page 73
.
‘ -Wall ’ turns on the following warning flags:
-Waddress
-Warray-bounds=1 (only with ‘-O2’ )
-Warray-compare
-Warray-parameter=2 (C and Objective-C only)
-Wbool-compare
-Wbool-operation
-Wc++11-compat -Wc++14-compat
-Wcatch-value (C ++ and Objective-C ++ only)
-Wchar-subscripts
-Wcomment
-Wdangling-pointer=2
-Wduplicate-decl-specifier (C and Objective-C only)
-Wenum-compare (in C/ObjC; this is on by default in C ++ )
-Wformat
-Wformat-overflow
-Wformat-truncation
-Wint-in-bool-context
-Wimplicit (C and Objective-C only)
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C ++ )
-Wlogical-not-parentheses
-Wmain (only for C/ObjC and unless ‘-ffreestanding’ )
-Wmaybe-uninitialized
-Wmemset-elt-size
-Wmemset-transposed-args
-Wmisleading-indentation (only for C/C ++ )
-Wmismatched-dealloc
-Wmismatched-new-delete (only for C/C ++ )
-Wmissing-attributes
-Wmissing-braces (only for C/ObjC)
92 Using the GNU Compiler Collection (GCC)
-Wmultistatement-macros
-Wnarrowing (only for C ++ )
-Wnonnull
-Wnonnull-compare
-Wopenmp-simd
-Wparentheses
-Wpessimizing-move (only for C ++ )
-Wpointer-sign
-Wrange-loop-construct (only for C ++ )
-Wreorder
-Wrestrict
-Wreturn-type
-Wsequence-point
-Wsign-compare (only in C ++ )
-Wsizeof-array-div
-Wsizeof-pointer-div
-Wsizeof-pointer-memaccess
-Wstrict-aliasing
-Wstrict-overflow=1
-Wswitch
-Wtautological-compare
-Wtrigraphs
-Wuninitialized
-Wunknown-pragmas
-Wunused-function
-Wunused-label
-Wunused-value
-Wunused-variable
-Wuse-after-free=3
-Wvla-parameter (C and Objective-C only)
-Wvolatile-register-var
-Wzero-length-bounds
Note that some warning flags are not implied by ‘ -Wall ’. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by ‘ -Wextra ’ but many of them must be enabled individually.
-Wextra This enables some extra warning flags that are not enabled by ‘ -Wall ’. (This option used to be called ‘ -W ’. The older name is still supported, but the newer name is more descriptive.)
-Wclobbered
-Wcast-function-type
-Wdeprecated-copy (C ++ only)
-Wempty-body
-Wenum-conversion (C only)
-Wignored-qualifiers
-Wimplicit-fallthrough=3
-Wmissing-field-initializers
-Wmissing-parameter-type (C only)
-Wold-style-declaration (C only)
-Woverride-init
-Wsign-compare (C only)
-Wstring-compare
-Wredundant-move (only for C ++ )
Chapter 3: GCC Command Options 93
-Wtype-limits
-Wuninitialized
-Wshift-negative-value (in C ++ 03 and in C99 and newer)
-Wunused-parameter (only with ‘-Wunused’ or ‘-Wall’ )
-Wunused-but-set-parameter (only with ‘-Wunused’ or ‘-Wall’ )
The option ‘ -Wextra ’ also prints warning messages for the following cases:
• A pointer is compared against integer zero with < , <= , > , or >= .
• (C ++ only) An enumerator and a non-enumerator both appear in a conditional expression.
• (C ++ only) Ambiguous virtual bases.
• (C ++ only) Subscripting an array that has been declared register .
• (C ++ only) Taking the address of a variable that has been declared register .
• (C ++ only) A base class is not initialized in the copy constructor of a derived class.
-Wabi (C, Objective-C, C ++ and Objective-C ++ only)
Warn about code affected by ABI changes. This includes code that may not be compatible with the vendor-neutral C ++ ABI as well as the psABI for the particular target.
Since G ++ now defaults to updating the ABI with each major release, normally
‘ -Wabi ’ warns only about C ++ ABI compatibility problems if there is a check added later in a release series for an ABI issue discovered since the initial release. ‘ -Wabi ’ warns about more things if an older ABI version is selected
(with ‘ -fabi-version= n ’).
‘ -Wabi ’ can also be used with an explicit version number to warn about C ++
ABI compatibility with a particular ‘ -fabi-version ’ level, e.g. ‘ -Wabi=2 ’ to warn about changes relative to ‘ -fabi-version=2 ’.
If an explicit version number is provided and ‘ -fabi-compat-version ’ is not specified, the version number from this option is used for compatibility aliases.
If no explicit version number is provided with this option, but
‘ -fabi-compat-version ’ is specified, that version number is used for C ++
ABI warnings.
Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G ++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G ++ may not be binary compatible with code generated by other compilers.
Known incompatibilities in ‘ -fabi-version=2 ’ (which was the default from
GCC 3.4 to 4.9) include:
• A template with a non-type template parameter of reference type was mangled incorrectly: extern int N;
94 Using the GNU Compiler Collection (GCC) template <int &> struct S {}; void n (S<N>) {2}
This was fixed in ‘ -fabi-version=3 ’.
• SIMD vector types declared using __attribute ((vector_size)) were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in ‘ -fabi-version=4 ’.
• __attribute ((const)) and noreturn were mangled as type qualifiers, and decltype of a plain declaration was folded away.
These mangling issues were fixed in ‘ -fabi-version=5 ’.
• Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing va_arg to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than int .
Also, the ABI changed the mangling of template argument packs, const_ cast , static_cast , prefix increment/decrement, and a class scope function used as a template argument.
These issues were corrected in ‘ -fabi-version=6 ’.
• Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of nullptr_t .
These issues were corrected in ‘ -fabi-version=7 ’.
• When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.
This was fixed in ‘ -fabi-version=8 ’, the default for GCC 5.1.
• decltype(nullptr) incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, as parameters have a minimum alignment.
This was fixed in ‘ -fabi-version=9 ’, the default for GCC 5.2.
• Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.
This was fixed in ‘ -fabi-version=10 ’, the default for GCC 6.1.
This option also enables warnings about psABI-related changes. The known psABI changes at this point include:
• For SysV/x86-64, unions with long double members are passed in memory as specified in psABI. Prior to GCC 4.4, this was not the case. For example: union U { long double ld;
}; int i; union U is now always passed in memory.
Chapter 3: GCC Command Options 95
-Wchar-subscripts
Warn if an array subscript has type char . This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by ‘ -Wall ’.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the ‘ -fprofile-use ’ option.
If a source file is changed between compiling with ‘ -fprofile-generate ’ and with ‘ -fprofile-use ’, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. ‘ -Wno-coverage-mismatch ’ can be used to disable the warning or ‘ -Wno-error=coverage-mismatch ’ can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.
-Wno-coverage-invalid-line-number
Warn in case a function ends earlier than it begins due to an invalid linenum macros. The warning is emitted only with ‘ --coverage ’ enabled.
By default, this warning is enabled and is treated as an error.
‘ -Wno-coverage-invalid-line-number ’ can be used to disable the warning or ‘ -Wno-error=coverage-invalid-line-number ’ can be used to disable the error.
-Wno-cpp (C, Objective-C, C ++ , Objective-C ++ and Fortran only)
Suppress warning messages emitted by #warning directives.
-Wdouble-promotion (C, C ++ , Objective-C and Objective-C ++ only)
Give a warning when a value of type float is implicitly promoted to double .
CPUs with a 32-bit “single-precision” floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation.
It is easy to accidentally do computations with double because floating-point literals are implicitly of type double . For example, in: float area(float radius)
{ return 3.14159 * radius * radius;
} the compiler performs the entire computation with double because the floatingpoint literal is a double .
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate const , volatile , restrict or _Atomic specifier. This warning is enabled by ‘ -Wall ’.
-Wformat
-Wformat= n
Check calls to printf and scanf , etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions
96 Using the GNU Compiler Collection (GCC) specified in the format string make sense. This includes standard functions, and others specified by format attributes (see
Section 6.33 [Function Attributes], page 548
), in the printf , scanf , strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by ‘ -ffreestanding ’ or ‘ -fno-builtin ’.
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library’s limitations.
However, if ‘ -Wpedantic ’ is used with ‘ -Wformat ’, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard). See
[Options Controlling C Dialect], page 43 .
-Wformat=1
-Wformat Option ‘ -Wformat ’ is equivalent to ‘ -Wformat=1 ’, and
‘ -Wno-format ’ is equivalent to ‘ -Wformat=0 ’.
Since ‘ -Wformat ’ also checks for null format arguments for several functions,
‘ -Wformat ’ also implies ‘ -Wnonnull ’.
Some aspects of this level of format checking can be disabled by the options:
‘ -Wno-format-contains-nul ’, ‘ -Wno-format-extra-args ’, and
‘ -Wno-format-zero-length ’. ‘ -Wformat ’ is enabled by ‘ -Wall ’.
-Wformat=2
Enable ‘ -Wformat ’ plus additional format checks. Currently equivalent to ‘ -Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k ’.
-Wno-format-contains-nul
If ‘ -Wformat ’ is specified, do not warn about format strings that contain NUL bytes.
-Wno-format-extra-args
If ‘ -Wformat ’ is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with ‘ $ ’ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
-Wformat-overflow
-Wformat-overflow= level
Warn about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. When the exact number
Chapter 3: GCC Command Options 97 of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of ‘ -Wformat-overflow ’ enabled by ‘ -Wformat ’ employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive’s precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ( ’\0’ ) appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow.
void f (int a, int b)
{ char buf [13]; sprintf (buf, "a = %i, b = %i\n", a, b);
}
-Wformat-overflow=2
Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level
2 , unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive’s precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long.
At level 2 , the call in the example above is again diagnosed, but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning.
An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the
98 Using the GNU Compiler Collection (GCC) format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes.
void f (int a, int b)
{ char buf [23]; sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If ‘ -Wformat ’ is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed.
-Wformat-nonliteral
If ‘ -Wformat ’ is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list .
-Wformat-security
If ‘ -Wformat ’ is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo); . This may be a security hole if the format string came from untrusted input and contains ‘ %n ’. (This is currently a subset of what ‘ -Wformat-nonliteral ’ warns about, but in future warnings may be added to ‘ -Wformat-security ’ that are not included in
‘ -Wformat-nonliteral ’.)
-Wformat-signedness
If ‘ -Wformat ’ is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa.
-Wformat-truncation
-Wformat-truncation= level
Warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic ‘ -Wformat-overflow ’.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of ‘ -Wformat-truncation ’ enabled by ‘ -Wformat ’ employs a conservative approach that warns only about calls to bounded
Chapter 3: GCC Command Options 99 functions whose return value is unused and that will most likely result in output truncation.
-Wformat-truncation=2
Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
-Wformat-y2k
If ‘ -Wformat ’ is specified, also warn about strftime formats that may yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.
‘ -Wnonnull ’ is included in ‘ -Wall ’ and ‘ -Wformat ’. It can be disabled with the
‘ -Wno-nonnull ’ option.
-Wnonnull-compare
Warn when comparing an argument marked with the nonnull function attribute against null inside the function.
‘ -Wnonnull-compare ’ is included in ‘ -Wall ’.
It can be disabled with the
‘ -Wno-nonnull-compare ’ option.
-Wnull-dereference
Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when
‘ -fdelete-null-pointer-checks ’ is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used.
-Winfinite-recursion
Warn about infinitely recursive calls. The warning is effective at all optimization levels but requires optimization in order to detect infinite recursion in calls between two or more functions. ‘ -Winfinite-recursion ’ is included in ‘ -Wall ’.
-Winit-self (C, C ++ , Objective-C and Objective-C ++ only)
Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the ‘ -Wuninitialized ’ option.
For example, GCC warns about i being uninitialized in the following snippet only when ‘ -Winit-self ’ has been specified: int f()
{ int i = i; return i;
}
This warning is enabled by ‘ -Wall ’ in C ++ .
-Wno-implicit-int (C and Objective-C only)
This option controls warnings when a declaration does not specify a type. This warning is enabled by default in C99 and later dialects of C, and also by ‘ -Wall ’.
100 Using the GNU Compiler Collection (GCC)
-Wno-implicit-function-declaration (C and Objective-C only)
This option controls warnings when a function is used before being declared.
This warning is enabled by default in C99 and later dialects of C, and also by
‘ -Wall ’. The warning is made into an error by ‘ -pedantic-errors ’.
-Wimplicit (C and Objective-C only)
Same as ‘ -Wimplicit-int ’ and ‘ -Wimplicit-function-declaration ’. This warning is enabled by ‘ -Wall ’.
-Wimplicit-fallthrough
‘ -Wimplicit-fallthrough ’ is the same as ‘ -Wimplicit-fallthrough=3 ’ and
‘ -Wno-implicit-fallthrough ’ is the same as ‘ -Wimplicit-fallthrough=0 ’.
-Wimplicit-fallthrough= n
Warn when a switch case falls through. For example: switch (cond)
{ case 1: a = 1; break; case 2: a = 2; case 3: a = 3; break;
}
This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. ‘ -Wimplicit-fallthrough= ’ also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.
switch (cond)
{ case 1: if (i > 3) { bar (5); break;
} else if (i < 1) { bar (0);
} else return; default:
...
}
Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)) , that is to be used along with a null statement to suppress this warning that would normally occur: switch (cond)
{ case 1: bar (0);
__attribute__ ((fallthrough)); default:
...
}
Chapter 3: GCC Command Options 101
C ++ 17 provides a standard way to suppress the ‘ -Wimplicit-fallthrough ’ warning using [[fallthrough]]; instead of the GNU attribute. In C ++ 11 or
C ++ 14 users can use [[gnu::fallthrough]]; , which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C ++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted:
• ‘ -Wimplicit-fallthrough=0 ’ disables the warning altogether.
• ‘ -Wimplicit-fallthrough=1 ’ matches .* regular expression, any comment is used as fallthrough comment.
• ‘ -Wimplicit-fallthrough=2 ’ case insensitively matches .*falls?[ \t-
]*thr(ough|u).* regular expression.
• ‘ -Wimplicit-fallthrough=3 ’ case sensitively matches one of the following regular expressions:
•
-fallthrough
• @fallthrough@
• lint -fallthrough[ \t]*
•
[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?
FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?
•
[ \t.!]*(Else,? |Intentional(ly)? )?
Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?
•
[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?
fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?
• ‘ -Wimplicit-fallthrough=4 ’ case sensitively matches one of the following regular expressions:
•
-fallthrough
• @fallthrough@
• lint -fallthrough[ \t]*
•
[ \t]*FALLTHR(OUGH|U)[ \t]*
• ‘ -Wimplicit-fallthrough=5 ’ doesn’t recognize any comments as fallthrough comments, only attributes disable the warning.
The comment needs to be followed after optional whitespace and other comments by case or default keywords or by a user label that precedes some case or default label.
switch (cond)
{ case 1: bar (0);
/* FALLTHRU */ default:
...
}
The ‘ -Wimplicit-fallthrough=3 ’ warning is enabled by ‘ -Wextra ’.
102 Using the GNU Compiler Collection (GCC)
-Wno-if-not-aligned (C, C ++ , Objective-C and Objective-C ++ only)
Control if warnings triggered by the warn_if_not_aligned attribute should be issued. These warnings are enabled by default.
-Wignored-qualifiers (C and C ++ only)
Warn if the return type of a function has a type qualifier such as const . For
ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C ++ , the warning is only emitted for scalar types or void .
ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option.
This warning is also enabled by ‘ -Wextra ’.
-Wno-ignored-attributes (C and C ++ only)
This option controls warnings when an attribute is ignored. This is different from the ‘ -Wattributes ’ option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default.
-Wmain Warn if the type of main is suspicious.
main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C ++ and is enabled by either ‘ -Wall ’ or ‘ -Wpedantic ’.
-Wmisleading-indentation (C and C ++ only)
Warn when the indentation of the code does not reflect the block structure.
Specifically, a warning is issued for if , else , while , and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation.
In the following example, the call to “bar” is misleadingly indented as if it were guarded by the “if” conditional.
if (some_condition ()) foo (); bar (); /* Gotcha: this is not guarded by the "if".
*/
In the case of mixed tabs and spaces, the warning uses the ‘ -ftabstop= ’ option to determine if the statements line up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor logic such as the following example.
if (flagA) foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD if (flagB)
#endif foo (1);
The warning is not issued after a #line directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.
This warning is enabled by ‘ -Wall ’ in C and C ++ .
-Wmissing-attributes
Warn when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the
Chapter 3: GCC Command Options 103 correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the ‘ -Wattribute-alias=2 ’ option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered include alloc_align , alloc_size , cold , const , hot , leaf , malloc , nonnull , noreturn , nothrow , pure , returns_nonnull , and returns_twice .
In C ++ , the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align , alloc_size , assume_aligned , format , format_arg , malloc , or nonnull is declared without it. Attributes deprecated , error , and warning suppress the warning.
(see
[Function Attributes], page 548
).
You can use the copy attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes.
This attribute can be applied to declarations of functions
(see
Section 6.33.1 [Common Function Attributes], page 549 ), variables
(see
Section 6.34.1 [Common Variable Attributes], page 614 ), or types (see
Section 6.35.1 [Common Type Attributes], page 629 ).
‘ -Wmissing-attributes ’ is enabled by ‘ -Wall ’.
For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes.
template <class T>
T* __attribute__ ((malloc, alloc_size (1))) allocate (size_t); template <> void* __attribute__ ((malloc)) // missing alloc_size allocate<void> (size_t);
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 }; int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by ‘ -Wall ’.
-Wmissing-include-dirs (C, C ++ , Objective-C, Objective-C ++ and Fortran only)
Warn if a user-supplied include directory does not exist. This opions is disabled by default for C, C ++ , Objective-C and Objective-C ++ . For Fortran, it is partially enabled by default by warning for -I and -J, only.
-Wno-missing-profile
This option controls warnings if feedback profiles are missing when using the
‘ -fprofile-use ’ option. This option diagnoses those cases where a new function or a new file is added between compiling with ‘ -fprofile-generate ’ and
104 Using the GNU Compiler Collection (GCC) with ‘ -fprofile-use ’, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform you that a profile generation step is due. Ignoring the warning can result in poorly optimized code. ‘ -Wno-missing-profile ’ can be used to disable the warning, but this is not recommended and should be done only when non-existent profile data is justified.
-Wmismatched-dealloc
Warn for calls to deallocation functions with pointer arguments returned from from allocations functions for which the former isn’t a suitable deallocator.
A pair of functions can be associated as matching allocators and deallocators by use of attribute malloc .
Unless disabled by the ‘ -fno-builtin ’ option the standard functions calloc , malloc , realloc , and free , as well as the corresponding forms of C ++ operator new and operator delete are implicitly associated as matching allocators and deallocators. In the following example mydealloc is the deallocator for pointers returned from myalloc .
void mydealloc (void*);
__attribute__ ((malloc (mydealloc, 1))) void* myalloc (size_t); void f (void)
{ void *p = myalloc (32);
// ...use p...
free (p); // warning: not a matching deallocator for myalloc mydealloc (p); // ok
}
In C ++ , the related option ‘ -Wmismatched-new-delete ’ diagnoses mismatches involving either operator new or operator delete .
Option ‘ -Wmismatched-dealloc ’ is included in ‘ -Wall ’.
-Wmultistatement-macros
Warn about unsafe multiple statement macros that appear to be guarded by a clause such as if , else , for , switch , or while , in which only the first statement is actually guarded after the macro is expanded.
For example:
#define DOIT x++; y++ if (c)
DOIT; will increment y unconditionally, not just when c holds. The can usually be fixed by wrapping the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0) if (c)
DOIT;
This warning is enabled by ‘ -Wall ’ in C and C ++ .
Chapter 3: GCC Command Options 105
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.
Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1
: 0) <= z , which is a different interpretation from that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?
: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead.
For C ++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}
This warning is enabled by ‘ -Wall ’.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C ++ standards.
The C and C ++ standards define the order in which expressions in a C/C ++ program are evaluated in terms of sequence points , which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a && , || , ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C ++ standards specify that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are a = a++; , a[n] = b[n++] and a[i++] = i; . Some more complicated cases are not diagnosed by this option,
106 Using the GNU Compiler Collection (GCC) and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.
The C ++ 17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C ++ .
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at https://gcc.gnu.org/readings.html
.
This warning is enabled by ‘ -Wall ’ for C and C ++ .
-Wno-return-local-addr
Do not warn about returning a pointer (or in C ++ , a reference) to a variable that goes out of scope after the function returns.
-Wreturn-type
Warn whenever a function is defined with a return type that defaults to int .
Also warn about any return statement with no return value in a function whose return type is not void (falling off the end of the function body is considered returning without a value).
For C only, warn about a return statement with an expression in a function whose return type is void , unless the expression type is also void . As a GNU extension, the latter case is accepted without a warning unless ‘ -Wpedantic ’ is used. Attempting to use the return value of a nonvoid function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined.
Unlike in C, in C ++ , flowing off the end of a nonvoid function other than main results in undefined behavior even when the value of the function is not used.
This warning is enabled by default in C ++ and by ‘ -Wall ’ otherwise.
-Wno-shift-count-negative
Controls warnings if a shift count is negative.
This warning is enabled by default.
-Wno-shift-count-overflow
Controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is enabled by ‘ -Wextra ’ in
C99 and C ++ 11 modes (and newer).
-Wno-shift-overflow
-Wshift-overflow= n
These options control warnings about left shift overflows.
Chapter 3: GCC Command Options 107
-Wshift-overflow=1
This is the warning level of ‘ -Wshift-overflow ’ and is enabled by default in C99 and C ++ 11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in
C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C ++ 20 mode (and newer), as signed left shifts always wrap.
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into the sign bit, unless C ++ 14 mode (or newer) is active.
-Wswitch Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used (even if there is a default label). This warning is enabled by ‘ -Wall ’.
-Wswitch-default
Warn whenever a switch statement does not have a default case.
-Wswitch-enum
Warn whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration.
case labels outside the enumeration range also provoke warnings when this option is used.
The only difference between ‘ -Wswitch ’ and this option is that this option gives a warning about an omitted enumeration code even if there is a default label.
-Wno-switch-bool
Do not warn when a switch statement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other than bool .
For example: switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C ++ programs.
-Wno-switch-outside-range
This option controls warnings when a switch case has a value that is outside of its respective type range. This warning is enabled by default for C and C ++ programs.
-Wno-switch-unreachable
Do not warn when a switch statement contains statements between the controlling expression and the first case label, which will never be executed. For example:
108 Using the GNU Compiler Collection (GCC) switch (cond)
{ i = 15;
...
case 5:
...
}
‘ -Wswitch-unreachable ’ does not warn if the statement between the controlling expression and the first case label is just a declaration: switch (cond)
{ int i;
...
case 5: i = 5;
...
}
This warning is enabled by default for C and C ++ programs.
-Wsync-nand (C and C ++ only)
Warn when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
To suppress this warning use the unused attribute (see
).
This warning is also enabled by ‘ -Wunused ’ together with ‘ -Wextra ’.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by ‘ -Wall ’.
To suppress this warning use the unused attribute (see
).
This warning is also enabled by ‘ -Wunused ’, which is enabled by ‘ -Wall ’.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by ‘ -Wall ’.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by
‘ -Wall ’.
To suppress this warning use the unused attribute (see
).
-Wunused-local-typedefs (C, Objective-C, C ++ and Objective-C ++ only)
Warn when a typedef locally defined in a function is not used. This warning is enabled by ‘ -Wall ’.
Chapter 3: GCC Command Options 109
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration.
To suppress this warning use the unused attribute (see
).
-Wno-unused-result
Do not warn if a caller of a function marked with attribute warn_unused_ result (see
Section 6.33 [Function Attributes], page 548 ) does not use its return
value. The default is ‘ -Wunused-result ’.
-Wunused-variable
Warn whenever a local or static variable is unused aside from its declaration.
This option implies ‘ -Wunused-const-variable=1 ’ for C, but not for C ++ . This warning is enabled by ‘ -Wall ’.
To suppress this warning use the unused attribute (see
).
-Wunused-const-variable
-Wunused-const-variable= n
Warn whenever a constant static variable is unused aside from its declaration.
‘ -Wunused-const-variable=1 ’ is enabled by ‘ -Wunused-variable ’ for C, but not for C ++ . In C this declares variable storage, but in C ++ this is not an error since const variables take the place of #define s.
To suppress this warning use the unused attribute (see
).
-Wunused-const-variable=1
This is the warning level that is enabled by ‘ -Wunused-variable ’ for C. It warns only about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.
-Wunused-const-variable=2
This warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of ‘ -Wunused-const-variable ’ and must be explicitly requested since in C ++ this isn’t an error and in C it might be harder to clean up all headers included.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to void . This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not.
This warning is enabled by ‘ -Wall ’.
-Wunused All the above ‘ -Wunused ’ options combined.
In order to get a warning about an unused function parameter, you must either specify ‘ -Wextra -Wunused ’ (note that ‘ -Wall ’ implies ‘ -Wunused ’), or separately specify ‘ -Wunused-parameter ’.
110 Using the GNU Compiler Collection (GCC)
-Wuninitialized
Warn if an object with automatic or allocated storage duration is used without having been initialized. In C ++ , also warn if a non-static reference or non-static const member appears in a class without constructors.
In addition, passing a pointer (or in C ++ , a reference) to an uninitialized object to a const -qualified argument of a built-in function known to read the object is also diagnosed by this warning.
(‘ -Wmaybe-uninitialized ’ is issued for ordinary functions.)
If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the ‘ -Winit-self ’ option.
These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized as a whole.
They do not occur for variables or elements declared volatile . Because these warnings depend on optimization, the exact variables or elements for which there are warnings depend on the precise optimization options and version of
GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.
In C ++ , this warning also warns about using uninitialized objects in memberinitializer-lists. For example, GCC warns about b being uninitialized in the following snippet: struct A { int a;
}; int b;
A() : a(b) { }
-Wno-invalid-memory-model
This option controls warnings for invocations of
Section 6.55 [ atomic Builtins], page 713
,
Section 6.54 [ sync Builtins], page 711 , and the C11 atomic generic
functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the memory_order enumeration. For example, since the __atomic_store and __atomic_store_n built-ins are only defined for the relaxed, release, and sequentially consistent memory orders the following code is diagnosed: void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
‘ -Winvalid-memory-model ’ is enabled by default.
-Wmaybe-uninitialized
For an object with automatic or allocated storage duration, if there exists a path from the function entry to a use of the object that is initialized, but there exist some other paths for which the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time.
Chapter 3: GCC Command Options 111
In addition, passing a pointer (or in C ++ , a reference) to an uninitialized object to a const -qualified function argument is also diagnosed by this warning.
(‘ -Wuninitialized ’ is issued for built-in functions known to read the object.)
Annotating the function with attribute access (none) indicates that the argument isn’t used to access the object and avoids the warning (see
[Common Function Attributes], page 549
).
These warnings are only possible in optimizing compilation, because otherwise
GCC does not keep track of the state of variables.
These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here is one example of how this can happen:
{ int x; switch (y)
{ case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5;
} foo (x);
}
If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn’t know this. To suppress the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to longjmp . The compiler sees only the calls to setjmp . It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn . See
Section 6.33 [Function Attributes], page 548
.
This warning is enabled by ‘ -Wall ’ or ‘ -Wextra ’.
-Wunknown-pragmas
Warn when a #pragma directive is encountered that is not understood by GCC.
If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the ‘ -Wall ’ command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also ‘ -Wunknown-pragmas ’.
-Wno-prio-ctor-dtor
Do not warn if a priority from 0 to 100 is used for constructor or destructor.
The use of constructor and destructor attributes allow you to assign a priority
112 Using the GNU Compiler Collection (GCC) to the constructor/destructor to control its order of execution before main is called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between 0–100 for the implementation.
-Wstrict-aliasing
This option is only active when ‘ -fstrict-aliasing ’ is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization.
The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in ‘ -Wall ’. It is equivalent to ‘ -Wstrict-aliasing=3 ’
-Wstrict-aliasing=n
This option is only active when ‘ -fstrict-aliasing ’ is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives).
Higher levels also correspond to more effort, similar to the way ‘ -O ’ works.
‘ -Wstrict-aliasing ’ is equivalent to ‘ -Wstrict-aliasing=3 ’.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but ‘ -fstrict-aliasing ’ still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced.
Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false positives
(not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for ‘ -Wstrict-aliasing ’): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun + dereference pattern in the front end: *(int*)&some_float . If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
-Wstrict-overflow
-Wstrict-overflow= n
This option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.
Chapter 3: GCC Command Options 113
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid.
For example the compiler simplifies x + 1 > x to 1 . This level of
‘ -Wstrict-overflow ’ is enabled by ‘ -Wall ’; higher levels are not, and must be explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a constant. For example: abs (x) >= 0 . This can only be simplified when signed integer overflow is undefined, because abs (INT_MIN) overflows to INT_MIN , which is less than zero. ‘ -Wstrict-overflow ’
(with no level) is the same as ‘ -Wstrict-overflow=2 ’.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified. For example: x + 1 > 1 is simplified to x > 0 .
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above cases. For example: (x * 10) / 5 is simplified to x * 2 .
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: x + 2 > y is simplified to x + 1 >= y . This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.
-Wstring-compare
Warn for calls to strcmp and strncmp whose result is determined to be either zero or non-zero in tests for such equality owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case of strncmp ). Such calls could be mistakes. For example, the call to strcmp below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array a .
extern char a[4]; void f (char *d)
{ strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true puts ("a and d are the same");
}
‘ -Wstring-compare ’ is enabled by ‘ -Wextra ’.
-Wno-stringop-overflow
-Wstringop-overflow
-Wstringop-overflow= type
Warn for calls to string manipulation functions such as memcpy and strcpy that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine
114 Using the GNU Compiler Collection (GCC) the size of the destination. See
Section 6.58 [Object Size Checking], page 720
.
The argument is meaningful only for functions that operate on character arrays but not for raw memory functions like memcpy which always make use of Object
Size type-0. The option also warns for calls that specify a size in excess of the largest possible object or at most SIZE_MAX / 2 bytes. The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins. For example, the option issues a warning for the strcpy call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow }; const char* f (enum Color clr)
{ static char buf [4]; const char *str; switch (clr)
{ case blue: str = "blue"; break; case purple: str = "purple"; break; case yellow: str = "yellow"; break;
}
} return strcpy (buf, str); // warning here
Option ‘ -Wstringop-overflow=2 ’ is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The ‘ -Wstringop-overflow=1 ’ option uses type-zero Object Size
Checking to determine the sizes of destination objects. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On
Linux systems, when optimization is enabled at this setting the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value.
-Wstringop-overflow=2
The ‘ -Wstringop-overflow=2 ’ option uses type-one Object Size
Checking to determine the sizes of destination objects. At this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. However, it does not warn for excessive writes to the same members of unknown objects referenced by pointers since they may point to arrays containing unknown numbers of elements. This is the default setting of the option.
Chapter 3: GCC Command Options 115
-Wstringop-overflow=3
The ‘ -Wstringop-overflow=3 ’ option uses type-two Object Size
Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code.
-Wstringop-overflow=4
The ‘ -Wstringop-overflow=4 ’ option uses type-three Object Size
Checking to determine the sizes of destination objects. At this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to
‘ -Wstringop-overflow=3 ’ this setting of the option may result in warnings for benign code.
-Wno-stringop-overread
Warn for calls to string manipulation functions such as memchr , or strcpy that are determined to read past the end of the source sequence.
Option ‘ -Wstringop-overread ’ is enabled by default.
-Wno-stringop-truncation
Do not warn for calls to bounded string manipulation functions such as strncat , strncpy , and stpncpy that may either truncate the copied string or leave the destination unchanged.
In the following example, the call to strncat specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use bufsize strlen (buf) - 1) as the bound.
void append (char *buf, size_t bufsize)
{ strncat (buf, ".txt", 3);
}
As another example, the following call to strncpy results in copying to d just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a common mistake, and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call memcpy instead.
void copy (char *d, const char *s)
{ strncpy (d, s, strlen (s));
}
In the following example, the call to strncpy specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size the result of the copy will not be NUL-terminated. Therefore, the call is also diagnosed. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL .
void copy (const char *s)
{
116 Using the GNU Compiler Collection (GCC)
} char buf[80]; strncpy (buf, s, sizeof buf);
...
In situations where a character array is intended to store a sequence of bytes with no terminating NUL such an array may be annotated with attribute nonstring to avoid this warning.
Such arrays, however, are not suitable arguments to functions that expect NUL -terminated strings.
To help detect accidental misuses of such arrays GCC issues warnings unless it can prove that the use is safe. See
Section 6.34.1 [Common Variable Attributes], page 614 .
-Wsuggest-attribute= [ pure|const|noreturn|format|cold|malloc ]
Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wmissing-noreturn
-Wsuggest-attribute=malloc
Warn about functions that might be candidates for attributes pure , const or noreturn or malloc . The compiler only warns for functions visible in other compilation units or (in the case of pure and const ) if it cannot prove that the function returns normally. A function returns normally if it doesn’t contain an infinite loop or return abnormally by throwing, calling abort or trapping. This analysis requires option ‘ -fipa-pure-const ’, which is enabled by default at ‘ -O ’ and higher. Higher optimization levels improve the accuracy of the analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for format attributes. Again, these are only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf , but this might not always be the case, and some functions for which format attributes are appropriate may not be detected.
Chapter 3: GCC Command Options 117
-Wsuggest-attribute=cold
Warn about functions that might be candidates for cold attribute.
This is based on static detection and generally only warns about functions which always leads to a call to another cold function such as wrappers of C ++ throw or fatal error reporting functions leading to abort .
-Walloc-zero
Warn about calls to allocation functions decorated with attribute alloc_size that specify zero bytes, including those to the built-in forms of the functions aligned_alloc , alloca , calloc , malloc , and realloc . Because the behavior of these functions when called with a zero size differs among implementations
(and in the case of realloc has been deprecated) relying on it may result in subtle portability bugs and should be avoided.
-Walloc-size-larger-than= byte-size
Warn about calls to functions decorated with attribute alloc_size that attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value of ‘ PTRDIFF_MAX ’ on the target.
‘ -Walloc-size-larger-than= ’‘ PTRDIFF_MAX ’ is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of
‘ SIZE_MAX ’ or more or by ‘ -Wno-alloc-size-larger-than ’. See
[Function Attributes], page 548
.
-Wno-alloc-size-larger-than
Disable ‘ -Walloc-size-larger-than= ’ warnings. The option is equivalent to
‘ -Walloc-size-larger-than= ’‘ SIZE_MAX ’ or larger.
-Walloca This option warns on all uses of alloca in the source.
-Walloca-larger-than= byte-size
This option warns on calls to alloca with an integer argument whose value is either zero, or that is not bounded by a controlling predicate that limits its value to at most byte-size . It also warns for calls to alloca where the bound value is unknown. Arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range.
For example, a bounded case of alloca could be: void func (size_t n)
{ void *p; if (n <= 1000) p = alloca (n); else p = malloc (n); f (p);
}
In the above example, passing -Walloca-larger-than=1000 would not issue a warning because the call to alloca is known to be at most 1000 bytes. However, if -Walloca-larger-than=500 were passed, the compiler would emit a warning.
118 Using the GNU Compiler Collection (GCC)
Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. For example: void func ()
{ void *p = alloca (n); f (p);
}
If -Walloca-larger-than=500 were passed, the above would trigger a warning, but this time because of the lack of bounds checking.
Note, that even seemingly correct code involving signed integers could cause a warning: void func (signed int n)
{ if (n < 500)
{ p = alloca (n); f (p);
}
}
In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call.
This option also warns when alloca is used in a loop.
‘ -Walloca-larger-than= ’‘ PTRDIFF_MAX ’ is enabled by default but is usually only effective when ‘ -ftree-vrp ’ is active (default for ‘ -O2 ’ and above).
See also ‘ -Wvla-larger-than= ’‘ byte-size ’.
-Wno-alloca-larger-than
Disable ‘ -Walloca-larger-than= ’ warnings.
The option is equivalent to
‘ -Walloca-larger-than= ’‘ SIZE_MAX ’ or larger.
-Warith-conversion
Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values.
This affects warnings from ‘ -Wconversion ’, ‘ -Wfloat-conversion ’, and
‘ -Wsign-conversion ’.
void f (char c, int i)
{ c = c + i; // warns with ‘-Wconversion’ c = c + 1; // only warns with ‘-Warith-conversion’
}
-Warray-bounds
-Warray-bounds= n
This option is only active when ‘ -ftree-vrp ’ is active (default for ‘ -O2 ’ and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by ‘ -Wall ’.
-Warray-bounds=1
This is the warning level of ‘ -Warray-bounds ’ and is enabled by
‘ -Wall ’; higher levels are not, and must be explicitly requested.
Chapter 3: GCC Command Options 119
-Warray-bounds=2
This warning level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers.
This warning level may give a larger number of false positives and is deactivated by default.
-Warray-compare
Warn about equality and relational comparisons between two operands of array type. This comparison was deprecated in C ++ 20. For example: int arr1[5]; int arr2[5]; bool same = arr1 == arr2;
‘ -Warray-compare ’ is enabled by ‘ -Wall ’.
-Warray-parameter
-Warray-parameter= n
Warn about redeclarations of functions involving arguments of array or pointer types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as ‘ -Warray-bounds ’.
If the first function declaration uses the array form the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed by warnings such as
‘ -Warray-bounds ’. At level 1 the warning diagnoses inconsistencies involving array parameters declared using the T[static N] form.
For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to f while the second one with the keyword static specifies that the array argument must have at least four elements.
void f (int[static 4]); void f (int[]); // warning (inconsistent array form) void g (void)
{ int *p = (int *)malloc (4); f (p); // warning (array too small)
...
}
At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning.
void g (int*); void g (int[]); void g (int[8]);
// no warning
// warning (inconsistent array bound)
‘ -Warray-parameter=2 ’ is included in ‘ -Wall ’. The ‘ -Wvla-parameter ’ option triggers warnings for similar inconsistencies involving Variable Length Array arguments.
120 Using the GNU Compiler Collection (GCC)
-Wattribute-alias= n
-Wno-attribute-alias
Warn about declarations using the alias and similar attributes whose target is incompatible with the type of the alias. See
Section 6.33 [Declaring Attributes of Functions], page 548 .
-Wattribute-alias=1
The default warning level of the ‘ -Wattribute-alias ’ option diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs.
-Wattribute-alias=2
At this level ‘ -Wattribute-alias ’ also diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the ‘ -Wmissing-attributes ’ option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.
Attributes considered include alloc_align , alloc_size , cold , const , hot , leaf , malloc , nonnull , noreturn , nothrow , pure , returns_nonnull , and returns_twice .
‘ -Wattribute-alias ’ is equivalent to ‘ -Wattribute-alias=1 ’. This is the default. You can disable these warnings with either ‘ -Wno-attribute-alias ’ or
‘ -Wattribute-alias=0 ’.
-Wbidi-chars= [ none|unpaired|any ]
Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is.
There are three levels of warning supported by GCC.
The default is
‘ -Wbidi-chars=unpaired ’, which warns about improperly terminated bidi contexts. ‘ -Wbidi-chars=none ’ turns the warning off. ‘ -Wbidi-chars=any ’ warns about any use of bidirectional control characters.
-Wbool-compare
Warn about boolean expression compared with an integer value different from true / false . For instance, the following comparison is always false: int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by ‘ -Wall ’.
Chapter 3: GCC Command Options 121
-Wbool-operation
Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For
C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C ++ , decrementing a boolean is always invalid.
Incrementing a boolean is invalid in C ++ 17, and deprecated otherwise.)
This warning is enabled by ‘ -Wall ’.
-Wduplicated-branches
Warn when an if-else has identical branches. This warning detects cases like if (p != NULL) return 0; else return 0;
It doesn’t warn when both branches contain just a null statement. This warning also warn for conditional operators: int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code: if (p->q != NULL) { ... } else if (p->q != NULL) { ... }
-Wframe-address
Warn when the ‘ __builtin_frame_address ’ or ‘ __builtin_return_address ’ is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in ‘ -Wall ’.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a const char * variable is passed to a function that takes a char * parameter. This option can be used to suppress such a warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a const int (*)[] variable is passed to a function that takes a int (*)[] parameter. This option can be used to suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by ‘ -Wno-pointer-sign ’, which warns for pointer argument passing or assignment with different signedness.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings ‘ -Wno-int-to-pointer-cast ’ and ‘ -Wno-pointer-to-int-cast ’ may be used.
122 Using the GNU Compiler Collection (GCC)
-Wzero-length-bounds
Warn about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accesses to them are undefined. See See
For example, the first two stores in function bad are diagnosed because the array elements overlap the subsequent members b and c . The third store is diagnosed by ‘ -Warray-bounds ’ because it is beyond the bounds of the enclosing object.
struct X { int a[0]; int b, c; }; struct X x; void bad (void)
{ x.a[0] = 0; // -Wzero-length-bounds x.a[1] = 1; // -Wzero-length-bounds x.a[2] = 2; // -Warray-bounds
}
Option ‘ -Wzero-length-bounds ’ is enabled by ‘ -Warray-bounds ’.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using
‘ -Wall ’ in conjunction with this option does not warn about unknown pragmas in system headers—for that, ‘ -Wunknown-pragmas ’ must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or false. This warning detects various mistakes such as: int i = 1;
...
if (i > i) { ... }
This warning also warns about bitwise comparisons that always evaluate to true or false, for instance: if ((a & 16) == 10) { ... } will always be false.
This warning is enabled by ‘ -Wall ’.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and
Chapter 3: GCC Command Options 123 thus requires the stack to be made executable in order for the program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that’s a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in traditional and ISO
C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.
• Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in
ISO C it does not.
• In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the ‘ # ’ appeared in column 1 on the line. Therefore ‘ -Wtraditional ’ warns about directives that traditional C understands but ignores because the ‘ # ’ does not appear as the first character on the line. It also suggests you hide directives like
#pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif , so this option suggests avoiding it altogether.
• A function-like macro that appears without arguments.
• The unary plus operator.
• The ‘ U ’ integer constant suffix, or the ‘ F ’ or ‘ L ’ floating-point constant suffixes. (Traditional C does support the ‘ L ’ suffix on integer constants.)
Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the ‘ _MIN ’/‘ _MAX ’ macros in <limits.h> . Use of these macros in user code might normally lead to spurious warnings, however
GCC’s integrated preprocessor has enough context to avoid warning in these cases.
• A function declared external in one block and then used after the end of the block.
• A switch statement has an operand of type long .
• A nonstatic function declaration follows a static one. This construct is not accepted by some traditional C compilers.
• The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the
124 Using the GNU Compiler Collection (GCC) constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
• Usage of ISO string concatenation is detected.
• Initialization of automatic aggregates.
• Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
• Initialization of unions. If the initializer is zero, the warning is omitted.
This is done under the assumption that the zero initializer in user code appears conditioned on e.g.
__STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
• Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional
C causes serious problems. This is a subset of the possible conversion warnings; for the full set use ‘ -Wtraditional-conversion ’.
• Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO
C features appear in your code when using libiberty’s traditional C compatibility macros, PARAMS and VPARAMS . This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block. This construct, known from C ++ , was introduced with ISO C99 and is by default allowed in
GCC. It is not supported by ISO C90. See
Section 6.32 [Mixed Labels and
-Wshadow Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C ++ ), or instance variable (in
Objective-C) or whenever a built-in function is shadowed. Note that in C ++ , the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it includes also all instances of local shadowing. This means that ‘ -Wno-shadow=local ’ and
‘ -Wno-shadow=compatible-local ’ are ignored when ‘ -Wshadow ’ is used. Same as ‘ -Wshadow=global ’.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance variable in an
Objective-C method.
-Wshadow=global
Warn for any shadowing. Same as ‘ -Wshadow ’.
Chapter 3: GCC Command Options 125
-Wshadow=local
Warn when a local variable shadows another local variable or parameter.
-Wshadow=compatible-local
Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C ++ , type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to ‘ -Wshadow=local ’) is based on the idea that when a local variable shadows another one of incompatible type, it is most likely intentional, not a bug or typo, as shown in the following example: for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{ for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable i in the example above have incompatible types, enabling only ‘ -Wshadow=compatible-local ’ does not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning.
Use of this flag instead of ‘ -Wshadow=local ’ can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means that shadowing const char *i by char *i does not emit a warning.
This warning is also enabled by ‘ -Wshadow=local ’.
-Wlarger-than= byte-size
Warn whenever an object is defined whose size exceeds byte-size .
‘ -Wlarger-than= ’‘ PTRDIFF_MAX ’ is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of ‘ SIZE_MAX ’ or more or by ‘ -Wno-larger-than ’.
Also warn for calls to bounded functions such as memchr or strnlen that specify a bound greater than the largest possible object, which is ‘ PTRDIFF_MAX ’ bytes by default. These warnings can only be disabled by ‘ -Wno-larger-than ’.
-Wno-larger-than
Disable ‘ -Wlarger-than= ’ warnings.
‘ -Wlarger-than= ’‘ SIZE_MAX ’ or larger.
The option is equivalent to
-Wframe-larger-than= byte-size
Warn if the size of a function frame exceeds byte-size . The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you do not get a warning. In addition, any space allocated via alloca , variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning. ‘ -Wframe-larger-than= ’‘ PTRDIFF_MAX ’ is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of ‘ SIZE_MAX ’ or more or by ‘ -Wno-frame-larger-than ’.
126 Using the GNU Compiler Collection (GCC)
-Wno-frame-larger-than
Disable ‘ -Wframe-larger-than= ’ warnings.
The option is equivalent to
‘ -Wframe-larger-than= ’‘ SIZE_MAX ’ or larger.
-Wfree-nonheap-object
Warn when attempting to deallocate an object that was either not allocated on the heap, or by using a pointer that was not returned from a prior call to the corresponding allocation function. For example, because the call to stpcpy returns a pointer to the terminating nul character and not to the begginning of the object, the call to free below is diagnosed.
void f (char *p)
{ p = stpcpy (p, "abc");
// ...
free (p); // warning
}
‘ -Wfree-nonheap-object ’ is included in ‘ -Wall ’.
-Wstack-usage= byte-size
Warn if the stack usage of a function might exceed byte-size . The computation done to determine the stack usage is conservative. Any space allocated via alloca , variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.
The message is in keeping with the output of ‘ -fstack-usage ’.
• If the stack usage is fully static but exceeds the specified amount, it’s: warning: stack usage is 1120 bytes
• If the stack usage is (partly) dynamic but bounded, it’s: warning: stack usage might be 1648 bytes
• If the stack usage is (partly) dynamic and not bounded, it’s: warning: stack usage might be unbounded
‘ -Wstack-usage= ’‘ PTRDIFF_MAX ’ is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of ‘ SIZE_MAX ’ or more or by ‘ -Wno-stack-usage ’.
-Wno-stack-usage
Disable ‘ -Wstack-usage= ’ warnings.
‘ -Wstack-usage= ’‘ SIZE_MAX ’ or larger.
The option is equivalent to
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With ‘ -funsafe-loop-optimizations ’ warn if the compiler makes such assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with ‘ -Wformat ’ and ‘ -pedantic ’ without GNU extensions, this option disables the warnings about non-ISO printf / scanf format width specifiers I32 , I64 , and I used on Windows targets, which depend on the MS runtime.
Chapter 3: GCC Command Options 127
-Wpointer-arith
Warn about anything that depends on the “size of” a function type or of void .
GNU C assigns these types a size of 1, for convenience in calculations with void
* pointers and pointers to functions. In C ++ , warn also when an arithmetic operation involves NULL . This warning is also enabled by ‘ -Wpedantic ’.
-Wno-pointer-compare
Do not warn if a pointer is compared with a zero character constant. This usually means that the pointer was meant to be dereferenced. For example: const char *p = foo (); if (p == ’\0’) return 42;
Note that the code above is invalid in C ++ 11.
This warning is enabled by default.
-Wtsan Warn about unsupported features in ThreadSanitizer.
ThreadSanitizer does not support std::atomic_thread_fence and can report false positives.
This warning is enabled by default.
-Wtype-limits
Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with < or >= . This warning is also enabled by ‘ -Wextra ’.
-Wabsolute-value (C and Objective-C only)
Warn for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, calling abs(3.14) triggers the warning because the appropriate function to call to compute the absolute value of a double argument is fabs . The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast and it is also enabled by ‘ -Wextra ’.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence ‘ /* ’ appears in a ‘ /* ’ comment, or whenever a backslash-newline appears in a ‘ // ’ comment.
This warning is enabled by ‘ -Wall ’.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines.
This option is implied by ‘ -Wall ’.
If ‘ -Wall ’ is not given, this option is still enabled unless trigraphs are enabled.
To get trigraph conversion without warnings, but get the other ‘ -Wall ’ warnings, use ‘ -trigraphs -Wall
-Wno-trigraphs ’.
128 Using the GNU Compiler Collection (GCC)
-Wundef Warn if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever ‘ defined ’ is encountered in the expansion of a macro (including the case where the macro is expanded by an ‘ #if ’ directive). Such usage is not portable. This warning is also enabled by ‘ -Wpedantic ’ and ‘ -Wextra ’.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in such a case, you might improve the scope of the macro’s definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an #else or an #endif are followed by text.
This sometimes happens in older programs with code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third FOO should be in comments. This warning is on by default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO C99. For instance, warn about use of variable length arrays, long long type, bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows
__extension__ .
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO C11. For instance, warn about use of anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__ .
Chapter 3: GCC Command Options 129
-Wc11-c2x-compat (C and Objective-C only)
Warn about features not present in ISO C11, but present in ISO C2X. For instance, warn about omitting the string in _Static_assert , use of ‘ [[]] ’ syntax for attributes, use of decimal floating-point types, and so on.
This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__ .
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C ++ , e.g. request for implicit conversion from void * to a pointer to nonvoid type.
-Wc++11-compat (C ++ and Objective-C ++ only)
Warn about C ++ constructs whose meaning differs between ISO C ++ 1998 and
ISO C ++ 2011, e.g., identifiers in ISO C ++ 1998 that are keywords in ISO C ++
2011. This warning turns on ‘ -Wnarrowing ’ and is enabled by ‘ -Wall ’.
-Wc++14-compat (C ++ and Objective-C ++ only)
Warn about C ++ constructs whose meaning differs between ISO C ++ 2011 and
ISO C ++ 2014. This warning is enabled by ‘ -Wall ’.
-Wc++17-compat (C ++ and Objective-C ++ only)
Warn about C ++ constructs whose meaning differs between ISO C ++ 2014 and
ISO C ++ 2017. This warning is enabled by ‘ -Wall ’.
-Wc++20-compat (C ++ and Objective-C ++ only)
Warn about C ++ constructs whose meaning differs between ISO C ++ 2017 and
ISO C ++ 2020. This warning is enabled by ‘ -Wall ’.
-Wno-c++11-extensions (C ++ and Objective-C ++ only)
Do not warn about C ++ 11 constructs in code being compiled using an older
C ++ standard. Even without this option, some C ++ 11 constructs will only be diagnosed if ‘ -Wpedantic ’ is used.
-Wno-c++14-extensions (C ++ and Objective-C ++ only)
Do not warn about C ++ 14 constructs in code being compiled using an older
C ++ standard. Even without this option, some C ++ 14 constructs will only be diagnosed if ‘ -Wpedantic ’ is used.
-Wno-c++17-extensions (C ++ and Objective-C ++ only)
Do not warn about C ++ 17 constructs in code being compiled using an older
C ++ standard. Even without this option, some C ++ 17 constructs will only be diagnosed if ‘ -Wpedantic ’ is used.
-Wno-c++20-extensions (C ++ and Objective-C ++ only)
Do not warn about C ++ 20 constructs in code being compiled using an older
C ++ standard. Even without this option, some C ++ 20 constructs will only be diagnosed if ‘ -Wpedantic ’ is used.
-Wno-c++23-extensions (C ++ and Objective-C ++ only)
Do not warn about C ++ 23 constructs in code being compiled using an older
C ++ standard. Even without this option, some C ++ 23 constructs will only be diagnosed if ‘ -Wpedantic ’ is used.
130 Using the GNU Compiler Collection (GCC)
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a const char * is cast to an ordinary char * .
Also warn when making a cast that introduces a type qualifier in an unsafe way.
For example, casting char ** to const char ** is unsafe, as in this example:
/* p is char ** value.
*/ const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK.
*/
*q = "string";
/* Now char** pointer points to read-only memory.
*/
**p = ’b’;
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.
-Wcast-align=strict
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * regardless of the target machine.
-Wcast-function-type
Warn when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list only the types of initial arguments that are provided are considered. Any parameter of pointertype matches any other pointer-type. Any benign differences in integral types are ignored, like int vs.
long on ILP32 targets. Likewise type qualifiers are ignored. The function type void (*) (void) is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types this warning warns whenever the type cast is changing the pointer to member type. This warning is enabled by ‘ -Wextra ’.
-Wwrite-strings
When compiling C, give string constants the type const char[ length ] so that copying the address of one into a nonconst char * pointer produces a warning.
These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using const in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make ‘ -Wall ’ request these warnings.
When compiling C ++ , warn about the deprecated conversion from string literals to char * . This warning is enabled by default for C ++ programs.
-Wclobbered
Warn for variables that might be changed by longjmp or vfork . This warning is also enabled by ‘ -Wextra ’.
-Wconversion
Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like abs (x) when x is double ; conversions between signed and unsigned, like unsigned ui = -1 ; and conversions to smaller types,
Chapter 3: GCC Command Options 131 like sqrtf (M_PI) . Do not warn for explicit casts like abs ((int) x) and ui
= (unsigned) -1 , or if the value is not changed by the conversion like in abs
(2.0) . Warnings about conversions between signed and unsigned integers can be disabled by using ‘ -Wno-sign-conversion ’.
For C ++ , also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void , the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in
C ++ unless ‘ -Wsign-conversion ’ is explicitly enabled.
Warnings about conversion from arithmetic on a small type back to that type are only given with ‘ -Warith-conversion ’.
-Wdangling-else
Warn about constructions where there may be confusion to which if statement an else branch belongs. Here is an example of such a case:
{
} if (a) if (b) foo (); else bar ();
In C/C ++ , every else branch belongs to the innermost possible if statement, which in this example is if (b) . This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else can belong to the enclosing if . The resulting code looks like this:
{ if (a)
{ if (b) foo (); else bar ();
}
}
This warning is enabled by ‘ -Wparentheses ’.
-Wdangling-pointer
-Wdangling-pointer= n
Warn about uses of pointers (or C ++ references) to objects with automatic storage duration after their lifetime has ended. This includes local variables declared in nested blocks, compound literals and other unnamed temporary objects. In addition, warn about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but may yield different results with optimization than without.
132 Using the GNU Compiler Collection (GCC)
-Wdangling-pointer=1
At level 1 the warning diagnoses only unconditional uses of dangling pointers. For example int f (int c1, int c2, x)
{ char *p = strchr ((char[]){ c1, c2 }, c3); return p ? *p : ’x’; // warning: dangling pointer to a compound literal
}
In the following function the store of the address of the local variable x in the escaped pointer *p also triggers the warning.
void g (int **p)
{ int x = 7;
*p = &x; // warning: storing the address of a local variable in *p
}
-Wdangling-pointer=2
At level 2, in addition to unconditional uses the warning also diagnoses conditional uses of dangling pointers.
For example, because the array a in the following function is out of scope when the pointer s that was set to point is used, the warning triggers at this level.
void f (char *s)
{ if (!s)
{ char a[12] = "tmpname"; s = a;
} strcat (s, ".tmp"); // warning: dangling pointer to a may be used
...
}
‘ -Wdangling-pointer=2 ’ is included in ‘ -Wall ’.
-Wdate-time
Warn when macros __TIME__ , __DATE__ or __TIMESTAMP__ are encountered as they might prevent bit-wise-identical reproducible compilations.
-Wempty-body
Warn if an empty body occurs in an if , else or do while statement. This warning is also enabled by ‘ -Wextra ’.
-Wno-endif-labels
Do not warn about stray tokens after #else and #endif .
-Wenum-compare
Warn about a comparison between values of different enumerated types. In
C ++ enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by ‘ -Wall ’.
-Wenum-conversion
Warn when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by ‘ -Wextra ’ in C.
Chapter 3: GCC Command Options 133
-Wjump-misses-init (C, Objective-C only)
Warn if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. This only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C ++ this sort of branch is an error in any case.
‘ -Wjump-misses-init ’ is included in ‘ -Wc++-compat ’. It can be disabled with the ‘ -Wno-jump-misses-init ’ option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. In C ++ , this warning is also enabled by ‘ -Wall ’. In C, it is also enabled by ‘ -Wextra ’.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled also by ‘ -Wconversion ’.
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values. This option is also enabled by ‘ -Wconversion ’.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar storage order.
-Wsizeof-array-div
Warn about divisions of two sizeof operators when the first one is applied to an array and the divisor does not equal the size of the array element. In such a case, the computation will not yield the number of elements in the array, which is likely what the user intended. This warning warns e.g. about int fn ()
{ int arr[10]; return sizeof (arr) / sizeof (short);
}
This warning is enabled by ‘ -Wall ’.
-Wsizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that divide the pointer size by the element size, which is the usual way to compute the array size but won’t work out correctly with pointers. This warning warns e.g. about sizeof
(ptr) / sizeof (ptr[0]) if ptr is not an array, but a pointer. This warning is enabled by ‘ -Wall ’.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory builtin functions if the argument uses sizeof . This warning triggers for example for memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and suggests a possible fix, or about memcpy (&foo, ptr, sizeof (&foo)); .
134 Using the GNU Compiler Collection (GCC)
‘ -Wsizeof-pointer-memaccess ’ also warns about calls to bounded string copy functions like strncat or strncpy that specify as the bound a sizeof expression of the source array. For example, in the following function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake and so the call is diagnosed.
void make_file (const char *name)
{ char path[PATH_MAX]; strncpy (path, name, sizeof path - 1); strncat (path, ".text", sizeof ".text");
...
}
The ‘ -Wsizeof-pointer-memaccess ’ option is enabled by ‘ -Wall ’.
-Wno-sizeof-array-argument
Do not warn when the sizeof operator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C ++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the memset built-in function, if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. This indicates that the user has omitted a multiplication by the element size. This warning is enabled by ‘ -Wall ’.
-Wmemset-transposed-args
Warn for suspicious calls to the memset built-in function where the second argument is not zero and the third argument is zero. For example, the call memset (buf, sizeof buf, 0) is diagnosed because memset (buf, 0, sizeof buf) was meant instead. The diagnostic is only emitted if the third argument is a literal zero. Otherwise, if it is an expression that is folded to zero, or a cast of zero to some type, it is far less likely that the arguments have been mistakenly transposed and no warning is emitted. This warning is enabled by ‘ -Wall ’.
-Waddress
Warn about suspicious uses of address expressions. These include comparing the address of a function or a declared object to the null pointer constant such as in void f (void); void g (void)
{ if (!func) // warning: expression evaluates to false abort ();
} comparisons of a pointer to a string literal, such as in void f (const char *x)
{ if (x == "abc") // warning: expression evaluates to false puts ("equal");
}
Chapter 3: GCC Command Options 135 and tests of the results of pointer addition or subtraction for equality to null, such as in void f (const int *p, int i)
{ return p + i == NULL;
}
Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception are weak symbols), so their use in a conditional might indicate missing parentheses in a function call or a missing dereference in an array expression. The subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as inptr_t or uinptr_t . Comparisons against string literals result in unspecified behavior and are not portable, and suggest the intent was to call strcmp . The warning is suppressed if the suspicious expression is the result of macro expansion. ‘ -Waddress ’ warning is enabled by ‘ -Wall ’.
-Wno-address-of-packed-member
Do not warn when the address of packed member of struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bit-wise operator is likely to be expected. Also warns when the operands of a logical operator are the same: extern int a; if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a comparison.
This option does not warn if the right operand is considered to be a boolean expression. Its purpose is to detect suspicious code like the following: int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into parentheses: if ((!a) > 1) { ... }
This warning is enabled by ‘ -Wall ’.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where you can return an array, this also elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler detects undefined behavior in some statement during one or more of the iterations.
-Wno-attributes
Do not warn if an unexpected __attribute__ is used, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.
136 Using the GNU Compiler Collection (GCC)
Additionally, using ‘ -Wno-attributes= ’, it is possible to suppress warnings about unknown scoped attributes (in C ++ 11 and C2X). For example,
‘ -Wno-attributes=vendor::attr ’ disables warning about the following declaration:
[[vendor::attr]] void f();
It is also possible to disable warning about all attributes in a namespace using ‘ -Wno-attributes=vendor:: ’ which prevents warning about both of these declarations:
[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();
Note that ‘ -Wno-attributes= ’ does not imply ‘ -Wno-attributes ’.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that does not include a prototype is called with arguments whose promoted types do not match those expected by the function. When ‘ -Wextra ’ is specified, also warn when a built-in function that takes arguments is declared without a prototype.
The ‘ -Wbuiltin-declaration-mismatch ’ warning is enabled by default. To avoid the warning include the appropriate header to bring the prototypes of built-in functions into scope.
For example, the call to memset below is diagnosed by the warning because the function expects a value of type size_t as its argument but the type of 32 is int . With ‘ -Wextra ’, the declaration of the function is diagnosed as well.
extern void* memset (); void f (void *d)
{ memset (d, ’\0’, 32);
}
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of __TIMESTAMP__ , __TIME__ , __DATE__ , __FILE__ , and
__BASE_FILE__ .
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the argument types.
(An old-style function definition is permitted without a warning if preceded by a declaration that specifies the argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static are not the first things in a declaration. This warning is also enabled by ‘ -Wextra ’.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is given even if there is a previous prototype. A definition using ‘ () ’ is not considered an old-style definition in C2X mode, because it is equivalent to ‘ (void) ’ in that case, but is considered an old-style definition for older standards.
Chapter 3: GCC Command Options 137
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in K&R-style functions: void foo(bar) { }
This warning is also enabled by ‘ -Wextra ’.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype declaration.
This warning is issued even if the definition itself provides a prototype. Use this option to detect global functions that do not have a matching prototype declaration in a header file. This option is not valid for C ++ because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an earlier declaration.
Use ‘ -Wmissing-declarations ’ to detect missing declarations in C ++ .
-Wmissing-declarations
Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use ‘ -Wmissing-prototypes ’ to detect missing prototypes. In C ++ , no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure’s initializer has some fields missing. For example, the following code causes such a warning, because x.h
is implicitly zero: struct s { int f, g, h; }; struct s x = { 3, 4 };
This option does not warn about designated initializers, so the following modification does not trigger a warning: struct s { int f, g, h; }; struct s x = { .f = 3, .g = 4 };
In C this option does not warn about the universal zero initializer ‘ { 0 } ’: struct s { int f, g, h; }; struct s x = { 0 };
Likewise, in C ++ this option does not warn about the empty { } initializer, for example: struct s { int f, g, h; }; s x = { };
This warning is included in ‘ -Wextra ’. To get other ‘ -Wextra ’ warnings without this one, use ‘ -Wextra -Wno-missing-field-initializers ’.
-Wno-missing-requires
By default, the compiler warns about a concept-id appearing as a C ++ 20 simplerequirement: bool satisfied = requires { C<T> };
Here ‘ satisfied ’ will be true if ‘ C<T> ’ is a valid expression, which it is for all
T. Presumably the user meant to write
138 Using the GNU Compiler Collection (GCC) bool satisfied = requires { requires C<T> }; so ‘ satisfied ’ is only true if concept ‘ C ’ is satisfied for type ‘ T ’.
This warning can be disabled with ‘ -Wno-missing-requires ’.
-Wno-missing-template-keyword
The member access tokens ., > and :: must be followed by the template keyword if the parent object is dependent and the member being named is a template.
template <class X> void DoStuff (X x)
{ x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}
In rare cases it is possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers: void NotATemplate (my_class t)
{ int N = 5; bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}
This warning can be disabled with ‘ -Wno-missing-template-keyword ’.
-Wno-multichar
Do not warn if a multicharacter constant (‘ ’FOOF’ ’) is used.
Usually they indicate a typo in the user’s code, as they have implementation-defined values, and should not be used in portable code.
-Wnormalized= [ none|id|nfc|nfkc ]
In ISO C and ISO C ++ , two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning.
There are four levels of warning supported by GCC.
The default is
‘ -Wnormalized=nfc ’, which warns about any identifier that is not in the ISO
10646 “C” normalized form, NFC . NFC is the recommended form for most uses. It is equivalent to ‘ -Wnormalized ’.
Unfortunately, there are some characters allowed in identifiers by ISO C and
ISO C ++ that, when turned into NFC, are not allowed in identifiers. That is, there’s no way to use these symbols in portable ISO C or C ++ and have all your identifiers in NFC. ‘ -Wnormalized=id ’ suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this, which is why this option is not the default.
Chapter 3: GCC Command Options 139
You can switch the warning off for all characters by writing
‘ -Wnormalized=none ’ or ‘ -Wno-normalized ’.
You should only do this if you are using some other normalization scheme (like “D”), because otherwise you can easily create bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance \u207F , “SUPERSCRIPT LATIN SMALL LETTER N”, displays just like a regular n that has been placed in a superscript. ISO 10646 defines the
NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use ‘ -Wnormalized=nfkc ’.
This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly.
-Wno-attribute-warning
Do not warn about usage of functions (see
Section 6.33 [Function Attributes], page 548 ) declared with
warning attribute.
By default, this warning is enabled. ‘ -Wno-attribute-warning ’ can be used to disable the warning or
‘ -Wno-error=attribute-warning ’ can be used to disable the error when compiled with ‘ -Werror ’ flag.
-Wno-deprecated
Do not warn about usage of deprecated features. See
.
-Wno-deprecated-declarations
Do not warn about uses of functions (see
Section 6.33 [Function Attributes], page 548 ), variables (see
Section 6.34 [Variable Attributes], page 614 ), and types
(see
Section 6.35 [Type Attributes], page 628
) marked as deprecated by using the deprecated attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Wno-odr Warn about One Definition Rule violations during link-time optimization. Enabled by default.
-Wopenacc-parallelism
Warn about potentially suboptimal choices related to OpenACC parallelism.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The ‘ -fsimd-cost-model=unlimited ’ option can be used to relax the cost model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when using designated initializers (see
Section 6.29 [Designated Initializers], page 545
).
This warning is included in ‘ -Wextra ’. To get other ‘ -Wextra ’ warnings without this one, use ‘ -Wextra -Wno-override-init ’.
140 Using the GNU Compiler Collection (GCC)
-Wno-override-init-side-effects (C and Objective-C only)
Do not warn if an initialized field with side effects is overridden when using designated initializers (see
Section 6.29 [Designated Initializers], page 545 ). This
warning is enabled by default.
-Wpacked Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable f.x
in struct bar is misaligned even though struct bar does not itself have the packed attribute: struct foo { int x; char a, b, c, d;
} __attribute__((packed)); struct bar { char z;
}; struct foo f;
-Wnopacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char . This was fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field a and b in this structure: struct foo
{ char a:4; char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use ‘ -Wno-packed-bitfield-compat ’ to disable this warning.
-Wpacked-not-aligned (C, C ++ , Objective-C and Objective-C ++ only)
Warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on struct S , like, warning: alignment 1 of ’struct S’ is less than 8 , in this code: struct __attribute__ ((aligned (8))) S8 { char a[8]; }; struct __attribute__ ((packed)) S { struct S8 s8;
};
This warning is enabled by ‘ -Wall ’.
-Wpadded Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing.
Chapter 3: GCC Command Options 141
-Wrestrict
Warn when an object referenced by a restrict -qualified parameter (or, in
C ++ , a __restrict -qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the strcpy function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into a[4] , the copies overlap and the call is diagnosed.
void foo (void)
{ char a[] = "abcd1234"; strcpy (a, a + 4);
...
}
The ‘ -Wrestrict ’ option detects some instances of simple overlap even without optimization but works best at ‘ -O2 ’ and above. It is included in ‘ -Wall ’.
-Wnested-externs (C and Objective-C only)
Warn if an extern declaration is encountered within a function.
-Winline Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by ‘ -Winline ’ to appear or disappear.
-Winterference-size
Warn about use of C ++ 17 std::hardware_destructive_interference_size without specifying its value with ‘ --param destructive-interference-size ’.
Also warn about questionable values for that option.
This variable is intended to be used for controlling class layout, to avoid false sharing in concurrent code: struct independent_fields {
}; alignas(std::hardware_destructive_interference_size) std::atomic<int> one; alignas(std::hardware_destructive_interference_size) std::atomic<int> two;
Here ‘ one ’ and ‘ two ’ are intended to be far enough apart that stores to one won’t require accesses to the other to reload the cache line.
By default, ‘ --param destructive-interference-size ’ and ‘ --param constructive-interference-size ’ are set based on the current ‘ -mtune ’ option, typically to the L1 cache line size for the particular target CPU, sometimes to a range if tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same ‘ -mtune ’ (or ‘ -mcpu ’).
If ABI stability is important, such as if the use is in a header for a library, you should probably not use the hardware interference size variables at all.
Alternatively, you can force a particular value with ‘ --param ’.
142 Using the GNU Compiler Collection (GCC)
If you are confident that your use of the variable does not affect ABI outside a single build of your project, you can turn off the warning with
‘ -Wno-interference-size ’.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean values are expected, such as conditional expressions (?:) using non-boolean integer constants in boolean context, like if (a <= b ? 2 : 3) . Or left shifting of signed integers in boolean context, like for (a = 0; 1 << a; a++); . Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by ‘ -Wall ’.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a different size.
In C ++ , casting to a pointer type of smaller size is an error.
‘ Wint-to-pointer-cast ’ is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a different size.
-Winvalid-pch
Warn if a precompiled header (see
Section 3.22 [Precompiled Headers], page 500
) is found in the search path but cannot be used.
-Wlong-long
Warn if long long type is used. This is enabled by either ‘ -Wpedantic ’ or
‘ -Wtraditional ’ in ISO C90 and C ++ 98 modes. To inhibit the warning messages, use ‘ -Wno-long-long ’.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode.
This is enabled by either
‘ -Wpedantic ’ or ‘ -Wtraditional ’.
To inhibit the warning messages, use
‘ -Wno-variadic-macros ’.
-Wno-varargs
Do not warn upon questionable usage of the macros used to handle variable arguments like va_start . These warnings are enabled by default.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. Vector operation can be implemented piecewise , which means that the scalar operation is performed on every vector element; in parallel , which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and as a single scalar , which means that vector fits into a scalar type.
-Wvla Warn if a variable-length array is used in the code. ‘ -Wno-vla ’ prevents the
‘ -Wpedantic ’ warning of the variable-length array.
Chapter 3: GCC Command Options 143
-Wvla-larger-than= byte-size
If this option is used, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes.
This is similar to how
‘ -Walloca-larger-than= ’ byte-size works, but with variable-length arrays.
Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays.
‘ -Wvla-larger-than= ’‘ PTRDIFF_MAX ’ is enabled by default but is typically only effective when ‘ -ftree-vrp ’ is active (default for ‘ -O2 ’ and above).
See also ‘ -Walloca-larger-than= byte-size ’.
-Wno-vla-larger-than
Disable ‘ -Wvla-larger-than= ’ warnings.
The option is equivalent to
‘ -Wvla-larger-than= ’‘ SIZE_MAX ’ or larger.
-Wvla-parameter
Warn about redeclarations of functions involving arguments of Variable Length
Array types of inconsistent kinds or forms, and enable the detection of out-ofbounds accesses to such parameters by warnings such as ‘ -Warray-bounds ’.
If the first function declaration uses the VLA form the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed.
For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to f while the second one specifies that the array argument must have at least n elements. In addition, calling f with the assotiated VLA bound parameter in excess of the actual VLA bound triggers a warning as well.
void f (int n, int[n]); void f (int, int[]); // warning: argument 2 previously declared as a VLA void g (int n)
{ if (n > 4) return; int a[n]; f (sizeof a, a);
...
}
// warning: access to a by f may be out of bounds
‘ -Wvla-parameter ’ is included in ‘ -Wall ’. The ‘ -Warray-parameter ’ option triggers warnings for similar problems involving ordinary array arguments.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by ‘ -Wall ’.
144 Using the GNU Compiler Collection (GCC)
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC’s optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different signedness.
This option is only supported for C and Objective-C. It is implied by ‘ -Wall ’ and by ‘ -Wpedantic ’, which can be disabled with ‘ -Wno-pointer-sign ’.
-Wstack-protector
This option is only active when ‘ -fstack-protector ’ is active. It warns about functions that are not protected against stack smashing.
-Woverlength-strings
Warn about string constants that are longer than the “minimum maximum” length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard’s minimum limit, but very portable programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to
4095. C ++ 98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C ++ .
This option is implied by ‘ -Wpedantic ’, and can be disabled with
‘ -Wno-overlength-strings ’.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a suffix. When used together with ‘ -Wsystem-headers ’ it warns about such constants in system header files. This can be useful when preparing code to use with the FLOAT_
CONST_DECIMAL64 pragma from the decimal floating-point extension to C99.
-Wno-lto-type-mismatch
During the link-time optimization, do not warn about type mismatches in global declarations from different compilation units. Requires ‘ -flto ’ to be enabled.
Enabled by default.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to initialize a structure that has been marked with the designated_init attribute.
3.9 Options That Control Static Analysis
-fanalyzer
This option enables an static analysis of program flow which looks for “interesting” interprocedural paths through the code, and issues warnings for problems found on them.
Chapter 3: GCC Command Options 145
This analysis is much more expensive than other GCC warnings.
Enabling this option effectively enables the following warnings:
-Wanalyzer-double-fclose
-Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap
-Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation
-Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference
-Wanalyzer-null-argument
-Wanalyzer-null-dereference
-Wanalyzer-shift-count-negative
-Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer
-Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index
-Wanalyzer-tainted-divisor
-Wanalyzer-tainted-offset
-Wanalyzer-tainted-size
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-write-to-const
-Wanalyzer-write-to-string-literal
This option is only available if GCC was configured with analyzer support enabled.
-Wanalyzer-too-complex
If ‘ -fanalyzer ’ is enabled, the analyzer uses various heuristics to attempt to explore the control flow and data flow in the program, but these can be defeated by sufficiently complicated code.
By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it reaches an internal limit.
The
‘ -Wanalyzer-too-complex ’ option warns if this occurs.
-Wno-analyzer-double-fclose
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-double-fclose ’ to disable it.
use
This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once.
-Wno-analyzer-double-free
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-double-free ’ to disable it.
use
This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once, either free , or a deallocator referenced by attribute malloc .
146 Using the GNU Compiler Collection (GCC)
-Wno-analyzer-exposure-through-output-file
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-exposure-through-output-file ’ to disable it.
use
This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file).
-Wno-analyzer-file-leak
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-file-leak ’ to disable it.
use
This diagnostic warns for paths through the code in which a <stdio.h> FILE
* stream object is leaked.
-Wno-analyzer-free-of-non-heap
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-free-of-non-heap ’ to disable it.
use
This diagnostic warns for paths through the code in which free is called on a non-heap pointer (e.g. an on-stack buffer, or a global).
-Wno-analyzer-malloc-leak
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-malloc-leak ’ to disable it.
use
This diagnostic warns for paths through the code in which a pointer allocated via an allocator is leaked: either malloc , or a function marked with attribute malloc .
-Wno-analyzer-mismatching-deallocation
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-mismatching-deallocation ’ to disable it.
use
This diagnostic warns for paths through the code in which the wrong deallocation function is called on a pointer value, based on which function was used to allocate the pointer value. The diagnostic will warn about mismatches between free , scalar delete and vector delete[] , and those marked as allocator/deallocator pairs using attribute malloc .
-Wno-analyzer-possible-null-argument
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-possible-null-argument ’ to disable it.
use
This diagnostic warns for paths through the code in which a possibly-NULL value is passed to a function argument marked with __attribute__
((nonnull)) as requiring a non-NULL value.
-Wno-analyzer-possible-null-dereference
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-possible-null-dereference ’ to disable it.
use
This diagnostic warns for paths through the code in which a possibly-NULL value is dereferenced.
-Wno-analyzer-null-argument
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-null-argument ’ to disable it.
use
Chapter 3: GCC Command Options 147
This diagnostic warns for paths through the code in which a value known to be NULL is passed to a function argument marked with __attribute__
((nonnull)) as requiring a non-NULL value.
-Wno-analyzer-null-dereference
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-null-dereference ’ to disable it.
use
This diagnostic warns for paths through the code in which a value known to be
NULL is dereferenced.
-Wno-analyzer-shift-count-negative
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-shift-count-negative ’ to disable it.
use
This diagnostic warns for paths through the code in which a shift is attempted with a negative count. It is analogous to the ‘ -Wshift-count-negative ’ diagnostic implemented in the C/C ++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree.
However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-shift-count-overflow
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-shift-count-overflow ’ to disable it.
use
This diagnostic warns for paths through the code in which a shift is attempted with a count greater than or equal to the precision of the operand’s type.
It is analogous to the ‘ -Wshift-count-overflow ’ diagnostic implemented in the C/C ++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-stale-setjmp-buffer
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-stale-setjmp-buffer ’ to disable it.
use
This diagnostic warns for paths through the code in which longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has returned.
When setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. The stack frame becomes invalid when the function containing the setjmp call returns. Attempting to rewind to it via longjmp would reference a stack frame that no longer exists, and likely lead to a crash (or worse).
-Wno-analyzer-tainted-allocation-size
This warning requires both ‘ -fanalyzer ’ and ‘ -fanalyzer-checker=taint ’ to enable it; use ‘ -Wno-analyzer-tainted-allocation-size ’ to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker’s control is used as the size of an allocation without being sanitized, so that an attacker could inject an excessively large allocation and potentially cause a denial of service attack.
See CWE-789: Memory Allocation with Excessive Size Value .
148 Using the GNU Compiler Collection (GCC)
-Wno-analyzer-tainted-array-index
This warning requires both ‘ -fanalyzer ’ and ‘ -fanalyzer-checker=taint ’ to enable it; use ‘ -Wno-analyzer-tainted-array-index ’ to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker’s control is used as the index of an array access without being sanitized, so that an attacker could inject an out-of-bounds access.
See CWE-129: Improper Validation of Array Index .
-Wno-analyzer-tainted-divisor
This warning requires both ‘ -fanalyzer ’ and ‘ -fanalyzer-checker=taint ’ to enable it; use ‘ -Wno-analyzer-tainted-divisor ’ to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker’s control is used as the divisor in a division or modulus operation without being sanitized, so that an attacker could inject a divisionby-zero.
-Wno-analyzer-tainted-offset
This warning requires both ‘ -fanalyzer ’ and ‘ -fanalyzer-checker=taint ’ to enable it; use ‘ -Wno-analyzer-tainted-offset ’ to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker’s control is used as a pointer offset without being sanitized, so that an attacker could inject an out-of-bounds access.
See CWE-823: Use of Out-of-range Pointer Offset .
-Wno-analyzer-tainted-size
This warning requires both ‘ -fanalyzer ’ and ‘ -fanalyzer-checker=taint ’ to enable it; use ‘ -Wno-analyzer-tainted-size ’ to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker’s control is used as the size of an operation such as memset without being sanitized, so that an attacker could inject an out-of-bounds access.
-Wno-analyzer-unsafe-call-within-signal-handler
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-unsafe-call-within-signal-handler it.
’ to use disable
This diagnostic warns for paths through the code in which a function known to be async-signal-unsafe (such as fprintf ) is called from a signal handler.
-Wno-analyzer-use-after-free
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-use-after-free ’ to disable it.
use
This diagnostic warns for paths through the code in which a pointer is used after a deallocator is called on it: either free , or a deallocator referenced by attribute malloc .
Chapter 3: GCC Command Options 149
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-use-of-pointer-in-stale-stack-frame ’ to it.
use disable
This diagnostic warns for paths through the code in which a pointer is dereferenced that points to a variable in a stale stack frame.
-Wno-analyzer-write-to-const
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-write-to-const ’ to disable it.
use
This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a pointer to a const object. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-write-to-string-literal
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-write-to-string-literal ’ to disable it.
use
This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a pointer to a string literal. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-use-of-uninitialized-value
This warning requires ‘ -fanalyzer ’, which enables it;
‘ -Wno-analyzer-use-of-uninitialized-value ’ to disable it.
use
This diagnostic warns for paths through the code in which an uninitialized value is used.
Pertinent parameters for controlling the exploration are: ‘ --param analyzer-bb-explosion-factor= value ’,
‘ --param analyzer-max-enodes-per-program-point= value ’, ‘ --param analyzer-max-recursion-depth= value ’, and ‘ --param analyzer-min-snodes-for-call-summary= value ’.
The following options control the analyzer.
-fanalyzer-call-summaries
Simplify interprocedural analysis by computing the effect of certain calls, rather than exploring all paths through the function from callsite to each possible return.
If enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated (as per ‘ --param analyzer-min-snodes-for-call-summary= value ’).
-fanalyzer-checker= name
Restrict the analyzer to run just the named checker, and enable it.
Some checkers are disabled by default (even with ‘ -fanalyzer ’), such as the taint checker that implements ‘ -Wanalyzer-tainted-array-index ’, and this option is required to enable them.
-fno-analyzer-feasibility
This option is intended for analyzer developers.
150 Using the GNU Compiler Collection (GCC)
By default the analyzer verifies that there is a feasible control flow path for each diagnostic it emits: that the conditions that hold are not mutually exclusive.
Diagnostics for which no feasible path can be found are rejected. This filtering can be suppressed with ‘ -fno-analyzer-feasibility ’, for debugging issues in this code.
-fanalyzer-fine-grained
This option is intended for analyzer developers.
Internally the analyzer builds an “exploded graph” that combines control flow graphs with data flow information.
By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With ‘ -fanalyzer-fine-grained ’, each statement gets its own edge.
-fanalyzer-show-duplicate-count
This option is intended for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic.
-fno-analyzer-state-merge
This option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its “exploded graph”. With
‘ -fno-analyzer-state-merge ’ this merging can be suppressed, for debugging state-handling issues.
-fno-analyzer-state-purge
This option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant e.g. the values of locals that aren’t accessed later in the function and which aren’t relevant to leak analysis.
With ‘ -fno-analyzer-state-purge ’ this purging of state can be suppressed, for debugging state-handling issues.
-fanalyzer-transitivity
This option enables transitivity of constraints within the analyzer.
-fanalyzer-verbose-edges
This option is intended for analyzer developers. It enables more verbose, lowerlevel detail in the descriptions of control flow within diagnostic paths.
-fanalyzer-verbose-state-changes
This option is intended for analyzer developers. It enables more verbose, lowerlevel detail in the descriptions of events relating to state machines within diagnostic paths.
-fanalyzer-verbosity= level
This option controls the complexity of the control flow paths that are emitted for analyzer diagnostics.
Chapter 3: GCC Command Options 151
‘
‘
‘
The level can be one of:
‘ 0 ’
1 ’
At this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a doublefree diagnostic, both calls to free will be shown.
As per the previous level, but also show events for the entry to each function.
‘ 2 ’
3
4 ’
’
As per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g. “true path taken” at a conditional).
This level is the default.
As per the previous level, but show all control flow events, not just significant ones.
This level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.
-fdump-analyzer
Dump internal details about what the analyzer is doing to
‘ file .analyzer.txt
’. This option is overridden by ‘ -fdump-analyzer-stderr ’.
-fdump-analyzer-stderr
Dump internal details about what the analyzer is doing to stderr. This option overrides ‘ -fdump-analyzer ’.
-fdump-analyzer-callgraph
Dump a representation of the call graph suitable for viewing with GraphViz to
‘ file .callgraph.dot
’.
-fdump-analyzer-exploded-graph
Dump a representation of the “exploded graph” suitable for viewing with
GraphViz to ‘ file .eg.dot
’. Nodes are color-coded based on state-machine states to emphasize state changes.
-fdump-analyzer-exploded-nodes
Emit diagnostics showing where nodes in the “exploded graph” are in relation to the program source.
-fdump-analyzer-exploded-nodes-2
Dump a textual representation of the “exploded graph” to ‘ file .eg.txt
’.
-fdump-analyzer-exploded-nodes-3
Dump a textual representation of the “exploded graph” to one dump file per node, to ‘ file .egid .txt
’. This is typically a large number of dump files.
-fdump-analyzer-exploded-paths
Dump a textual representation of the “exploded path” for each diagnostic to
‘ file .
idx .
kind .epath.txt
’.
152 Using the GNU Compiler Collection (GCC)
-fdump-analyzer-feasibility
Dump internal details about the analyzer’s search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the form ‘ file .*.fg.dot
’ and ‘ file .*.tg.dot
’.
-fdump-analyzer-json
Dump a compressed JSON representation of analyzer internals to
‘ file .analyzer.json.gz
’. The precise format is subject to change.
-fdump-analyzer-state-purge
As per ‘ -fdump-analyzer-supergraph ’, dump a representation of the “supergraph” suitable for viewing with GraphViz, but annotate the graph with information on what state will be purged at each node. The graph is written to
‘ file .state-purge.dot
’.
-fdump-analyzer-supergraph
Dump representations of the “supergraph” suitable for viewing with GraphViz to ‘ file .supergraph.dot
’ and to ‘ file .supergraph-eg.dot
’.
These show all of the control flow graphs in the program, with interprocedural edges for calls and returns. The second dump contains annotations showing nodes in the
“exploded graph” and diagnostics associated with them.
3.10 Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add ‘ -g ’ to your other options. Some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command line option to your other options.
GCC allows you to use ‘ -g ’ with ‘ -O ’.
The shortcuts taken by optimized code may occasionally be surprising: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. Nevertheless it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.
If you are not using some other optimization option, consider using ‘ -Og ’ (see
) with ‘ -g ’. With no ‘ -O ’ option at all, some compiler passes that collect information useful for debugging do not run at all, so that ‘ -Og ’ may result in a better debugging experience.
-g Produce debugging information in the operating system’s native format (stabs,
COFF, XCOFF, or DWARF). GDB can work with this debugging information.
On most systems that use stabs format, ‘ -g ’ enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use ‘ -gstabs+ ’, ‘ -gstabs ’, ‘ -gxcoff+ ’, ‘ -gxcoff ’, or ‘ -gvms ’ (see below).
Chapter 3: GCC Command Options 153
-ggdb Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
-gdwarf
-gdwarfversion
Produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is
5 (with the exception of VxWorks, TPF and Darwin/Mac OS X, which default to version 2, and AIX, which defaults to version 4).
Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and ‘ -fvar-tracking-assignments ’ for maximum benefit. Version 5 requires GDB 8.0 or higher.
GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as ‘ -fno-dwarf2-cfi-asm ’) retain a reference to DWARF Version
2 in their names, but apply to all currently-supported versions of DWARF.
-gbtf Request BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.
-gctf
-gctf level
Request CTF debug information and use level to specify how much CTF debug information should be produced. If ‘ -gctf ’ is specified without a value for level, the default level of CTF debug information is 2.
CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.
Level 0 produces no CTF debug information at all. Thus, ‘ -gctf0 ’ negates
‘ -gctf ’.
Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information.
Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.
-gstabs Produce debugging information in stabs format (if that is supported), without
GDB extensions.
This is the format used by DBX on most BSD systems.
On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX. On System V Release 4 systems this option requires the GNU assembler.
-gstabs+ Produce debugging information in stabs format (if that is supported), using
GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program.
154 Using the GNU Compiler Collection (GCC)
-gxcoff Produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.
-gxcoff+ Produce debugging information in XCOFF format (if that is supported), using
GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program, and may cause assemblers other than the GNU assembler (GAS) to fail with an error.
-gvms Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems.
-g level
-ggdb level
-gstabs level
-gxcoff level
-gvms level
Request debugging information and also use level to specify how much information. The default level is 2.
Level 0 produces no debug information at all. Thus, ‘ -g0 ’ negates ‘ -g ’.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don’t plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use ‘ -g3 ’.
If you use multiple ‘ -g ’ options, with or without level numbers, the last such option is the one that is effective.
‘ -gdwarf ’ does not accept a concatenated debug level, to avoid confusion with
‘ -gdwarflevel ’. Instead use an additional ‘ -g level ’ option to change the debug level for DWARF.
-fno-eliminate-unused-debug-symbols
By default, no debug information is produced for symbols that are not actually used. Use this option if you want debug information for all symbols.
-femit-class-debug-always
Instead of emitting debugging information for a C ++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes because using this option increases the size of debugging information by as much as a factor of two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.
Chapter 3: GCC Command Options 155
-fdebug-prefix-map= old = new
When compiling files residing in directory ‘ old ’, record debugging information describing them as if the files resided in directory ‘ new ’ instead. This can be used to replace a build-time path with an install-time path in the debug info.
It can also be used to change an absolute path to a relative path by using ‘ .
’ for new . This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files.
See also ‘ -ffile-prefix-map ’.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization (‘ -Os ’, ‘ -O ’, ‘ -O2 ’,
. . . ), debugging information (‘ -g ’) and the debug info format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of ‘ -gdwarf-4 ’ is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. By default, this flag is enabled together with ‘ -fvar-tracking ’, except when selective scheduling is enabled.
-gsplit-dwarf
If DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension ‘ .dwo
’. This option allows the build system to avoid linking files with debug information.
To be useful, this option requires a debugger capable of reading ‘ .dwo
’ files.
-gdwarf32
-gdwarf64
If DWARF debugging information is enabled, the ‘ -gdwarf32 ’ selects the 32-bit
DWARF format and the ‘ -gdwarf64 ’ selects the 64-bit DWARF format. The default is target specific, on most targets it is ‘ -gdwarf32 ’ though. The 32-bit
DWARF format is smaller, but can’t support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet.
-gdescribe-dies
Add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs.
-gpubnames
Generate DWARF .debug_pubnames
and .debug_pubtypes
sections.
156 Using the GNU Compiler Collection (GCC)
-ggnu-pubnames
Generate .debug_pubnames
and .debug_pubtypes
sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into their own
.debug_types
section instead of making them part of the .debug_info
section.
It is more efficient to put them in a separate comdat section since the linker can then remove duplicates. But not all DWARF consumers support .debug_
types sections yet and on some objects .debug_types
produces larger instead of smaller debugging information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW AT producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also ‘ -frecord-gcc-switches ’ for another way of storing compiler options into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected with
‘ -gdwarfversion ’. On most targets using non-conflicting DWARF extensions from later standard versions is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with
‘ -gdwarfversion ’.
-gas-loc-support
Inform the compiler that the assembler supports .loc
directives. It may then use them for the assembler to generate DWARF2 + line number tables.
This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself.
This option will be enabled by default if, at GCC configure time, the assembler was found to support such directives.
-gno-as-loc-support
Force GCC to generate DWARF2 + line number tables internally, if DWARF2 + line number tables are to be generated.
-gas-locview-support
Inform the compiler that the assembler supports view assignment and reset assertion checking in .loc
directives.
This option will be enabled by default if, at GCC configure time, the assembler was found to support them.
-gno-as-locview-support
Force GCC to assign view numbers internally, if ‘ -gvariable-location-views ’ are explicitly requested.
Chapter 3: GCC Command Options 157
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging information, rather than just file and line. This option is enabled by default.
-gstatement-frontiers
-gno-statement-frontiers
This option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. This is enabled by default when compiling with optimization
(‘ -Os ’, ‘ -O1 ’, ‘ -O2 ’, . . . ), and outputting DWARF 2 debug information at the normal level.
-gvariable-location-views
-gvariable-location-views=incompat5
-gno-variable-location-views
Augment variable location lists with progressive view numbers implied from the line number table. This enables debug information consumers to inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this will cause the compiler to emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backwardcompatible, so they can be consumed by debug information consumers that are not aware of these augmentations, but they won’t derive any benefit from them either.
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support,
‘ -fvar-tracking-assignments ’ is enabled and ‘ -gstrict-dwarf ’ is not.
When assembler support is not available, this may still be enabled, but it will force GCC to output internal line number tables, and if
‘ -ginternal-reset-location-views ’ is not enabled, that will most certainly lead to silently mismatching location views.
There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with ‘ -gvariable-location-views=incompat5 ’. This option may be removed in the future, is only provided as a reference implementation of the proposed representation. Debug information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it.
-ginternal-reset-location-views
-gno-internal-reset-location-views
Attempt to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate insn length estimates, which isn’t always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists. The
158 Using the GNU Compiler Collection (GCC)
GNU assembler will flag any such error as a view number mismatch . This is only enabled on ports that define a reliable estimation function.
-ginline-points
-gno-inline-points
Generate extended debug information for inlined functions.
Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug information. This can be enabled independently of location views, in which case the view numbers won’t be output, but it can only be enabled along with statement frontiers, and it is only enabled by default if location views are enabled.
-gz [ = type ]
Produce compressed debug sections in DWARF format, if that is supported. If type is not given, the default type depends on the capabilities of the assembler and linker used.
type may be one of ‘ none ’ (don’t compress debug sections),
‘ zlib ’ (use zlib compression in ELF gABI format), or ‘ zlib-gnu ’ (use zlib compression in traditional GNU format). If the linker doesn’t support writing compressed debug sections, the option is rejected. Otherwise, if the assembler does not support them, ‘ -gz ’ is silently ignored when producing object files.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined.
This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger.
See ‘ -femit-struct-debug-reduced ’ for a less aggressive option.
See
‘ -femit-struct-debug-detailed ’ for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header.
‘
This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger.
See
-femit-struct-debug-baseonly ’ for a more aggressive option.
‘ -femit-struct-debug-detailed ’ for more detailed control.
See
This option works only with DWARF debug output.
-femit-struct-debug-detailed [ = spec-list ]
Specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.
This option is a detailed version of ‘ -femit-struct-debug-reduced ’ and
‘ -femit-struct-debug-baseonly ’, which serves for most needs.
A specification has the syntax
[‘ dir: ’ | ‘ ind: ’][‘ ord: ’ | ‘ gen: ’](‘ any ’ | ‘ sys ’ | ‘ base ’ | ‘ none ’)
Chapter 3: GCC Command Options 159
The optional first word limits the specification to structs that are used directly
(‘ dir: ’) or used indirectly (‘ ind: ’). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs.
That is, when use of an incomplete struct is valid, the use is indirect. An example is ‘ struct one direct; struct two * indirect; ’.
The optional second word limits the specification to ordinary structs (‘ ord: ’) or generic structs (‘ gen: ’). Generic structs are a bit complicated to explain. For
C ++ , these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but
‘ -femit-struct-debug-detailed ’ does not yet implement them.
The third word specifies the source files for those structs for which the compiler should emit debug information. The values ‘ none ’ and ‘ any ’ have the normal meaning. The value ‘ base ’ means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling ‘ foo.c
’, debug information is generated for types declared in that file and ‘ foo.h
’, but not other header files. The value ‘ sys ’ means those types satisfying ‘ base ’ or declared in system or compiler headers.
You may need to experiment to determine the best settings for your application.
The default is ‘ -femit-struct-debug-detailed=all ’.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated .eh_frame
section instead of using GAS .cfi_* directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled.
Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared).
More often, however, this results in a significant amount of wasted space.
3.11 Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
160 Using the GNU Compiler Collection (GCC)
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Most optimizations are completely disabled at ‘ -O0 ’ or if an ‘ -O ’ level is not set on the command line, even if individual optimization flags are specified. Similarly, ‘ -Og ’ suppresses many optimization passes.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each ‘ -O ’ level than those listed here. You can invoke GCC with ‘ -Q --help=optimizers ’ to find out the exact set of optimizations that are enabled at each level. See
Section 3.2 [Overall Options], page 33 , for examples.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With ‘ -O ’, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
‘ -O ’ turns on the following optimization flags:
-fauto-inc-dec
-fbranch-count-reg
-fcombine-stack-adjustments
-fcompare-elim
-fcprop-registers
-fdce
-fdefer-pop
-fdelayed-branch
-fdse
-fforward-propagate
-fguess-branch-probability
-fif-conversion
-fif-conversion2
-finline-functions-called-once
-fipa-modref
-fipa-profile
-fipa-pure-const
-fipa-reference
-fipa-reference-addressable
-fmerge-constants
-fmove-loop-invariants
-fmove-loop-stores
-fomit-frame-pointer
-freorder-blocks
-fshrink-wrap
-fshrink-wrap-separate
-fsplit-wide-types
-fssa-backprop
-fssa-phiopt
-ftree-bit-ccp
-ftree-ccp
-ftree-ch
-ftree-coalesce-vars
-ftree-copy-prop
Chapter 3: GCC Command Options 161
-O2
-ftree-dce
-ftree-dominator-opts
-ftree-dse
-ftree-forwprop
-ftree-fre
-ftree-phiprop
-ftree-pta
-ftree-scev-cprop
-ftree-sink
-ftree-slsr
-ftree-sra
-ftree-ter
-funit-at-a-time
Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to ‘ -O ’, this option increases both compilation time and the performance of the generated code.
‘ -O2 ’ turns on all optimization flags specified by ‘ -O1 ’. It also turns on the following optimization flags:
-falign-functions -falign-jumps
-falign-labels -falign-loops
-fcaller-saves
-fcode-hoisting
-fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks
-fdelete-null-pointer-checks
-fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations
-ffinite-loops
-fgcse -fgcse-lm
-fhoist-adjacent-loads
-finline-functions
-finline-small-functions
-findirect-inlining
-fipa-bit-cp -fipa-cp -fipa-icf
-fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference
-flra-remat
-foptimize-sibling-calls
-foptimize-strlen
-fpartial-inlining
-fpeephole2
-freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop
-fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec
-fstore-merging
-fstrict-aliasing
-fthread-jumps
-ftree-builtin-call-dce
-ftree-loop-vectorize
-ftree-pre
-ftree-slp-vectorize
-ftree-switch-conversion -ftree-tail-merge
-ftree-vrp
-fvect-cost-model=very-cheap
162 Using the GNU Compiler Collection (GCC)
-O3
-O0
-Os
-Ofast
-Og
-Oz
Please note the warning under ‘ -fgcse ’ about invoking ‘ -O2 ’ on programs that use computed gotos.
Optimize yet more. ‘ -O3 ’ turns on all optimizations specified by ‘ -O2 ’ and also turns on the following optimization flags:
-fgcse-after-reload
-fipa-cp-clone -floop-interchange
-floop-unroll-and-jam
-fpeel-loops
-fpredictive-commoning
-fsplit-loops
-fsplit-paths
-ftree-loop-distribution
-ftree-partial-pre
-funswitch-loops
-fvect-cost-model=dynamic
-fversion-loops-for-strides
Reduce compilation time and make debugging produce the expected results.
This is the default.
Optimize for size. ‘ -Os ’ enables all ‘ -O2 ’ optimizations except those that often increase code size:
-falign-functions -falign-jumps
-falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stc
It also enables ‘ -finline-functions ’, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size.
Disregard strict standards compliance.
‘ -Ofast ’ enables all ‘ -O3 ’ optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on ‘ -ffast-math ’, ‘ -fallow-store-data-races ’ and the
Fortran-specific ‘ -fstack-arrays ’, unless ‘ -fmax-stack-var-size ’ is specified, and ‘ -fno-protect-parens ’. It turns off ‘ -fsemantic-interposition ’.
Optimize debugging experience. ‘ -Og ’ should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.
It is a better choice than ‘ -O0 ’ for producing debuggable code because some compiler passes that collect debug information are disabled at ‘ -O0 ’.
Like ‘ -O0 ’, ‘ -Og ’ completely disables a number of optimization passes so that individual options controlling them have no effect. Otherwise ‘ -Og ’ enables all
‘ -O1 ’ optimization flags except for those that may interfere with debugging:
-fbranch-count-reg -fdelayed-branch
-fdse -fif-conversion -fif-conversion2
-finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra
Optimize aggressively for size rather than speed. This may increase the number of instructions executed if those instructions require fewer bytes to encode. ‘ -Oz ’ behaves similarly to ‘ -Os ’ including enabling most ‘ -O2 ’ optimizations.
Chapter 3: GCC Command Options 163
If you use multiple ‘ -O ’ options, with or without level numbers, the last such option is the one that is effective.
Options of the form ‘ -f flag ’ specify machine-independent flags. Most flags have both positive and negative forms; the negative form of ‘ -ffoo ’ is ‘ -fno-foo ’. In the table below, only one of the forms is listed—the one you typically use. You can figure out the other form by either removing ‘ no’ or adding it.
The following options control specific optimizations. They are either activated by ‘ -O ’ options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.
-fno-defer-pop
For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels ‘ -O1 ’ and higher,
‘ -fdefer-pop ’ is the default; this allows the compiler to let arguments accumulate on the stack for several function calls and pop them all at once.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-ffp-contract= style
‘ -ffp-contract=off ’ disables floating-point expression contraction.
‘ -ffp-contract=fast ’ enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them.
‘ -ffp-contract=on ’ enables floating-point expression contraction if allowed by the language standard.
This is currently not implemented and treated equal to ‘ -ffp-contract=off ’.
The default is ‘ -ffp-contract=fast ’.
-fomit-frame-pointer
Omit the frame pointer in functions that don’t need one.
This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available.
On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.
Note that ‘ -fno-omit-frame-pointer ’ doesn’t guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.
Enabled by default at ‘ -O1 ’ and higher.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-foptimize-strlen
Optimize various standard C string functions (e.g.
strlen , strchr or strcpy ) and their _FORTIFY_SOURCE counterparts into faster alternatives.
164 Using the GNU Compiler Collection (GCC)
Enabled at levels ‘ -O2 ’, ‘ -O3 ’.
-fno-inline
Do not expand any functions inline apart from those marked with the always_ inline attribute. This is the default when not optimizing.
Single functions can be exempted from inlining by marking them with the noinline attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating in this way. This inlining applies to all functions, even those not declared inline.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-findirect-inlining
Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option has any effect only when inlining itself is turned on by the ‘ -finline-functions ’ or ‘ -finline-small-functions ’ options.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-finline-functions
Consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides which functions are worth integrating in this way.
If all calls to a given function are integrated, and the function is declared static , then the function is normally not output as assembler code in its own right.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’. Also enabled by ‘ -fprofile-use ’ and
‘ -fauto-profile ’.
-finline-functions-called-once
Consider all static functions called once for inlining into their caller even if they are not marked inline . If a call to a given function is integrated, then the function is not output as assembler code in its own right.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’ and ‘ -Os ’, but not ‘ -Og ’.
-fearly-inlining
Inline functions marked by always_inline and functions whose body seems smaller than the function call overhead early before doing
‘ -fprofile-generate ’ instrumentation and real inlining pass. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’ and ‘ -Os ’.
Chapter 3: GCC Command Options 165
-finline-limit= n
By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit.
n is the size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by using ‘ --param name = value ’. The ‘ -finline-limit= n ’ option sets some of these parameters as follows: max-inline-insns-single is set to n /2.
max-inline-insns-auto is set to n /2.
See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value to ‘ -finline-limit ’ that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function’s size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of ‘ -fkeep-inline-functions ’, which applies only to functions that are declared using the dllexport attribute or declspec. See
Section 6.33 [Declaring Attributes of Functions], page 548
.
-fkeep-inline-functions
In C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90. In C ++ , emit any and all inline functions into the object file.
-fkeep-static-functions
Emit static functions into the object file, even if the function is never used.
-fkeep-static-consts
Emit variables declared static const when optimization isn’t turned on, even if the variables aren’t referenced.
GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the ‘ -fno-keep-static-consts ’ option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating-point constants) across compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use ‘ -fno-merge-constants ’ to inhibit this behavior.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
166 Using the GNU Compiler Collection (GCC)
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies ‘ -fmerge-constants ’. In addition to ‘ -fmerge-constants ’ this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C ++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass.
This pass looks at innermost loops and reorders their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis.
This option is effective only with ‘ -fmodulo-sched ’ enabled.
-fno-branch-count-reg
Disable the optimization pass that scans for opportunities to use “decrement and branch” instructions on a count register instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the
‘ -fno-branch-count-reg ’ option doesn’t remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes.
The default is ‘ -fbranch-count-reg ’ at ‘ -O1 ’ and higher, except for ‘ -Og ’.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function contain the function’s address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
The default is ‘ -ffunction-cse ’
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to the data section—e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
The default is ‘ -fzero-initialized-in-bss ’.
-fthread-jumps
Perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is
Chapter 3: GCC Command Options 167 redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fsplit-wide-types
When using a type that occupies multiple registers, such as long long on a
32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fsplit-wide-types-early
Fully split wide types early, instead of very late. This option has no effect unless
‘ -fsplit-wide-types ’ is turned on.
This is the default on some targets.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fcse-skip-blocks
This is similar to ‘ -fcse-follow-jumps ’, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, ‘ -fcse-skip-blocks ’ causes CSE to follow the jump around the body of the if .
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations are performed.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fgcse Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding ‘ -fno-gcse ’ to the command line.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fgcse-lm
When ‘ -fgcse-lm ’ is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves.
This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop.
Enabled by default when ‘ -fgcse ’ is enabled.
168 Using the GNU Compiler Collection (GCC)
-fgcse-sm
When ‘ -fgcse-sm ’ is enabled, a store motion pass is run after global common subexpression elimination. This pass attempts to move stores out of loops.
When used in conjunction with ‘ -fgcse-lm ’, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When ‘ -fgcse-las ’ is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When ‘ -fgcse-after-reload ’ is enabled, a redundant load elimination pass is performed after reload. The purpose of this pass is to clean up redundant spilling.
Enabled by ‘ -O3 ’, ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations.
This option is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared in common blocks (e.g.
Fortran) may later be overridden with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that do not have instructions to support this. Enabled by default at ‘ -O1 ’ and higher on architectures that support this.
-fdce Perform dead code elimination (DCE) on RTL. Enabled by default at ‘ -O1 ’ and higher.
-fdse Perform dead store elimination (DSE) on RTL. Enabled by default at ‘ -O1 ’ and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents.
This includes use of conditional moves, min, max, set flags and abs instructions, and
Chapter 3: GCC Command Options 169 some tricks doable by standard arithmetics. The use of conditional execution on chips where it is available is controlled by ‘ -fif-conversion2 ’.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’, but not with ‘ -Og ’.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’, but not with ‘ -Og ’.
-fdeclone-ctor-dtor
The C ++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function. With this option, the base and complete variants are changed to be thunks that call a common implementation.
Enabled by ‘ -Os ’.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. In addition, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not true.
Use ‘ -fno-delete-null-pointer-checks ’ to disable this optimization for programs that depend on that behavior.
This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR, CR16, and MSP430, this option is completely disabled.
Passes that use the dataflow information are enabled independently at different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls.
This is done both within a procedure and interprocedurally as part of indirect inlining (‘ -findirect-inlining ’) and interprocedural constant propagation
(‘ -fipa-cp ’). Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size 1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, such as inlining. When they seem useless after further optimization, they are converted back into original form.
170 Using the GNU Compiler Collection (GCC)
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-free Attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.
Enabled for Alpha, AArch64 and x86 at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fno-lifetime-dse
In C ++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination will take advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor, you can use ‘ -flifetime-dse=1 ’.
The default behavior can be explicitly selected with ‘ -flifetime-dse=2 ’.
‘ -flifetime-dse=0 ’ is equivalent to
‘ -fno-lifetime-dse ’.
-flive-range-shrinkage
Attempt to decrease register pressure through register live range shrinkage.
This is helpful for fast processors with small or moderate size register sets.
-fira-algorithm= algorithm
Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be ‘ priority ’, which specifies Chow’s priority coloring, or ‘ CB ’, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code.
-fira-region= region
Use specified regions for the integrated register allocator. The region argument should be one of the following:
‘ all ’ Use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
‘ mixed ’ Use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (‘ -O ’, ‘ -O2 ’, . . . ).
Chapter 3: GCC Command Options 171
‘ one ’ Use all functions as a single region. This typically results in the smallest code size, and is enabled by default for ‘ -Os ’ or ‘ -O0 ’.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down.
This option is enabled at level ‘ -Os ’ for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files ( > = 32 registers), but it can slow the compiler down.
This option is enabled at level ‘ -O3 ’ for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard register gets a separate stack slot, and as a result function stack frames are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers.
Each pseudoregister that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it is profitable.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’, but not at ‘ -Og ’.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’.
-fschedule-insns2
Similar to ‘ -fschedule-insns ’, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
172 Using the GNU Compiler Collection (GCC)
-fno-sched-interblock
Disable instruction scheduling across basic blocks, which is normally enabled when scheduling before register allocation, i.e. with ‘ -fschedule-insns ’ or at
‘ -O2 ’ or higher.
-fno-sched-spec
Disable speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with ‘ -fschedule-insns ’ or at
‘ -O2 ’ or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before register allocation.
This only makes sense when scheduling before register allocation is enabled, i.e. with ‘ -fschedule-insns ’ or at ‘ -O2 ’ or higher. Usage of this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and subsequent spills in register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with ‘ -fschedule-insns ’ or at
‘ -O2 ’ or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with ‘ -fschedule-insns ’ or at
‘ -O2 ’ or higher.
-fsched-stalled-insns
-fsched-stalled-insns= n
Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass.
‘ -fno-sched-stalled-insns ’ means that no insns are moved prematurely,
‘ -fsched-stalled-insns=0 ’ means there is no limit on how many queued insns can be moved prematurely. ‘ -fsched-stalled-insns ’ without a value is equivalent to ‘ -fsched-stalled-insns=1 ’.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep= n
Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for premature removal from the queue of stalled insns.
This has an effect only during the second scheduling pass, and only if ‘ -fsched-stalled-insns ’ is used.
‘ -fno-sched-stalled-insns-dep ’ is equivalent to
‘ -fsched-stalled-insns-dep=0 ’.
‘ -fsched-stalled-insns-dep ’ without a value is equivalent to ‘ -fsched-stalled-insns-dep=1 ’.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, resulting in faster schedules. This
Chapter 3: GCC Command Options 173 option is experimental, as not all machine descriptions used by GCC model the
CPU closely enough to avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with
‘ -fschedule-insns2 ’ or at ‘ -O2 ’ or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. This is enabled by default when scheduling is enabled, i.e. with ‘ -fschedule-insns ’ or ‘ -fschedule-insns2 ’ or at ‘ -O2 ’ or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path. This is enabled by default when scheduling is enabled, i.e. with ‘ -fschedule-insns ’ or ‘ -fschedule-insns2 ’ or at ‘ -O2 ’ or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with ‘ -fschedule-insns ’ or
‘ -fschedule-insns2 ’ or at ‘ -O2 ’ or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block with greater size or frequency.
This is enabled by default when scheduling is enabled, i.e. with ‘ -fschedule-insns ’ or
‘ -fschedule-insns2 ’ or at ‘ -O2 ’ or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with ‘ -fschedule-insns ’ or ‘ -fschedule-insns2 ’ or at ‘ -O2 ’ or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with ‘ -fschedule-insns ’ or
‘ -fschedule-insns2 ’ or at ‘ -O2 ’ or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.
174 Using the GNU Compiler Collection (GCC)
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective scheduling.
This option has no effect unless one of ‘ -fselective-scheduling ’ or
‘ -fselective-scheduling2 ’ is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline outer loops.
This option has no effect unless ‘ -fsel-sched-pipelining ’ is turned on.
-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by the dynamic linker. This means that for symbols exported from the DSO, the compiler cannot perform interprocedural propagation, inlining and other optimizations in anticipation that the function or variable in question may change. While this feature is useful, for example, to rewrite memory allocation functions by a debugging implementation, it is expensive in the terms of code quality. With
‘ -fno-semantic-interposition ’ the compiler assumes that if interposition happens for functions the overwriting function will have precisely the same semantics (and side effects). Similarly if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it is never allowed for interposition to change semantics) and for symbols explicitly declared weak.
-fshrink-wrap
Emit function prologues only before parts of the function that need it, rather than at the top of the function. This flag is enabled by default at ‘ -O ’ and higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue separately, so that those parts are only executed when needed. This option is on by default, but has no effect unless ‘ -fshrink-wrap ’ is also turned on and the target supports this.
-fcaller-saves
Enable allocation of values to registers that are clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls.
Such allocation is done only when it seems to result in better code.
This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to combine them.
Enabled by default at ‘ -O1 ’ and higher.
-fipa-ra Use caller save registers for allocation if those registers are not used by any called function. In that case it is not necessary to save and restore them around calls.
This is only possible if called functions are part of same compilation unit as current function and they are compiled before it.
Chapter 3: GCC Command Options 175
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’, however the option is disabled if generated code will be instrumented for profiling (‘ -p ’, or ‘ -pg ’) or if callee’s register usage cannot be known exactly (this happens on targets that do not expose prologues and epilogues in RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes the program slower. This option implies setting the
‘ large-stack-frame ’ parameter to 100 and the ‘ large-stack-frame-growth ’ parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at ‘ -O1 ’ and higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed as well.
This flag is enabled by default at ‘ -O2 ’ and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at ‘ -O2 ’ and ‘ -O3 ’.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at ‘ -O3 ’.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default at ‘ -O1 ’ and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference between
FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at ‘ -O1 ’ and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at ‘ -O1 ’ and higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at ‘ -O2 ’ and higher.
-ftree-copy-prop
Perform copy propagation on trees.
This pass eliminates unnecessary copy operations. This flag is enabled by default at ‘ -O1 ’ and higher.
176 Using the GNU Compiler Collection (GCC)
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default at ‘ -O1 ’ and higher.
-fipa-reference
Discover which static variables do not escape the compilation unit. Enabled by default at ‘ -O1 ’ and higher.
-fipa-reference-addressable
Discover read-only, write-only and non-addressable static variables. Enabled by default at ‘ -O1 ’ and higher.
-fipa-stack-alignment
Reduce stack alignment on call sites if possible. Enabled by default.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level.
-fipa-profile
Perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as cold , noreturn , static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at ‘ -O1 ’ and higher.
-fipa-modref
Perform interprocedural mod/ref analysis. This optimization analyzes the side effects of functions (memory locations that are modified or referenced) and enables better optimization across the function call boundary.
This flag is enabled by default at ‘ -O1 ’ and higher.
-fipa-cp Perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly.
This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at ‘ -O2 ’, ‘ -Os ’ and ‘ -O3 ’. It is also enabled by ‘ -fprofile-use ’ and
‘ -fauto-profile ’.
-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger.
When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments.
Because this optimization can create multiple copies of functions, it may significantly increase code size (see ‘ --param ipa-cp-unit-growth= value ’). This flag is enabled by default at ‘ -O3 ’. It is also enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
Chapter 3: GCC Command Options 177
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at ‘ -O2 ’ and by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
It requires that ‘ -fipa-cp ’ is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at ‘ -O2 ’. It requires that ‘ -fipa-cp ’ is enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-only variables.
The optimization reduces code size and may disturb unwind stacks by replacing a function by equivalent one with a different name. The optimization works more effectively with link-time optimization enabled.
Although the behavior is similar to the Gold Linker’s ICF optimization, GCC
ICF works on different levels and thus the optimizations are not same - there are equivalences that are found only by GCC and equivalences found only by
Gold.
This flag is enabled by default at ‘ -O2 ’ and ‘ -Os ’.
-flive-patching= level
Control GCC’s optimizations to produce output suitable for live-patching.
If the compiler’s optimization uses a function’s body or information extracted from its body to optimize/change another function, the latter is called an impacted function of the former. If a function is patched, its impacted functions should be patched too.
The impacted functions are determined by the compiler’s interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function’s pureness/constness information to optimize its direct or indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In order to control the number of impacted functions and more easily compute the list of impacted function, IPA optimizations can be partially enabled at two different levels.
The level argument should be one of the following:
‘ inline-clone ’
Only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. As a result, when patching a function, all its callers and its clones’ callers are impacted, therefore need to be patched as well.
‘ -flive-patching=inline-clone ’ disables the following optimization flags:
-fwhole-program -fipa-pta -fipa-reference -fipa-ra
-fipa-icf -fipa-icf-functions -fipa-icf-variables
-fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable
-fipa-stack-alignment -fipa-modref
178 Using the GNU Compiler Collection (GCC)
‘ inline-only-static ’
Only enable inlining of static functions. As a result, when patching a static function, all its callers are impacted and so need to be patched as well.
In addition to all the flags that ‘ -flive-patching=inline-clone ’ disables, ‘ -flive-patching=inline-only-static ’ disables the following additional optimization flags:
-fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When ‘ -flive-patching ’ is specified without any value, the default value is inline-clone .
This flag is disabled by default.
Note that ‘ -flive-patching ’ is not supported with link-time optimization
(‘ -flto ’).
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at ‘ -O2 ’ and higher and depends on ‘ -fdelete-null-pointer-checks ’ also being enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a returns_nonnull or nonnull attribute.
Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by ‘ -O2 ’ in the future.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at ‘ -O1 ’ and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only operates on local scalar variables and is enabled by default at ‘ -O1 ’ and higher, except for ‘ -Og ’. It requires that
‘ -ftree-ccp ’ is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at ‘ -O1 ’ and higher.
-fssa-backprop
Propagate information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. The flag is enabled by default at ‘ -O1 ’ and higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional code.
This pass is enabled by default at ‘ -O1 ’ and higher, except for ‘ -Og ’.
Chapter 3: GCC Command Options 179
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at ‘ -O2 ’ and higher.
-ftree-tail-merge
Look for identical code sequences.
When found, replace one with a jump to the other. This optimization is known as tail merging or cross jumping.
This flag is enabled by default at ‘ -O2 ’ and higher. The compilation time in this pass can be limited using ‘ max-tail-merge-comparisons ’ parameter and
‘ max-tail-merge-iterations ’ parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at ‘ -O1 ’ and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-in functions that may set errno but are otherwise free of side effects. This flag is enabled by default at ‘ -O2 ’ and higher if ‘ -Os ’ is not also specified.
-ffinite-loops
Assume that a loop with an exit will eventually take the exit and not loop indefinitely. This allows the compiler to remove loops that otherwise have no side-effects, not considering eventual endless looping as such.
This option is enabled by default at ‘ -O2 ’ for C ++ with -std=c ++ 11 or higher.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce jumps to jumps). This flag is enabled by default at ‘ -O1 ’ and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that is later overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This flag is enabled by default at ‘ -O1 ’ and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump. This flag is enabled by default at ‘ -O1 ’ and higher. It is not enabled for ‘ -Os ’, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at ‘ -O1 ’ and higher.
180 Using the GNU Compiler Collection (GCC)
-ftree-loop-linear
-floop-strip-mine
-floop-block
Perform loop nest optimizations. Same as ‘ -floop-nest-optimize ’. To use this code transformation, GCC has to be configured with ‘ --with-isl ’ to enable the Graphite loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple.
Using
‘ -fgraphite-identity ’ we can check the costs or benefits of the GIMPLE
> GRAPHITE > GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization algorithms.
It calculates a loop structure optimized for data-locality and parallelism. This option is experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA representation, attempt to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries. This may severely limit the ability to debug an optimized program compiled with ‘ -fno-var-tracking-assignments ’. In the negated form, this flag prevents SSA coalescing of user variables. This option is enabled by default if optimization is enabled, and it does very little otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops.
This is enabled by default if vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
Chapter 3: GCC Command Options 181
DO I = 1, N
D(I) = E(I) * F
ENDDO
This flag is enabled by default at ‘ -O3 ’. It is also enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with calls to a library.
This flag is enabled by default at ‘ -O2 ’ and higher, and by
‘ -fprofile-use ’ and ‘ -fauto-profile ’.
This pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO and the initialization loop is transformed into a call to memset zero. This flag is enabled by default at ‘ -O3 ’. It is also enabled by ‘ -fprofile-use ’ and
‘ -fauto-profile ’.
-floop-interchange
Perform loop interchange outside of graphite. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. For example, the loop for (int i = 0; i < N; i++) for (int j = 0; j < N; j++) for (int k = 0; k < N; k++) c[i][j] = c[i][j] + a[i][k]*b[k][j]; is transformed to for (int i = 0; i < N; i++) for (int k = 0; k < N; k++) for (int j = 0; j < N; j++) c[i][j] = c[i][j] + a[i][k]*b[k][j];
This flag is enabled by default at ‘ -O3 ’. It is also enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-floop-unroll-and-jam
Apply unroll and jam transformations on feasible loops. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops.
This flag is enabled by default at ‘ -O3 ’. It is also enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to non-
182 Using the GNU Compiler Collection (GCC) trivial sequences of insns). With ‘ -funswitch-loops ’ it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling.
-ftree-scev-cprop
Perform final value replacement. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, replace uses of the final value by such a computation, provided it is sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at ‘ -O1 ’ and higher.
-fivopts Perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies ‘ -pthread ’, and thus is only supported on targets that have support for ‘ -pthread ’.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is enabled by default at ‘ -O1 ’ and higher, except for ‘ -Og ’.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at ‘ -O1 ’ and higher, except for ‘ -Og ’.
-fstore-merging
Perform merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores to reduce the number of instructions. This is enabled by default at ‘ -O2 ’ and higher as well as ‘ -Os ’.
-ftree-ter
Perform temporary expression replacement during the SSA> normal phase. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at ‘ -O1 ’ and higher.
Chapter 3: GCC Command Options 183
-ftree-slsr
Perform straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. This is enabled by default at ‘ -O1 ’ and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables ‘ -ftree-loop-vectorize ’ and ‘ -ftree-slp-vectorize ’ if not explicitly specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by default at ‘ -O2 ’ and by ‘ -ftree-vectorize ’, ‘ -fprofile-use ’, and ‘ -fauto-profile ’.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at
‘ -O2 ’ and by ‘ -ftree-vectorize ’, ‘ -fprofile-use ’, and ‘ -fauto-profile ’.
-ftrivial-auto-var-init= choice
Initialize automatic variables with either a pattern or with zeroes to increase the security and predictability of a program by preventing uninitialized memory disclosure and use. GCC still considers an automatic variable that doesn’t have an explicit initializer as uninitialized, ‘ -Wuninitialized ’ will still report warning messages on such automatic variables. With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes.
The three values of choice are:
• ‘ uninitialized ’ doesn’t initialize any automatic variables. This is C and
C ++ ’s default.
• ‘ pattern ’ Initialize automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump and without being values that programmers can rely on for useful program semantics. The current value is byte-repeatable pattern with byte
" 0xFE " . The values used for pattern initialization might be changed in the future.
• ‘ zero ’ Initialize automatic variables with zeroes.
The default is ‘ uninitialized ’.
You can control this behavior for a specific variable by using the variable attribute uninitialized (see
Section 6.34 [Variable Attributes], page 614 ).
-fvect-cost-model= model
Alter the cost model used for vectorization. The model argument should be one of ‘ unlimited ’, ‘ dynamic ’, ‘ cheap ’ or ‘ very-cheap ’. With the ‘ unlimited ’ model the vectorized code-path is assumed to be profitable while with the
‘ dynamic ’ model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. The ‘ cheap ’ model disables vectorization of loops where doing so would be cost prohibitive for example due to required runtime checks for data dependence or alignment but otherwise is equal to the ‘ dynamic ’ model.
184 Using the GNU Compiler Collection (GCC)
The ‘ very-cheap ’ model only allows vectorization if the vector code would entirely replace the scalar code that is being vectorized. For example, if each iteration of a vectorized loop would only be able to handle exactly four iterations of the scalar loop, the ‘ very-cheap ’ model would only allow vectorization if the scalar iteration count is known to be a multiple of four.
The default cost model depends on other optimization flags and is either
‘ dynamic ’ or ‘ cheap ’.
-fsimd-cost-model= model
Alter the cost model used for vectorization of loops marked with the
OpenMP simd directive. The model argument should be one of ‘ unlimited ’,
‘ dynamic ’, ‘ cheap ’.
All values of model have the same meaning as described in ‘ -fvect-cost-model ’ and by default a cost model defined with
‘ -fvect-cost-model ’ is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at ‘ -O2 ’ and higher. Null pointer check elimination is only done if ‘ -fdelete-null-pointer-checks ’ is enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. This is enabled by default at ‘ -O3 ’ and above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes.
A combination of ‘ -fweb ’ and CSE is often sufficient to obtain the same effect.
However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code.
This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.
-fpartial-inlining
Inline parts of functions. This option has any effect only when inlining itself is turned on by the ‘ -finline-functions ’ or ‘ -finline-small-functions ’ options.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
Chapter 3: GCC Command Options 185
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops.
This option is enabled at level ‘ -O3 ’. It is also enabled by ‘ -fprofile-use ’ and
‘ -fauto-profile ’.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.
This option may generate better or worse code; results are highly dependent on the structure of loops within the source code.
Disabled at level ‘ -Os ’.
-fno-printf-return-value
Do not substitute constants for known return value of formatted output functions such as sprintf , snprintf , vsprintf , and vsnprintf (but not printf of fprintf ). This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when
‘ -fprintf-return-value ’ is in effect, both the branch and the body of the if statement (but not the call to snprint ) can be optimized away when i is a
32-bit or smaller integer because the return value is guaranteed to be at most
8.
char buf[9]; if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The ‘ -fprintf-return-value ’ option relies on other optimizations and yields best results with ‘ -O2 ’ and above.
It works in tandem with the ‘ -Wformat-overflow ’ and ‘ -Wformat-truncation
‘ -fprintf-return-value ’ option is enabled by default.
’ options.
The
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between
‘ -fno-peephole ’ and ‘ -fno-peephole2 ’ is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.
‘ -fpeephole ’ is enabled by default.
‘ -fpeephole2 ’ enabled at levels ‘ -O2 ’,
‘ -O3 ’, ‘ -Os ’.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (‘ -fprofile-arcs ’). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_ expect , then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex,
186 Using the GNU Compiler Collection (GCC) and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand.
It is also possible to specify expected probability of the expression with __ builtin_expect_with_probability built-in function.
The default is ‘ -fguess-branch-probability ’ at levels ‘ -O ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-freorder-blocks-algorithm= algorithm
Use the specified algorithm for basic block reordering. The algorithm argument can be ‘ simple ’, which does not increase code size (except sometimes due to secondary effects like alignment), or ‘ stc ’, the “software trace cache” algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.
The default is ‘ simple ’ at levels ‘ -O1 ’, ‘ -Os ’, and ‘ stc ’ at levels ‘ -O2 ’, ‘ -O3 ’.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and ‘ .o
’ files, to improve paging and cache locality performance.
This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections. When
‘ -fsplit-stack ’ is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker).
Enabled for x86 at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-freorder-functions
Reorder functions in the object file in order to improve code locality. This is implemented by using special subsections .text.hot
for most frequently executed functions and .text.unlikely
for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way.
This option isn’t effective unless you either provide profile feedback (see
‘ -fprofile-arcs ’ for details) or manually annotate functions with hot or cold attributes (see
Section 6.33.1 [Common Function Attributes], page 549
).
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C ++ ), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types
Chapter 3: GCC Command Options 187 are almost the same. For example, an unsigned int can alias an int , but not a void* or a double . A character type may alias any other type.
Pay special attention to code like this: union a_union { int i;
}; double d; int f() { union a_union t; t.d = 3.0; return t.i;
}
The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common.
Even with
‘ -fstrict-aliasing ’, type-punning is allowed, provided the memory is accessed through the union type. So, the code above works as expected. See
Section 4.9 [Structures unions enumerations and bit-fields implementation], page 513
. However, this code might not: int f() { union a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip;
}
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.: int f() { double d = 3.0; return ((union a_union *) &d)->i;
}
The ‘ -fstrict-aliasing ’ option is enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fipa-strict-aliasing
Controls whether rules of ‘ -fstrict-aliasing ’ are applied across function boundaries.
Note that if multiple functions gets inlined into a single function the memory accesses are no longer considered to be crossing a function boundary.
The ‘ -fipa-strict-aliasing ’ option is enabled by default and is effective only in combination with ‘ -fstrict-aliasing ’.
-falign-functions
-falign-functions= n
-falign-functions= n : m
-falign-functions= n : m : n2
-falign-functions= n : m : n2 : m2
Align the start of functions to the next power-of-two greater than or equal to n , skipping up to m -1 bytes. This ensures that at least the first m bytes of
188 Using the GNU Compiler Collection (GCC) the function can be fetched by the CPU without crossing an n -byte alignment boundary.
If m is not specified, it defaults to n .
Examples: ‘ -falign-functions=32 ’ aligns functions to the next 32-byte boundary, ‘ -falign-functions=24 ’ aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, ‘ -falign-functions=32:7 ’ aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.
The second pair of n2 : m2 values allows you to specify a secondary alignment:
‘ -falign-functions=64:7:32:3 ’ aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2 .
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
‘ -fno-align-functions ’ and ‘ -falign-functions=1 ’ are equivalent and mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by ‘ -falign-functions ’, but not to skip more bytes than the size of the function.
-falign-labels
-falign-labels= n
-falign-labels= n : m
-falign-labels= n : m : n2
-falign-labels= n : m : n2 : m2
Align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the ‘ -falign-functions ’ option.
‘ -fno-align-labels ’ and ‘ -falign-labels=1 ’ are equivalent and mean that labels are not aligned.
If ‘ -falign-loops ’ or ‘ -falign-jumps ’ are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be ‘ 1 ’, meaning no alignment. The maximum allowed n option value is 65536.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’.
Chapter 3: GCC Command Options 189
-falign-loops
-falign-loops= n
-falign-loops= n : m
-falign-loops= n : m : n2
-falign-loops= n : m : n2 : m2
Align loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions.
If ‘ -falign-labels ’ is greater than this value, then its value is used instead.
Parameters of this option are analogous to the ‘ -falign-functions ’ option.
‘ -fno-align-loops ’ and ‘ -falign-loops=1 ’ are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’.
-falign-jumps
-falign-jumps= n
-falign-jumps= n : m
-falign-jumps= n : m : n2
-falign-jumps= n : m : n2 : m2
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping. In this case, no dummy operations need be executed.
If ‘ -falign-labels ’ is greater than this value, then its value is used instead.
Parameters of this option are analogous to the ‘ -falign-functions ’ option.
‘ -fno-align-jumps ’ and ‘ -falign-jumps=1 ’ are equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’.
-fno-allocation-dce
Do not remove unused C ++ allocations in dead code elimination.
-fallow-store-data-races
Allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable cannot be concurrently accessed by other threads. Does not affect optimization of local data. It is safe to use this option if it is known that global data will not be accessed by multiple threads.
Examples of optimizations enabled by ‘ -fallow-store-data-races ’ include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization.
Enabled at level ‘ -Ofast ’.
190 Using the GNU Compiler Collection (GCC)
-funit-at-a-time
This option is left for compatibility reasons.
‘ -funit-at-a-time ’ has no effect, while ‘ -fno-unit-at-a-time ’ implies ‘ -fno-toplevel-reorder ’ and
‘ -fno-section-anchors ’.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and asm statements. Output them in the same order that they appear in the input file.
When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible.
‘ -ftoplevel-reorder ’ is the default at ‘ -O1 ’ and higher, and also at ‘ -O0 ’ if ‘ -fsection-anchors ’ is explicitly requested.
Additionally
‘ -fno-toplevel-reorder ’ implies ‘ -fno-section-anchors ’.
-fweb Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a “home register”.
Enabled by default with ‘ -funroll-loops ’.
-fwhole-program
Assume that the current compilation unit represents the whole program being compiled. All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with ‘ -flto ’. Instead relying on a linker plugin should provide safer and more precise information.
-flto[= n ]
This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC’s internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit.
To use the link-time optimizer, ‘ -flto ’ and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example: gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside ‘ foo.o
’ and ‘ bar.o
’. The final invocation reads the GIMPLE bytecode from ‘ foo.o
’ and ‘ bar.o
’, merges the two files into a
Chapter 3: GCC Command Options 191 single internal image, and compiles the result as usual. Since both ‘ foo.o
’ and ‘ bar.o
’ are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in ‘ bar.o
’ into functions in ‘ foo.o
’ and vice-versa.
Another (simpler) way to enable link-time optimization is: gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for ‘ foo.c
’ and ‘ bar.c
’, merges them together into a single GIMPLE representation and optimizes them as usual to produce
‘ myprog ’.
The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the ‘ -flto ’ command-line option. You can always override the automatic decision to do link-time optimization by passing ‘ -fno-lto ’ to the link command.
To make whole program optimization effective, it is necessary to make certain whole program assumptions.
The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit.
When supported by the linker, the linker plugin (see ‘ -fuse-linker-plugin ’) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available,
‘ -fwhole-program ’ should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.
When a file is compiled with ‘ -flto ’ without ‘ -fuse-linker-plugin ’, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see ‘ -ffat-lto-objects ’). This means that object files with LTO information can be linked as normal object files; if
‘ -fno-lto ’ is passed to the linker, no interprocedural optimizations are applied.
Note that when ‘ -fno-fat-lto-objects ’ is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them.
When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.
Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.
If you do not specify an optimization level option ‘ -O ’ at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for
192 Using the GNU Compiler Collection (GCC) effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: ‘ -fcommon ’, ‘ -fexceptions ’, ‘ -fnon-call-exceptions ’, ‘ -fgnu-tm ’ and all the ‘ -m ’ target flags.
The following options ‘ -fPIC ’, ‘ -fpic ’, ‘ -fpie ’ and ‘ -fPIE ’ are combined based on the following scheme:
‘-fPIC’ + ‘-fpic’ = ‘-fpic’
‘-fPIC’ + ‘-fno-pic’ = ‘-fno-pic’
‘-fpic/-fPIC’ + (no option) = (no option)
‘-fPIC’ + ‘-fPIE’ = ‘-fPIE’
‘-fpic’ + ‘-fPIE’ = ‘-fpie’
‘-fPIC/-fpic’ + ‘-fpie’ = ‘-fpie’
Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as ‘ -freg-struct-return ’ and ‘ -fpcc-struct-return ’.
Other options such as ‘ -ffp-contract ’, ‘ -fno-strict-overflow ’, ‘ -fwrapv ’,
‘ -fno-trapv ’ or ‘ -fno-strict-aliasing ’ are passed through to the link stage and merged conservatively for conflicting translation units.
Specifically ‘ -fno-strict-overflow ’, ‘ -fwrapv ’ and ‘ -fno-trapv ’ take precedence; and for example ‘ -ffp-contract=off ’ takes precedence over
‘ -ffp-contract=fast ’. You can override them at link time.
Diagnostic options such as ‘ -Wstringop-overflow ’ are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization.
Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions if code not consistent with the setting at compile time.
When you need to pass options to the assembler via ‘ -Wa ’ or ‘ -Xassembler ’ make sure to either compile such translation units with ‘ -fno-lto ’ or consistently use the same assembler options on all translation units.
You can alternatively also specify assembler options at LTO link time.
To enable debug info generation you need to supply ‘ -g ’ at compile time. If any of the input files at link time were built with debug info generation enabled the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level ‘ -gdwarf-5 ’ need to be explicitly repeated at the linker command line and mixing different settings in different translation units is discouraged.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages: gcc -c -flto foo.c
Chapter 3: GCC Command Options 193 g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C ++ runtime libraries and
‘ -lgfortran ’ is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say
‘ libfoo.a
’, it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib ; to show the symbols of object files with GIMPLE bytecode, use gcc-nm . Those commands require that ar , ranlib and nm have been compiled with plugin support. At link time, use the flag ‘ -fuse-linker-plugin ’ to ensure that the library participates in the LTO optimization process: gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files from ‘ libfoo.a
’ and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside ‘ libfoo.a
’ are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with ‘ -flto ’ ‘ -ffat-lto-objects ’.
Link-time optimizations do not require the presence of the whole program to operate. If the program does not require any symbols to be exported, it is possible to combine ‘ -flto ’ and ‘ -fwhole-program ’ to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of ‘ -fwhole-program ’ is not needed when linker plugin is active (see ‘ -fuse-linker-plugin ’).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.
If you specify the optional n , the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used.
You can also specify ‘ -flto=jobserver ’ to use GNU make’s job server mode to determine the number of parallel jobs. This is useful when the Makefile calling
GCC is already executing in parallel. You must prepend a ‘ + ’ to the command recipe in the parent Makefile for this to work. This option likely only works if
MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make’s job server.
194 Using the GNU Compiler Collection (GCC)
Use ‘ -flto=auto ’ to use GNU make’s job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system.
-flto-partition= alg
Specify the partitioning algorithm used by the link-time optimizer. The value is either ‘ 1to1 ’ to specify a partitioning mirroring the original source files or
‘ balanced ’ to specify partitioning into equally sized chunks (whenever possible) or ‘ max ’ to create new partition for every symbol where possible. Specifying
‘ none ’ as an algorithm disables partitioning and streaming completely. The default value is ‘ balanced ’. While ‘ 1to1 ’ can be used as an workaround for various code ordering issues, the ‘ max ’ partitioning is intended for internal testing only.
The value ‘ one ’ specifies that exactly one partition should be used while the value ‘ none ’ bypasses partitioning and executes the link-time optimization step directly from the WPA phase.
-flto-compression-level= n
This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (‘ -flto ’). GCC currently supports two LTO compression algorithms.
For zstd, valid values are 0 (no compression) to 19 (maximum compression), while zlib supports values from 0 to 9. Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld
2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to ‘ -fwhole-program ’. See ‘ -flto ’ for a description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and
GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with ‘ -flto ’ and is ignored at link time.
‘ -fno-fat-lto-objects ’ improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm , ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar , gcc-nm , gcc-ranlib
Chapter 3: GCC Command Options 195 wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into ‘ $libdir/bfd-plugins ’ has the same effect as usage of the command wrappers ( gcc-ar , gcc-nm and gcc-ranlib ).
The default is ‘ -fno-fat-lto-objects ’ on targets with linker plugin support.
-fcompare-elim
After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fcprop-registers
After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy.
Enabled at levels ‘ -O1 ’, ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.
This option is enabled by ‘ -fauto-profile ’.
-fprofile-partial-training
With -fprofile-use all portions of programs not executed during train run are optimized agressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, program may contain functions specific for a given hardware and trianing may not cover all hardware configurations program is run on.) With -fprofile-partialtraining profile feedback will be ignored for all functions not executed during the train run leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when train run is not representative but also leads to significantly bigger code.
-fprofile-use
-fprofile-use= path
Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values
-funroll-loops -fpeel-loops -ftracer -fvpt
-finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp
-fpredictive-commoning -fsplit-loops -funswitch-loops
-fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize
196 Using the GNU Compiler Collection (GCC)
-fvect-cost-model=dynamic -ftree-loop-distribute-patterns
-fprofile-reorder-functions
Before you can use this option, you must first generate profiling information.
See
Section 3.12 [Instrumentation Options], page 232
, for information about the ‘ -fprofile-generate ’ option.
By default, GCC emits an error message if the feedback profiles do not match the source code.
This error can be turned into a warning by using
‘ -Wno-error=coverage-mismatch ’. Note this may result in poorly optimized code.
Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see ‘ -Wmissing-profile ’).
If path is specified, GCC looks at the path to find the profile feedback data files. See ‘ -fprofile-dir ’.
-fauto-profile
-fauto-profile= path
Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values
-funroll-loops -fpeel-loops -ftracer -fvpt
-finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp
-fpredictive-commoning -fsplit-loops -funswitch-loops
-fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize
-fvect-cost-model=dynamic -ftree-loop-distribute-patterns
-fprofile-correction path is the name of a file containing AutoFDO profile information. If omitted, it defaults to ‘ fbdata.afdo
’ in the current directory.
Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see https://perf.wiki.kernel.org/ .
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See https://github.com/google/autofdo .
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-point arithmetic.
These options trade off between speed and correctness. All must be specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double
Chapter 3: GCC Command Options 197 is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use ‘ -ffloat-store ’ for such programs, after modifying them to store all pertinent intermediate computations into variables.
-fexcess-precision= style
This option allows further control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types.
By default,
‘ -fexcess-precision=fast ’ is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C, if
‘ -fexcess-precision=standard ’ is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas ‘ -ffloat-store ’ only affects assignments). This option is enabled by default for C if a strict conformance option such as ‘ -std=c99 ’ is used.
‘ -ffast-math ’ enables
‘ -fexcess-precision=fast ’ by default regardless of whether a strict conformance option is used.
‘ -fexcess-precision=standard ’ is not implemented for languages other than
C. On the x86, it has no effect if ‘ -mfpmath=sse ’ or ‘ -mfpmath=sse+387 ’ is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable.
-ffast-math
Sets the options ‘ -fno-math-errno ’, ‘ -funsafe-math-optimizations ’,
‘ -ffinite-math-only ’, ‘ -fno-rounding-math ’, ‘ -fno-signaling-nans ’,
‘ -fcx-limited-range ’ and ‘ -fexcess-precision=fast ’.
This option causes the preprocessor macro __FAST_MATH__ to be defined.
This option is not turned on by any ‘ -O ’ option besides ‘ -Ofast ’ since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
-fno-math-errno
Do not set errno after calling math functions that are executed with a single instruction, e.g., sqrt . A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any ‘ -O ’ option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is ‘ -fmath-errno ’.
198 Using the GNU Compiler Collection (GCC)
On Darwin systems, the math library never sets errno . There is therefore no reason for the compiler to consider the possibility that it might, and
‘ -fno-math-errno ’ is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards.
When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.
This option is not turned on by any ‘ -O ’ option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
Enables ‘ -fno-signed-zeros ’, ‘ -fno-trapping-math ’, ‘ -fassociative-math ’ and ‘ -freciprocal-math ’.
The default is ‘ -fno-unsafe-math-optimizations ’.
-fassociative-math
Allow re-association of operands in series of floating-point operations. This violates the ISO C and C ++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52 . May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both ‘ -fno-signed-zeros ’ and
‘ -fno-trapping-math ’ be in effect. Moreover, it doesn’t make much sense with
‘ -frounding-math ’. For Fortran the option is automatically enabled when both
‘ -fno-signed-zeros ’ and ‘ -fno-trapping-math ’ are in effect.
The default is ‘ -fno-associative-math ’.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example x / y can be replaced with x * (1/y) , which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value.
The default is ‘ -fno-reciprocal-math ’.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or + -Infs.
This option is not turned on by any ‘ -O ’ option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.
The default is ‘ -fno-finite-math-only ’.
Chapter 3: GCC Command Options 199
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct + 0.0 and − 0.0 values, which then prohibits simplification of expressions such as x + 0.0 or 0.0*x (even with ‘ -ffinite-math-only ’). This option implies that the sign of a zero result isn’t significant.
The default is ‘ -fsigned-zeros ’.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate uservisible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that ‘ -fno-signaling-nans ’ be in effect. Setting this option may allow faster code if one relies on “non-stop”
IEEE arithmetic, for example.
This option should never be turned on by any ‘ -O ’ option since it can result in incorrect output for programs that depend on an exact implementation of
IEEE or ISO rules/specifications for math functions.
The default is ‘ -ftrapping-math ’.
-frounding-math
Disable transformations and optimizations that assume default floating-point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.
The default is ‘ -fno-rounding-math ’.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99’s FENV_ACCESS pragma. This command-line option will be used to specify the default state for FENV_ACCESS .
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs.
This option implies ‘ -ftrapping-math ’.
This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.
The default is ‘ -fno-signaling-nans ’.
This option is experimental and does not currently guarantee to disable all
GCC optimizations that affect signaling NaN behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions ceil , floor , round and trunc , and their float and long double variants, to generate code that raises the “inexact”
200 Using the GNU Compiler Collection (GCC) floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the “inexact” exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C2X, does not allow these functions to do so.
The default is ‘ -ffp-int-builtin-inexact ’, allowing the exception to be raised, unless C2X or a later C standard is selected. This option does nothing unless ‘ -ftrapping-math ’ is in effect.
Even if ‘ -fno-fp-int-builtin-inexact ’ is used, if the functions generate a call to a library function then the “inexact” exception may be raised if the library implementation does not follow TS 18661.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN , with an attempt to rescue the situation in that case. The default is ‘ -fno-cx-limited-range ’, but is enabled by ‘ -ffast-math ’.
This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN , with an attempt to rescue the situation in that case.
The default is ‘ -fno-cx-fortran-rules ’.
The following options control optimizations that may improve performance, but are not enabled by any ‘ -O ’ options. This section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with ‘ -fprofile-arcs ’ (see
[Instrumentation Options], page 232
), you can compile it a second time using ‘ -fbranch-probabilities ’, to improve optimizations based on the number of times each branch was taken.
When a program compiled with ‘ -fprofile-arcs ’ exits, it saves arc execution counts to a file called
‘ sourcename .gcda
’ for each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations.
With ‘ -fbranch-probabilities ’, GCC puts a ‘ REG_BR_PROB ’ note on each
‘ JUMP_INSN ’ and ‘ CALL_INSN ’. These can be used to improve optimization.
Currently, they are only used in one place: in ‘ reorg.c
’, instead of guessing which path a branch is most likely to take, the ‘ REG_BR_PROB ’ values are used to exactly determine which path is taken more often.
Chapter 3: GCC Command Options 201
Enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-fprofile-values
If combined with ‘ -fprofile-arcs ’, it adds code so that some data about values of expressions in the program is gathered.
With ‘ -fbranch-probabilities ’, it reads back the data gathered from profiling values of expressions for usage in optimizations.
Enabled by ‘ -fprofile-generate ’, ‘ -fprofile-use ’, and ‘ -fauto-profile ’.
-fprofile-reorder-functions
Function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order.
Enabled with ‘ -fprofile-use ’.
-fvpt If combined with ‘ -fprofile-arcs ’, this option instructs the compiler to add code to gather information about values of expressions.
With ‘ -fbranch-probabilities ’, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator.
Enabled with ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a “home register”.
Enabled by default with ‘ -funroll-loops ’.
-fschedule-fusion
Performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow.
Enabled at levels ‘ -O2 ’, ‘ -O3 ’, ‘ -Os ’.
-ftracer Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job.
Enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. ‘ -funroll-loops ’ implies ‘ -frerun-cse-after-loop ’,
‘ -fweb ’ and ‘ -frename-registers ’. It also turns on complete loop peeling (i.e.
complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster.
Enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
202 Using the GNU Compiler Collection (GCC)
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. ‘ -funroll-all-loops ’ implies the same options as ‘ -funroll-loops ’.
-fpeel-loops
Peels loops for which there is enough information that they do not roll much
(from profile feedback or static analysis). It also turns on complete loop peeling
(i.e. complete removal of loops with small constant number of iterations).
Enabled by ‘ -O3 ’, ‘ -fprofile-use ’, and ‘ -fauto-profile ’.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level ‘ -O1 ’ and higher, except for ‘ -Og ’.
-fmove-loop-stores
Enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop in exchange for carrying the stored value in a register across the iteration. Note for this option to have an effect
‘ -ftree-loop-im ’ has to be enabled as well. Enabled at level ‘ -O1 ’ and higher, except for ‘ -Og ’.
-fsplit-loops
Split a loop into two if it contains a condition that’s always true for one side of the iteration space and false for the other.
Enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).
Enabled by ‘ -fprofile-use ’ and ‘ -fauto-profile ’.
-fversion-loops-for-strides
If a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one. For example: for (int i = 0; i < n; ++i) x[i * stride] = ...; becomes: if (stride == 1) for (int i = 0; i < n; ++i) x[i] = ...; else for (int i = 0; i < n; ++i) x[i * stride] = ...;
This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at ‘ -O3 ’.
It is also enabled by ‘ -fprofile-use ’ and
‘ -fauto-profile ’.
Chapter 3: GCC Command Options 203
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section’s name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the
ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.
Together with a linker garbage collection (linker ‘ --gc-sections ’ option) these options may lead to smaller statically-linked executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.
Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.
-fstdarg-opt
Optimize the prologue of variadic argument functions with respect to usage of those arguments.
-fsection-anchors
Try to reduce the number of symbolic address calculations by using shared
“anchor” symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.
For example, the implementation of the following function foo : static int a, b, c; int foo (void) { return a + b + c; } usually calculates the addresses of all three variables, but if you compile it with
‘ -fsection-anchors ’, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn’t valid C): int foo (void)
{ register int *xr = &x; return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
-fzero-call-used-regs= choice
Zero call-used registers at function return to increase program security by either mitigating Return-Oriented Programming (ROP) attacks or preventing information leakage through registers.
204 Using the GNU Compiler Collection (GCC)
The possible values of choice are the same as for the zero_call_used_regs attribute (see
Section 6.33 [Function Attributes], page 548 ). The default is
‘ skip ’.
You can control this behavior for a specific function by using the function attribute zero_call_used_regs (see
Section 6.33 [Function Attributes], page 548
).
--param name = value
In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the ‘ --param ’ option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.
In order to get minimal, maximal and default value of a parameter, one can use
‘ --help=param -Q ’ options.
In each case, the value is an integer. The following choices of name are recognized for all targets: predictable-branch-outcome
When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions.
This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.
max-rtl-if-conversion-predictable-cost
RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions.
These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the
GCC internal seq cost metric. The compiler will try to provide a reasonable default for this parameter using the BRANCH COST target macro.
max-crossjump-edges
The maximum number of incoming edges to consider for crossjumping. The algorithm used by ‘ -fcrossjumping ’ is O ( N 2 ) in the number of edges incoming to each block.
Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size.
Chapter 3: GCC Command Options 205 min-crossjump-insns
The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O ( N 2 ) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-gotoduplication-insns are unfactored.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.
max-pending-list-length
The maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.
206 Using the GNU Compiler Collection (GCC) max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time.
max-inline-functions-called-once-loop-depth
Maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once.
max-inline-functions-called-once-insns
Maximal estimated size of functions produced while inlining functions called once.
max-inline-insns-single
Several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC’s internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C ++ ).
max-inline-insns-auto
When you use ‘ -finline-functions ’ (included in ‘ -O3 ’), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied
(‘ --param max-inline-insns-auto ’).
max-inline-insns-small
This is bound applied to calls which are considered relevant with
‘ -finline-small-functions ’.
max-inline-insns-size
This is bound applied to calls which are optimized for size. Small growth may be desirable to anticipate optimization oppurtunities exposed by inlining.
uninlined-function-insns
Number of instructions accounted by inliner for function overhead such as function prologue and epilogue.
uninlined-function-time
Extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue.
inline-heuristics-hint-percent
The scale (in percents) applied to ‘ inline-insns-single ’,
‘ inline-insns-single-O2 ’, ‘ inline-insns-auto ’ when inline heuristics hints that inlining is very profitable (will enable later optimizations).
Chapter 3: GCC Command Options 207 uninlined-thunk-insns uninlined-thunk-time
Same as ‘ --param uninlined-function-insns ’ and ‘ --param uninlined-function-time ’ but applied to function thunks.
inline-min-speedup
When estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on ‘ --param max-inline-insns-single ’ and ‘ --param max-inline-insns-auto ’.
large-function-insns
The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by ‘ --param large-function-growth ’. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end.
large-function-growth
Specifies maximal growth of large function caused by inlining in percents. For example, parameter value 100 limits large function growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit.
Growth caused by inlining of units larger than this limit is limited by ‘ --param inline-unit-growth ’. For small units this might be too tight.
For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is 300 \ % and yet such inlining is very sane.
For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size.
Thus for smaller units, the size is increased to ‘ --param large-unit-insns ’ before applying
‘ --param inline-unit-growth ’.
lazy-modules
Maximum number of concurrently open C ++ module files when lazy loading.
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.2
times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size.
ipa-cp-unit-growth
Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value 10 limits unit growth to 1.1 times the original size.
208 Using the GNU Compiler Collection (GCC) ipa-cp-large-unit-insns
The size of translation unit that IPA-CP pass considers large.
large-stack-frame
The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value 1000 limits large stack frame growth to 11 times the original size.
max-inline-insns-recursive max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining.
‘ --param max-inline-insns-recursive ’ applies to functions declared inline.
For functions not declared inline, recursive inlining happens only when ‘ -finline-functions ’ (included in ‘ -O3 ’) is enabled; ‘ --param max-inline-insns-recursive-auto ’ applies instead.
max-inline-recursive-depth max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive inlining.
‘ --param max-inline-recursive-depth ’ applies to functions declared inline.
For functions not declared inline, recursive inlining happens only when ‘ -finline-functions ’ (included in ‘ -O3 ’) is enabled; ‘ --param max-inline-recursive-depth-auto ’ applies instead.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers.
When profile feedback is available (see ‘ -fprofile-generate ’) the actual recursion depth can be guessed from the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents).
early-inlining-insns
Specify growth that the early inliner can make. In effect it increases the amount of inlining for code having a large abstraction penalty.
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining.
Chapter 3: GCC Command Options 209 comdat-sharing-probability
Probability (in percent) that C ++ inline function with comdat visibility are shared across multiple compilation units.
modref-max-bases modref-max-refs modref-max-accesses
Specifies the maximal number of base pointers, references and accesses stored for a single function by mod/ref analysis.
modref-max-tests
Specifies the maxmal number of tests alias oracle can perform to disambiguate memory locations using the mod/ref information. This parameter ought to be bigger than ‘ --param modref-max-bases ’ and ‘ --param modref-max-refs ’.
modref-max-depth
Specifies the maximum depth of DFS walk used by modref escape analysis. Setting to 0 disables the analysis completely.
modref-max-escape-points
Specifies the maximum number of escape points tracked by modref per SSA-name.
modref-max-adjustments
Specifies the maximum number the access range is enlarged during modref dataflow analysis.
profile-func-internal-id
A parameter to control whether to use function internal id in profile database lookup. If the value is 0, the compiler uses an id that is based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc.
min-vect-loop-bound
The minimum number of iterations under which loops are not vectorized when ‘ -ftree-vectorize ’ is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e., the expressions that have cost less than ‘ gcse-unrestricted-cost ’. Specifying 0 disables hoisting of simple expressions.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only
210 Using the GNU Compiler Collection (GCC) in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances.
max-hoist-depth
The depth of search in the dominator tree for expressions to hoist.
This is used to avoid quadratic behavior in hoisting algorithm. The value of 0 does not limit on the search, but may slow down compilation of huge functions.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function.
This is used to limit compilation time in tree tail merging.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if it is legal to do so.
max-stores-to-merge
The maximum number of stores to attempt to merge into wider stores in the store merging pass.
max-store-chains-to-track
The maximum number of store chains to track at the same time in the attempt to merge them into wider stores in the store merging pass.
max-stores-to-track
The maximum number of stores to track at the same time in the attemt to to merge them into wider stores in the store merging pass.
max-unrolled-insns
The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled.
Chapter 3: GCC Command Options 211 max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through the peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.
min-loop-cond-split-prob
When FDO profile information is available, ‘ min-loop-cond-split-prob ’ specifies minimum threshold for probability of semi-invariant condition statement to trigger loop split.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations.
If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that contain more induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times.
212 Using the GNU Compiler Collection (GCC) dse-max-alias-queries-per-store
Maximum number of queries into the alias oracle per store. Larger values result in larger compilation times and may result in more removed dead stores.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer.
Large expressions slow the analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked with simd pragma.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means no limit.
max-iterations-to-track
The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.
hot-bb-count-fraction
The denominator n of fraction 1/n of the maximal execution count of a basic block in the entire program that a basic block needs to at least have in order to be considered hot. The default is 10000, which means that a basic block is considered hot if its execution count is greater than 1/10000 of the maximal execution count. 0 means that it is never considered hot. Used in non-LTO mode.
hot-bb-count-ws-permille
The number of most executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program to which the execution count of a basic block must be part of in order to be considered hot.
The default is 990, which means that a basic block is considered hot if its execution count contributes to the upper 990 permilles, or 99.0%, of the profiled execution of the entire program. 0 means that it is never considered hot. Used in LTO mode.
Chapter 3: GCC Command Options 213 hot-bb-frequency-fraction
The denominator n of fraction 1/n of the execution frequency of the entry block of a function that a basic block of this function needs to at least have in order to be considered hot. The default is 1000, which means that a basic block is considered hot in a function if it is executed more frequently than 1/1000 of the frequency of the entry block of the function. 0 means that it is never considered hot.
unlikely-bb-count-fraction
The denominator n of fraction 1/n of the number of profiled runs of the entire program below which the execution count of a basic block must be in order for the basic block to be considered unlikely executed. The default is 20, which means that a basic block is considered unlikely executed if it is executed in fewer than 1/20, or
5%, of the runs of the program. 0 means that it is always considered unlikely executed.
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds appears artificially cold relative to the other one.
builtin-expect-probability
Control the probability of the expression having the specified value.
This parameter takes a percentage (i.e. 0 ... 100) as input.
builtin-string-cmp-inline-length
The maximum length of a constant string for a builtin string cmp call eligible for inlining.
align-threshold
Select fraction of the maximal frequency of executions of a basic block in a function to align the basic block.
align-loop-iterations
A loop expected to iterate at least the selected number of iterations is aligned.
tracer-dynamic-coverage tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.
The ‘ tracer-dynamic-coverage-feedback ’ parameter is used only when profile feedback is available.
The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value.
214 Using the GNU Compiler Collection (GCC) tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
tracer-min-branch-probability tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower than this threshold.
Similarly to ‘ tracer-dynamic-coverage ’ two parameters are provided.
‘ tracer-min-branch-probability-feedback ’ is used for compilation with profile feedback and
‘ tracer-min-branch-probability ’ compilation without.
The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.
stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks.
stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is allocated.
This param controls the maximum distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks.
max-cse-path-length
The maximum number of basic blocks on path that CSE considers.
max-cse-insns
The maximum number of instructions CSE processes before flushing.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector’s heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM > = 1GB. If getrlimit is available, the notion of “RAM” is the smallest of actual RAM and RLIMIT_DATA or
Chapter 3: GCC Command Options 215
RLIMIT_AS . If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ‘ ggc-min-heapsize ’ to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector’s heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by ‘ ggc-min-expand ’% beyond ‘ ggc-min-heapsize ’.
Again, tuning this may improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT RSS, or a limit that tries to ensure that RLIMIT DATA or RLIMIT AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ‘ ggc-min-expand ’ to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into account.
Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance.
max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in the selective scheduler.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling.
216 Using the GNU Compiler Collection (GCC) max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in the selective scheduler.
min-spec-prob
The minimum probability (in percents) of reaching a source block for interblock speculative scheduling.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions.
A value of 0 disables region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that speculative insns are scheduled.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its state across it.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same memory locations.
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling.
It is a depth of search for available instructions.
selsched-max-sched-times
The maximum number of times that an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined.
selsched-insns-to-rename
The maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler.
sms-min-sc
The minimum value of stage count that swing modulo scheduler generates.
max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register.
max-combine-insns
The maximum number of instructions the RTL combiner tries to combine.
Chapter 3: GCC Command Options 217 integer-share-limit
Small integer constants can use a shared data structure, reducing the compiler’s memory usage and increasing its speed. This sets the maximum value of a shared integer constant.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack smashing protection when ‘ -fstack-protector ’ is used.
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing when not optimizing.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field sensitive manner during pointer analysis.
prefetch-latency
Estimate on average number of instructions that are executed before prefetch finishes. The distance prefetched ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see ‘ simultaneous-prefetches ’).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 data cache, in bytes.
l1-cache-size
The size of L1 data cache, in kilobytes.
l2-cache-size
The size of L2 data cache, in kilobytes.
prefetch-dynamic-strides
Whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. In some cases this may be beneficial, though the fact the stride is non-constant may make it hard to predict when there is clear benefit to issuing these hints.
Set to 1 if the prefetch hints should be issued for nonconstant strides.
Set to 0 if prefetch hints should be issued only for strides that are known to be constant and below
‘ prefetch-minimum-stride ’.
prefetch-minimum-stride
Minimum constant stride, in bytes, to start using prefetch hints for.
If the stride is less than this threshold, prefetch hints will not be issued.
218 Using the GNU Compiler Collection (GCC)
This setting is useful for processors that have hardware prefetchers, in which case there may be conflicts between the hardware prefetchers and the software prefetchers. If the hardware prefetchers have a maximum stride they can handle, it should be used here to improve the use of software prefetchers.
A value of -1 means we don’t have a threshold and therefore prefetch hints can be issued for any constant stride.
This setting is only useful for strides that are known and constant.
destructive-interference-size constructive-interference-size
The values for the C ++ 17 variables std::hardware_destructive_ interference_size interference_size .
and
The std::hardware_constructive_ destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects; the constructive interference size is the maximum recommended size of contiguous memory accessed together. Typically both will be the size of an L1 cache line for the target, in bytes. For a generic target covering a range of L1 cache line sizes, typically the constructive interference size will be the small end of the range and the destructive size will be the large end.
The destructive interference size is intended to be used for layout, and thus has ABI impact. The default value is not expected to be stable, and on some targets varies with ‘ -mtune ’, so use of this variable in a context where ABI stability is important, such as the public interface of a library, is strongly discouraged; if it is used in that context, users can stabilize the value using this option.
The constructive interference size is less sensitive, as it is typically only used in a ‘ static_assert ’ to make sure that a type fits within a cache line.
See also ‘ -Winterference-size ’.
loop-interchange-max-num-stmts
The maximum number of stmts in a loop to be interchanged.
loop-interchange-stride-ratio
The minimum ratio between stride of two loops for interchange to be profitable.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop.
Chapter 3: GCC Command Options 219 use-canonical-types
Whether the compiler should use the “canonical” type system.
Should always be 1, which uses a more efficient internal mechanism for comparing types in C ++ and Objective-C ++ . However, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that are bigger than ‘ switch-conversion-max-branch-ratio ’ times the number of branches in the switch.
max-partial-antic-length
Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (‘ -ftree-pre ’) when optimizing at ‘ -O3 ’ and above. For some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine. This parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior. Setting a value of 0 for this parameter allows an unlimited set length.
rpo-vn-max-loop-depth
Maximum loop depth that is value-numbered optimistically. When the limit hits the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically and the remaining ones not.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit is hit the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry.
ira-max-loops-num
IRA uses regional register allocation by default. If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require excessive amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table.
220 Using the GNU Compiler Collection (GCC) ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see ‘ -O3 ’). The number of available registers reserved for some other purposes is given by this parameter. Default of the parameter is the best found from numerous experiments.
ira-consider-dup-in-all-alts
Make IRA to consider matching constraint (duplicated operand number) heavily in all available alternatives for preferred register class. If it is set as zero, it means IRA only respects the matching constraint when it’s in the only available alternative with an appropriate register class. Otherwise, it means IRA will check all available alternatives for preferred register class even if it has found some choice with an appropriate register class and respect the found qualified matching constraint.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance. EBB is used as a region to do this optimization. The parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory, with very large loops. Loops with more basic blocks than this parameter won’t have loop invariant motion optimization performed on them.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. These large loops are no handled by the optimizations using loop data dependencies.
max-vartrack-size
Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions.
This trades compilation time for more complete debug information.
Chapter 3: GCC Command Options 221
If this is set too low, value expressions that are available and could be represented in debug information may end up not being used; setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow.
max-debug-marker-count
Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to
RTL. If a function has more such gimple stmts than the set limit, such stmts will be dropped from the inlined copy of a function, and from its RTL expansion.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by ‘ -fvar-tracking-assignments ’, but debug insns may get (nonoverlapping) uids above it if the reserved range is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ‘ ipa-sra-ptr-growth-factor ’ times the size of the original pointer parameter.
ipa-sra-max-replacements
Maximum pieces of an aggregate that IPA-SRA tracks. As a consequence, it is also the maximum number of replacements of a formal parameter.
sra-max-scalarization-size-Ospeed sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and
IPA-SRA) aim to replace scalar parts of aggregates with uses of independent scalar variables.
These parameters control the maximum size, in storage units, of aggregate which is considered for replacement when compiling for speed (‘ sra-max-scalarization-size-Ospeed ’) or size
(‘ sra-max-scalarization-size-Osize ’) respectively.
sra-max-propagations
The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, in order to facilitate copy propagation.
tm-max-aggregate-size
When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs. This option only applies when using ‘ -fgnu-tm ’.
222 Using the GNU Compiler Collection (GCC) graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded.
A value of zero can be used to lift the bound. A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with
‘ -floop-block ’ or ‘ -floop-strip-mine ’, strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the ‘ loop-block-tile-size ’ parameter.
ipa-jump-function-lookups
Specifies number of statements visited during jump function offset discovery.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a function’s parameter in order to propagate them and perform devirtualization. ‘ ipa-cp-value-list-size ’ is the maximum number of values and types it stores per one formal parameter of a function.
ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed
‘ ipa-cp-eval-threshold ’.
ipa-cp-max-recursive-depth
Maximum depth of recursive cloning for self-recursive function.
ipa-cp-min-recursive-probability
Recursive cloning only when the probability of call being executed exceeds the parameter.
ipa-cp-profile-count-base
When using ‘ -fprofile-use ’ option, IPA-CP will consider the measured execution count of a call graph edge at this percentage position in their histogram as the basis for its heuristics calculation.
ipa-cp-recursive-freq-factor
The number of times interprocedural copy propagation expects recursive functions to call themselves.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when they are evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to another function will receive when they are evaluated for cloning.
Chapter 3: GCC Command Options 223 ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values passed in an aggregate. ‘ ipa-max-agg-items ’ controls the maximum number of such values per one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of
‘ ipa-cp-loop-hint-bonus ’ to the profitability score of the candidate.
ipa-max-loop-predicates
The maximum number of different predicates IPA will use to describe when loops in a function have known properties.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by function parameters.
In order not spend too much time analyzing huge functions, it gives up and consider all memory clobbered after examining
‘ ipa-max-aa-steps ’ statements modifying memory.
ipa-max-switch-predicate-bounds
Maximal number of boundary endpoints of case ranges of switch statement. For switch exceeding this limit, IPA-CP will not construct cloning cost predicate, which is used to estimate cloning benefit, for default case of the switch statement.
ipa-max-param-expr-ops
IPA-CP will analyze conditional statement that references some function parameter to estimate benefit for cloning upon certain constant value. But if number of operations in a parameter expression exceeds ‘ ipa-max-param-expr-ops ’, the expression is treated as complicated one, and is not handled by IPA analysis.
lto-partitions
Specify desired number of partitions produced during WHOPR compilation. The number of partitions should exceed the number of CPUs used for compilation.
lto-min-partition
Size of minimal partition for WHOPR (in estimated instructions).
This prevents expenses of splitting very small programs into too many partitions.
lto-max-partition
Size of max partition for WHOPR (in estimated instructions). to provide an upper bound for individual size of partition. Meant to be used only with balanced partitioning.
lto-max-streaming-parallelism
Maximal number of parallel processes used for LTO streaming.
224 Using the GNU Compiler Collection (GCC) cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C ++ name lookup fails for an identifier.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block relative to a statement’s original block to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking. A small positive adjustment is applied for statements with memory operands as those are even more profitable so sink.
max-stores-to-sink
The maximum number of conditional store pairs that can be sunk.
Set to 0 if either vectorization (‘ -ftree-vectorize ’) or if-conversion (‘ -ftree-loop-if-convert ’) is disabled.
case-values-threshold
The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine.
jump-table-max-growth-ratio-for-size
The maximum code size growth ratio when expanding into a jump table (in percent). The parameter is used when optimizing for size.
jump-table-max-growth-ratio-for-speed
The maximum code size growth ratio when expanding into a jump table (in percent).
The parameter is used when optimizing for speed.
tree-reassoc-width
Set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides target dependent heuristics used by default if has non zero value.
sched-pressure-algorithm
Choose between the two available implementations of
‘ -fsched-pressure ’. Algorithm 1 is the original implementation and is the more likely to prevent instructions from being reordered.
Algorithm 2 was designed to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes.
See ‘ haifa-sched.c
’ in the GCC sources for more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate.
Chapter 3: GCC Command Options 225 asan-globals
Enable buffer overflow detection for global objects.
This kind of protection is enabled by default if you are using
‘ -fsanitize=address ’ option.
To disable global objects protection use ‘ --param asan-globals=0 ’.
asan-stack
Enable buffer overflow detection for stack objects. This kind of protection is enabled by default when using ‘ -fsanitize=address ’.
To disable stack protection use ‘ --param asan-stack=0 ’ option.
asan-instrument-reads
Enable buffer overflow detection for memory reads.
This kind of protection is enabled by default when using
‘ -fsanitize=address ’. To disable memory reads protection use
‘ --param asan-instrument-reads=0 ’.
asan-instrument-writes
Enable buffer overflow detection for memory writes.
This kind of protection is enabled by default when using
‘ -fsanitize=address ’. To disable memory writes protection use
‘ --param asan-instrument-writes=0 ’ option.
asan-memintrin
Enable detection for built-in functions. This kind of protection is enabled by default when using ‘ -fsanitize=address ’. To disable built-in functions protection use ‘ --param asan-memintrin=0 ’.
asan-use-after-return
Enable detection of use-after-return.
This kind of protection is enabled by default when using the ‘ -fsanitize=address ’ option.
To disable it use ‘ --param asan-use-after-return=0 ’.
Note: By default the check is disabled at run time. To enable it, add detect_stack_use_after_return=1 to the environment variable
ASAN_OPTIONS .
asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented is greater or equal to this number, use callbacks instead of inline checks.
E.g.
to disable inline code use ‘ --param asan-instrumentation-with-call-threshold=0 ’.
hwasan-instrument-stack
Enable hwasan instrumentation of statically sized stack-allocated variables. This kind of instrumentation is enabled by default when using ‘ -fsanitize=hwaddress ’ and disabled by default when using
‘ -fsanitize=kernel-hwaddress ’. To disable stack instrumentation use ‘ --param hwasan-instrument-stack=0 ’, and to enable it use ‘ --param hwasan-instrument-stack=1 ’.
226 Using the GNU Compiler Collection (GCC) hwasan-random-frame-tag
When using stack instrumentation, decide tags for stack variables using a deterministic sequence beginning at a random tag for each frame.
With this parameter unset tags are chosen using the same sequence but beginning from 1.
This is enabled by default for ‘ -fsanitize=hwaddress ’ and unavailable for
‘ -fsanitize=kernel-hwaddress ’.
To disable it use ‘ --param hwasan-random-frame-tag=0 ’.
hwasan-instrument-allocas
Enable hwasan instrumentation of dynamically sized stackallocated variables.
This kind of instrumentation is enabled by default when using ‘ -fsanitize=hwaddress ’ and disabled by default when using ‘ -fsanitize=kernel-hwaddress ’.
To disable instrumentation of such variables use ‘ --param hwasan-instrument-allocas=0 ’, and to enable it use ‘ --param hwasan-instrument-allocas=1 ’.
hwasan-instrument-reads
Enable hwasan checks on memory reads.
Instrumentation of reads is enabled by default for both ‘ -fsanitize=hwaddress ’ and
‘ -fsanitize=kernel-hwaddress ’.
To disable checking memory reads use ‘ --param hwasan-instrument-reads=0 ’.
hwasan-instrument-writes
Enable hwasan checks on memory writes.
Instrumentation of writes is enabled by default for both ‘ -fsanitize=hwaddress ’ and
‘ -fsanitize=kernel-hwaddress ’.
To disable checking memory writes use ‘ --param hwasan-instrument-writes=0 ’.
hwasan-instrument-mem-intrinsics
Enable hwasan instrumentation of builtin functions. Instrumentation of these builtin functions is enabled by default for both
‘ -fsanitize=hwaddress ’ and ‘ -fsanitize=kernel-hwaddress ’.
To disable instrumentation of builtin functions use ‘ --param hwasan-instrument-mem-intrinsics=0 ’.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks.
tsan-distinguish-volatile
Emit special instrumentation for accesses to volatiles.
tsan-instrument-func-entry-exit
Emit instrumentation calls to tsan func exit().
tsan func entry() and max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.
Chapter 3: GCC Command Options 227 max-fsm-thread-length
Maximum number of basic blocks on a jump thread path.
threader-debug threader-debug=[none | all] Enables verbose dumping of the threader solver.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by parloops.
parloops-schedule
Schedule type of omp schedule for loops parallelized by parloops
(static, dynamic, guided, auto, runtime).
parloops-min-per-thread
The minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. Note that for a parallelized loop nest the minimum number of iterations of the outermost loop per thread is two.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion corresponds to following a use-def chain.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively.
max-vrp-switch-assertions
The maximum number of assertions to add along the default edge of a switch statement during VRP.
evrp-sparse-threshold
Maximum number of basic blocks before EVRP uses a sparse cache.
evrp-mode
Specifies the mode Early VRP should operate in.
vrp1-mode
Specifies the mode VRP pass 1 should operate in.
vrp2-mode
Specifies the mode VRP pass 2 should operate in.
ranger-debug
Specifies the type of debug output to be issued for ranges.
evrp-switch-limit
Specifies the maximum number of switch cases before EVRP ignores a switch.
228 Using the GNU Compiler Collection (GCC) unroll-jam-min-percent
The minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable.
unroll-jam-max-unroll
The maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation.
max-rtl-if-conversion-unpredictable-cost
Maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable.
max-variable-expansions-in-unroller
If ‘ -fvariable-expansion-in-unroller ’ is used, the maximum number of times that an individual variable will be expanded during loop unrolling.
partial-inlining-entry-probability
Maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make partial inlining happen.
max-tracked-strlens
Maximum number of strings for which strlen optimization pass will track string lengths.
gcse-after-reload-partial-fraction
The threshold ratio for performing partial redundancy elimination after reload.
gcse-after-reload-critical-fraction
The threshold ratio of critical edges execution count that permit performing redundancy elimination after reload.
max-loop-header-insns
The maximum number of insns in loop header duplicated by the copy loop headers pass.
vect-epilogues-nomask
Enable loop epilogue vectorization using smaller vector size.
vect-partial-vector-usage
Controls when the loop vectorizer considers using partial vector loads and stores as an alternative to falling back to scalar code. 0 stops the vectorizer from ever using partial vector loads and stores.
1 allows partial vector loads and stores if vectorization removes the need for the code to iterate. 2 allows partial vector loads and stores in all loops. The parameter only has an effect on targets that support partial vector loads and stores.
Chapter 3: GCC Command Options 229 vect-inner-loop-cost-factor
The maximum factor which the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized.
The factor applied is the maximum of the estimated number of iterations of the inner loop and this parameter. The default value of this parameter is 50.
avoid-fma-max-bits
Maximum number of bits for which we avoid creating FMAs.
sms-loop-average-count-threshold
A threshold on the average loop count considered by the swing modulo scheduler.
sms-dfa-history
The number of cycles the swing modulo scheduler considers when checking conflicts using DFA.
graphite-allow-codegen-errors
Whether codegen errors should be ICEs when ‘ -fchecking ’.
sms-max-ii-factor
A factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop.
lra-max-considered-reload-pseudos
The max number of reload pseudos which are considered during spilling a non-reload pseudo.
max-pow-sqrt-depth
Maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant.
max-dse-active-local-stores
Maximum number of active local stores in RTL dead store elimination.
asan-instrument-allocas
Enable asan allocas/VLAs protection.
max-iterations-computation-cost
Bound on the cost of an expression to compute the number of iterations.
max-isl-operations
Maximum number of isl operations, 0 means unlimited.
graphite-max-arrays-per-scop
Maximum number of arrays per scop.
max-vartrack-reverse-op-size
Max. size of loc list for which reverse ops should be added.
fsm-scale-path-stmts
Scale factor to apply to the number of statements in a threading path when comparing to the number of (scaled) blocks.
230 Using the GNU Compiler Collection (GCC) uninit-control-dep-attempts
Maximum number of nested calls to search for control dependencies during uninitialized variable analysis.
fsm-scale-path-blocks
Scale factor to apply to the number of blocks in a threading path when comparing to the number of (scaled) statements.
sched-autopref-queue-depth
Hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the model looks into; at ’ ’ only enable instruction sorting heuristic.
loop-versioning-max-inner-insns
The maximum number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy.
loop-versioning-max-outer-insns
The maximum number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning.
ssa-name-def-chain-limit
The maximum number of SSA NAME assignments to follow in determining a property of a variable such as its value. This limits the number of iterations or recursive calls GCC performs when optimizing certain statements or when determining their validity prior to issuing diagnostics.
store-merging-max-size
Maximum size of a single store merging region in bytes.
hash-table-verification-limit
The number of elements for which hash table verification is done for each searched element.
max-find-base-term-values
Maximum number of VALUEs handled during a single find base term call.
analyzer-max-enodes-per-program-point
The maximum number of exploded nodes per program point within the analyzer, before terminating analysis of that point.
analyzer-max-constraints
The maximum number of constraints per state.
analyzer-min-snodes-for-call-summary
The minimum number of supernodes within a function for the analyzer to consider summarizing its effects at call sites.
analyzer-max-enodes-for-full-dump
The maximum depth of exploded nodes that should appear in a dot dump before switching to a less verbose format.
Chapter 3: GCC Command Options 231 analyzer-max-recursion-depth
The maximum number of times a callsite can appear in a call stack within the analyzer, before terminating analysis of a call that would recurse deeper.
analyzer-max-svalue-depth
The maximum depth of a symbolic value, before approximating the value as unknown.
analyzer-max-infeasible-edges
The maximum number of infeasible edges to reject before declaring a diagnostic as infeasible.
gimple-fe-computed-hot-bb-threshold
The number of executions of a basic block which is considered hot.
The parameter is used only in GIMPLE FE.
analyzer-bb-explosion-factor
The maximum number of ’after supernode’ exploded nodes within the analyzer per supernode, before terminating analysis.
ranger-logical-depth
Maximum depth of logical expression evaluation ranger will look through when evaluating outgoing edge ranges.
openacc-kernels
Specify mode of OpenACC ‘kernels’ constructs handling.
With
‘ --param=openacc-kernels=decompose ’, OpenACC ‘kernels’ constructs are decomposed into parts, a sequence of compute constructs, each then handled individually. This is work in progress.
With ‘ --param=openacc-kernels=parloops ’, OpenACC ‘kernels’ constructs are handled by the ‘ parloops ’ pass, en bloc. This is the current default.
openacc-privatization
Specify mode of OpenACC privatization diagnostics for
‘ -fopt-info-omp-note ’ and applicable ‘ -fdump-tree-*-details ’.
With ‘ --param=openacc-privatization=quiet ’, don’t diagnose.
This is the current default.
‘ --param=openacc-privatization=noisy ’, do diagnose.
With
The following choices of name are available on AArch64 targets: aarch64-sve-compare-costs
When vectorizing for SVE, consider using “unpacked” vectors for smaller elements and use the cost model to pick the cheapest approach. Also use the cost model to choose between SVE and Advanced SIMD vectorization.
Using unpacked vectors includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores.
232 Using the GNU Compiler Collection (GCC) aarch64-float-recp-precision
The number of Newton iterations for calculating the reciprocal for float type. The precision of division is proportional to this param when division approximation is enabled. The default value is 1.
aarch64-double-recp-precision
The number of Newton iterations for calculating the reciprocal for double type. The precision of division is propotional to this param when division approximation is enabled. The default value is 2.
aarch64-autovec-preference
Force an ISA selection strategy for auto-vectorization.
Accepts values from 0 to 4, inclusive.
‘ 0 ’
‘ 1 ’
Use the default heuristics.
Use only Advanced SIMD for auto-vectorization.
‘
‘
2
3
’
’
Use only SVE for auto-vectorization.
Use both Advanced SIMD and SVE. Prefer Advanced
SIMD when the costs are deemed equal.
‘ 4 ’ Use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.
The default value is 0.
aarch64-loop-vect-issue-rate-niters
The tuning for some AArch64 CPUs tries to take both latencies and issue rates into account when deciding whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or not vectorized at all. If this parameter is set to n , GCC will not use this heuristic for loops that are known to execute in fewer than n
Advanced SIMD iterations.
3.12 Program Instrumentation Options
GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. For example, one purpose of instrumentation is collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C ++ vtable hijacking. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes.
-p
-pg Generate extra code to write profile information suitable for the analysis program prof (for ‘ -p ’) or gprof (for ‘ -pg ’). You must use this option when compiling the source files you want data about, and you must also use it when linking.
Chapter 3: GCC Command Options 233
You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options.
See
Section 6.33.1 [Common Function Attributes], page 549 .
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C ++ constructors (and destructors) of classes which are used as a type of a global variable.
When the compiled program exits it saves this data to a file called
‘ auxname .gcda
’ for each source file. The data may be used for profile-directed optimizations (‘ -fbranch-probabilities ’), or for test coverage analysis
(‘ -ftest-coverage ’). Each object file’s auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. ‘ foo.gcda
’ for input file ‘ dir/foo.c
’, or ‘ dir/foo.gcda
’ for output file specified as ‘ -o dir/foo.o
’).
See
Section 10.5 [Cross-profiling], page 927
.
--coverage
This option is used to compile and link code instrumented for coverage analysis.
The option is a synonym for ‘ -fprofile-arcs ’ ‘ -ftest-coverage ’ (when compiling) and ‘ -lgcov ’ (when linking). See the documentation for those options for more details.
• Compile the source files with ‘ -fprofile-arcs ’ plus optimization and code generation options. For test coverage analysis, use the additional
‘ -ftest-coverage ’ option. You do not need to profile every source file in a program.
• Compile the source files additionally with ‘ -fprofile-abs-path ’ to create absolute path names in the ‘ .gcno
’ files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
• Link your object files with ‘ -lgcov ’ or ‘ -fprofile-arcs ’ (the latter implies the former).
• Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict
ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting.
Moreover, an object file can be recompiled multiple times and the corresponding ‘ .gcda
’ file merges as long as the source file and the compiler options are unchanged.
• For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus
234 Using the GNU Compiler Collection (GCC)
‘ -fbranch-probabilities ’ (see
Section 3.11 [Options that Control
• For test coverage analysis, use gcov to produce human readable information from the ‘ .gcno
’ and ‘ .gcda
’ files. Refer to the gcov documentation for further information.
With ‘ -fprofile-arcs ’, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility (see
Chapter 10 [ gcov — a Test Coverage Program], page 915
) can use to show program coverage. Each source file’s note file is called ‘ auxname .gcno
’. Refer to the ‘ -fprofile-arcs ’ option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize.
-fprofile-abs-path
Automatically convert relative source file names to absolute path names in the
‘ .gcno
’ files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
-fprofile-dir= path
Set the directory to search for the profile data files in to path .
This option affects only the profile data generated by ‘ -fprofile-generate ’,
‘ -ftest-coverage ’, ‘ -fprofile-arcs ’ and used by ‘ -fprofile-use ’ and
‘ -fbranch-probabilities ’ and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path , thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the ‘ sourcename .gcda
’ file and use it as the file name of a ‘ .gcda
’ file. See similar option ‘ -fprofile-note ’.
When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:
%p process ID.
%q{VAR} value of environment variable VAR
-fprofile-generate
-fprofile-generate= path
Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use ‘ -fprofile-generate ’ both when compiling and when linking your program.
Chapter 3: GCC Command Options 235
The following options are enabled: ‘ -fprofile-arcs ’, ‘ -fprofile-values ’,
‘ -finline-functions ’, and ‘ -fipa-bit-cp ’.
If path is specified, GCC looks at the path to find the profile feedback data files. See ‘ -fprofile-dir ’.
To optimize the program based on the collected profile information, use
‘ -fprofile-use ’.
See
Section 3.11 [Optimize Options], page 159 , for more
information.
-fprofile-info-section
-fprofile-info-section= name
Register the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to .gcov_info
. A pointer to the profile information generated by ‘ -fprofile-arcs ’ is placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. This option is not intended to be used in hosted environments such as GNU/Linux. It targets free-standing environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O.
The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:
.gcov_info
{
:
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}
The program could dump the profiling information registered in this linker set for example like this:
#include <gcov.h>
#include <stdio.h>
#include <stdlib.h> extern const struct gcov_info *__gcov_info_start[]; extern const struct gcov_info *__gcov_info_end[]; static void filename (const char *f, void *arg)
{ puts (f);
} static void dump (const void *d, unsigned n, void *arg)
{ const unsigned char *c = d;
} for (unsigned i = 0; i < n; ++i) printf ("%02x", c[i]);
236 Using the GNU Compiler Collection (GCC) static void * allocate (unsigned length, void *arg)
{ return malloc (length);
} static void dump_gcov_info (void)
{ const struct gcov_info **info = __gcov_info_start; const struct gcov_info **end = __gcov_info_end;
/* Obfuscate variable to prevent compiler optimizations.
*/
__asm__ ("" : "+r" (info));
} while (info != end)
{ void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg); putchar (’\n’);
++info;
} int main()
{ dump_gcov_info(); return 0;
}
-fprofile-note= path
If path is specified, GCC saves ‘ .gcno
’ file into path location. If you combine the option with multiple source files, the ‘ .gcno
’ file will be overwritten.
-fprofile-prefix-path= path
This option can be used in combination with ‘ profile-generate= ’ profile dir and ‘ profile-use= ’ profile dir to inform GCC where is the base directory of built source tree. By default profile dir will contain files with mangled absolute paths of all object files in the built project.
This is not desirable when directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback because the profile data will not be found during the optimized build.
In such setups
‘ -fprofile-prefix-path= ’ path with path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory.
-fprofile-prefix-map= old = new
When compiling files residing in directory ‘ old ’, record profiling information
(with ‘ --coverage ’) describing them as if the files resided in directory ‘ new ’ instead. See also ‘ -ffile-prefix-map ’.
Chapter 3: GCC Command Options 237
-fprofile-update= method
Alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one of ‘ single ’, ‘ atomic ’ or ‘ prefer-atomic ’. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code.
Warning: When an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted.
Using ‘ prefer-atomic ’ would be transformed either to ‘ atomic ’, when supported by a target, or to ‘ single ’ otherwise. The GCC driver automatically selects ‘ prefer-atomic ’ when ‘ -pthread ’ is present in the command line.
-fprofile-filter-files= regex
Instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons).
For example, ‘ -fprofile-filter-files=main\.c;module.*\.c
’ will instrument only ‘ main.c
’ and all C files starting with ’module’.
-fprofile-exclude-files= regex
Instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons).
For example, ‘ -fprofile-exclude-files=/usr/.* ’ will prevent instrumentation of all files that are located in the ‘ /usr/ ’ folder.
-fprofile-reproducible= [ multithreaded|parallel-runs|serial ]
Control level of reproducibility of profile gathered by -fprofile-generate .
This makes it possible to rebuild program with same outcome which is useful, for example, for distribution packages.
With ‘ -fprofile-reproducible=serial ’ the profile gathered by
‘ -fprofile-generate ’ is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before fork function is invoked.
Note that it is quite common that execution counts of some part of programs depends, for example, on length of temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-reproducible part of programs may be annotated by no_instrument_function function attribute.
gcov-dump with ‘ -l ’ can be used to dump gathered data and verify that they are indeed reproducible.
With ‘ -fprofile-reproducible=parallel-runs ’ collected profile stays reproducible regardless the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of instrumented program in parallel
(such as with make -j ). This reduces quality of gathered data, in particular of indirect call profiling.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector.
Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs.
238 Using the GNU Compiler Collection (GCC)
The option enables ‘ -fsanitize-address-use-after-scope ’. See https:// github.com/google/sanitizers/wiki/AddressSanitizer for more details.
The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to help=1 , the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/ wiki / AddressSanitizerFlags # run-time-flags for a list of supported options.
The option cannot be combined with ‘ -fsanitize=thread ’ or
‘ -fsanitize=hwaddress ’. Note that the only target ‘ -fsanitize=hwaddress ’ is currently supported on is AArch64.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See https://github.com/google/ kasan for more details.
-fsanitize=hwaddress
Enable Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer to allow the detection of memory errors with a low memory overhead.
Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs.
The option enables ‘ -fsanitize-address-use-after-scope ’.
See https: / / clang .
llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html
for more details. The run-time behavior can be influenced using the HWASAN_OPTIONS environment variable. When set to help=1 , the available options are shown at startup of the instrumented program.
The option cannot be combined with ‘ -fsanitize=thread ’ or ‘ -fsanitize=address ’, and is currently only available on AArch64.
-fsanitize=kernel-hwaddress
Enable Hardware-assisted AddressSanitizer for compilation of the Linux kernel.
Similar to ‘ -fsanitize=kernel-address ’ but using an alternate instrumentation method, and similar to ‘ -fsanitize=hwaddress ’ but with instrumentation differences necessary for compiling the Linux kernel. These differences are to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte.
Note: This option has different defaults to the ‘ -fsanitize=hwaddress ’.
Instrumenting the stack and alloca calls are not on by default but are still possible by specifying the command-line options ‘ --param hwasan-instrument-stack=1 ’ and ‘ --param hwasan-instrument-allocas=1 ’ respectively.
Using a random frame tag is not implemented for kernel instrumentation.
-fsanitize=pointer-compare
Instrument comparison operation ( < , < =, > , > =) with pointer operands.
The option must be combined with either ‘ -fsanitize=kernel-address ’ or ‘ -fsanitize=address ’ The option cannot be combined with
‘ -fsanitize=thread ’. Note: By default the check is disabled at run time.
To enable it, add detect_invalid_pointer_pairs=2 to the environment variable ASAN_OPTIONS .
Using detect_invalid_pointer_pairs=1 detects invalid operation only when both pointers are non-null.
Chapter 3: GCC Command Options 239
-fsanitize=pointer-subtract
Instrument subtraction with pointer operands. The option must be combined with either ‘ -fsanitize=kernel-address ’ or ‘ -fsanitize=address ’ The option cannot be combined with ‘ -fsanitize=thread ’. Note: By default the check is disabled at run time. To enable it, add detect_invalid_pointer_ pairs=2 to the environment variable ASAN_OPTIONS . Using detect_invalid_ pointer_pairs=1 detects invalid operation only when both pointers are nonnull.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access instructions are instrumented to detect data race bugs.
See https: / / github . com / google/sanitizers/wiki#threadsanitizer for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https: / / github . com / google / sanitizers / wiki / ThreadSanitizerFlags for a list of supported options.
The option cannot be combined with
‘ -fsanitize=address ’, ‘ -fsanitize=leak ’.
Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions
(‘ -fnon-call-exceptions ’).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables and the executable is linked against a library that overrides malloc and other allocator functions. See https://github.com/google/ sanitizers /wiki /AddressSanitizerLeakSanitizer for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot be combined with ‘ -fsanitize=thread ’.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are instrumented to detect undefined behavior at runtime.
See https: / / clang . llvm . org / docs / UndefinedBehaviorSanitizer . html for more details. The run-time behavior can be influenced using the UBSAN_
OPTIONS environment variable. Current suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift operation is not undefined.
Note that what exactly is considered undefined differs slightly between C and C ++ , as well as between
ISO C90 and C99, etc.
This option has two suboptions,
‘ -fsanitize=shift-base ’ and ‘ -fsanitize=shift-exponent ’.
-fsanitize=shift-exponent
This option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument.
-fsanitize=shift-base
If the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. Note that what
240 Using the GNU Compiler Collection (GCC) exactly is considered undefined differs slightly between C and C ++ , as well as between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero.
-fsanitize=unreachable
With this option, the compiler turns the __builtin_unreachable call into a diagnostics message call instead. When reaching the
__builtin_unreachable call, the behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size of a variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.
-fsanitize=return
This option enables return statement checking.
Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C ++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We check that the result of + , * , and both unary and binary does not overflow in the signed arithmetics. This also detects INT_MIN / -1 signed division. Note, integer promotion rules must be taken into account.
That is, the following is not an overflow: signed char a = SCHAR_MAX; a++;
-fsanitize=bounds
This option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented.
-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage are not instrumented.
-fsanitize=alignment
This option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned
Chapter 3: GCC Command Options 241 target, or when a method or constructor is invoked on insufficiently aligned object.
-fsanitize=object-size
This option enables instrumentation of memory references using the
__builtin_object_size function. Various out of bounds pointer accesses are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero.
Unlike other similar options, ‘ -fsanitize=float-divide-by-zero ’ is not enabled by
‘ -fsanitize=undefined ’, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow.
Unlike other similar options, ‘ -fsanitize=float-cast-overflow ’ is not enabled by ‘ -fsanitize=undefined ’. This option does not work well with FE_INVALID exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the nonnull function attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in functions marked with returns_nonnull function attribute, to detect returning of null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum type.
If a value outside the range of values for the enum type is loaded, a run-time error is issued.
-fsanitize=vptr
This option enables instrumentation of C ++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type.
-fsanitize=pointer-overflow
This option enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued.
-fsanitize=builtin
This option enables instrumentation of arguments to selected builtin functions. If an invalid value is passed to such arguments,
242 Using the GNU Compiler Collection (GCC) a run-time error is issued.
E.g. passing 0 as the argument to
__builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option.
While ‘ -ftrapv ’ causes traps for signed overflows to be emitted,
‘ -fsanitize=undefined ’ gives a diagnostic message.
This currently works only for the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers. ‘ -fsanitize=all ’ is not allowed, as some sanitizers cannot be used together.
-fasan-shadow-offset= number
This option forces GCC to use custom shadow offset in AddressSanitizer checks.
It is useful for experimenting with different shadow memory layouts in Kernel
AddressSanitizer.
-fsanitize-sections= s1 , s2 ,...
Sanitize global variables in selected user-defined sections.
si may contain wildcards.
-fsanitize-recover [ = opts ]
‘ -fsanitize-recover= ’ controls error recovery mode for sanitizers mentioned in comma-separated list of opts . Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened.
This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. The ‘ -fno-sanitize-recover= ’ option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code.
Currently this feature only works for ‘ -fsanitize=undefined ’ (and its suboptions except for ‘ -fsanitize=unreachable ’ and ‘ -fsanitize=return ’),
‘ -fsanitize=float-cast-overflow ’, ‘ -fsanitize=float-divide-by-zero ’,
‘ -fsanitize=bounds-strict ’, ‘ -fsanitize=kernel-address ’ and
‘ -fsanitize=address ’.
For these sanitizers error recovery is turned on by default, except ‘ -fsanitize=address ’, for which this feature is experimental.
‘ -fsanitize-recover=all ’ and ‘ -fno-sanitize-recover=all ’ is also accepted, the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.
Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while default value for AddressSanitizer is halt_on_ error=1 . This can be overridden through setting the halt_on_error flag in the corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of: undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
Chapter 3: GCC Command Options 243
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope bugs. The option sets ‘ -fstack-reuse ’ to ‘ none ’.
-fsanitize-undefined-trap-on-error
The ‘ -fsanitize-undefined-trap-on-error ’ option instructs the compiler to report undefined behavior using __builtin_trap rather than a libubsan library routine. The advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation.
Inserts a call to __ sanitizer_cov_trace_pc into every basic block.
-fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation.
Inserts a call to __sanitizer_cov_trace_cmp1 , __sanitizer_cov_trace_cmp2 ,
__sanitizer_cov_trace_cmp4 or __sanitizer_cov_trace_cmp8 for integral comparison with both operands variable or __sanitizer_cov_trace_ const_cmp1 , __sanitizer_cov_trace_const_cmp2 , __sanitizer_cov_ trace_const_cmp4 or __sanitizer_cov_trace_const_cmp8 for integral comparison with one operand constant, __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons and
__sanitizer_cov_trace_switch for switch statements.
-fcf-protection= [ full|branch|return|none|check ]
Enable code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions
(such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. This is intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP).
The value branch tells the compiler to implement checking of validity of controlflow transfer at the point of indirect branch instructions, i.e. call/jmp instructions. The value return implements checking of validity at the point of returning from a function. The value full is an alias for specifying both branch and return . The value none turns off instrumentation.
The value check is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different ‘ -fcf-protection ’ values. The value check is ignored at the compile time.
The macro __CET__ is defined when ‘ -fcf-protection ’ is used. The first bit of __CET__ is set to 1 for the value branch and the second bit of __CET__ is set to 1 for the return .
You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation (see
Section 6.33 [Function Attributes], page 548
).
Currently the x86 GNU/Linux target provides an implementation based on Intel
Control-flow Enforcement Technology (CET) which works for i686 processor or newer.
244 Using the GNU Compiler Collection (GCC)
-fharden-compares
For every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call __builtin_trap if the results do not match.
Use with ‘ -fharden-conditional-branches ’ to cover all conditionals.
-fharden-conditional-branches
For every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call __ builtin_trap if the result is unexpected. Use with ‘ -fharden-compares ’ to cover all conditionals.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks.
This is done by adding a guard variable to functions with vulnerable objects.
This includes functions that call alloca , and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don’t count.
-fstack-protector-all
Like ‘ -fstack-protector ’ except that all functions are protected.
-fstack-protector-strong
Like ‘ -fstack-protector ’ but includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don’t count.
-fstack-protector-explicit
Like ‘ -fstack-protector ’ but only protects those functions which have the stack_protect attribute.
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack.
You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: ‘ no ’ means no checking,
‘ generic ’ means force the use of old-style checking, ‘ specific ’ means use the best checking method and is equivalent to bare ‘ -fstack-check ’.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:
Chapter 3: GCC Command Options 245
1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.
Note that old-style stack checking is also the fallback method for ‘ specific ’ if no target support has been added in the compiler.
‘ -fstack-check= ’ is designed for Ada’s needs to detect infinite recursion and stack overflows. ‘ specific ’ is an excellent choice when compiling Ada code.
It is not generally sufficient to protect against stack-clash attacks. To protect against those you want ‘ -fstack-clash-protection ’.
-fstack-clash-protection
Generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system.
Most targets do not fully support stack clash protection. However, on those targets ‘ -fstack-clash-protection ’ will protect dynamic stack allocations.
‘ -fstack-clash-protection ’ may also provide limited protection for static stack allocations if the target supports ‘ -fstack-check=specific ’.
-fstack-limit-register= reg
-fstack-limit-symbol= sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.
For instance, if the stack starts at absolute address ‘ 0x80000000 ’ and grows downwards, you can use the flags ‘ -fstack-limit-symbol=__stack_limit ’ and ‘ -Wl,--defsym,__stack_limit=0x7ffe0000 ’ to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the no_stack_limit function attribute (see
Section 6.33 [Function Attributes], page 548
).
-fsplit-stack
Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running
GNU/Linux.
246 Using the GNU Compiler Collection (GCC)
When code compiled with ‘ -fsplit-stack ’ calls code compiled without
‘ -fsplit-stack ’, there may not be much stack space available for the latter code to run.
If compiling all code, including library code, with
‘ -fsplit-stack ’ is not an option, then the linker can fix up these calls so that the code compiled without ‘ -fsplit-stack ’ always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later.
-fvtable-verify= [ std|preinit|none ]
This option is only available when compiling C ++ code. It turns on (or off, if using ‘ -fvtable-verify=none ’) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten.
If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted.
This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers.
The options ‘ std ’ and
‘ preinit ’ control the timing of when these data structures are built. In both cases the data structures are built before execution reaches main .
Using
‘ -fvtable-verify=std ’ causes the data structures to be built after shared libraries have been loaded and initialized. ‘ -fvtable-verify=preinit ’ causes them to be built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with different values specified, ‘ none ’ takes highest priority over both ‘ std ’ and ‘ preinit ’; ‘ preinit ’ takes priority over ‘ std ’.
-fvtv-debug
When used in conjunction with ‘ -fvtable-verify=std ’ or
‘ -fvtable-verify=preinit ’, causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. This information is written to a file named ‘ vtv_set_ptr_data.log
’ in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one.
-fvtv-counts
This is a debugging flag.
When used in conjunction with
‘ -fvtable-verify=std ’ or ‘ -fvtable-verify=preinit ’, this causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. The compiler writes this information to a file named
‘ vtv_count_data.log
’ in the directory named by the environment variable
VTV_LOGS_DIR if that is defined or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to ‘ vtv_class_set_sizes.log
’ in the same directory.
Chapter 3: GCC Command Options 247
Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms, __builtin_return_address does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.) void __cyg_profile_func_enter (void *this_fn, void *call_site); void __cyg_profile_func_exit (void *this_fn, void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use extern inline in your
C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute no_instrument_function , in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). See
[Common Function Attributes], page 549
.
-finstrument-functions-exclude-file-list= file , file ,...
Set the list of functions that are excluded from instrumentation (see the description of ‘ -finstrument-functions ’). If the file that contains a function definition matches with one of file , then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys excludes any inline function defined in files whose pathnames contain
‘ /bits/stl ’ or ‘ include/sys ’.
If, for some reason, you want to include letter ‘ , ’ in one of sym , write ‘ \, ’. For example, ‘ -finstrument-functions-exclude-file-list=’\,\,tmp’ ’ (note the single quote surrounding the option).
248 Using the GNU Compiler Collection (GCC)
-finstrument-functions-exclude-function-list= sym , sym ,...
This is similar to ‘ -finstrument-functions-exclude-file-list ’, but this option sets the list of function names to be excluded from instrumentation.
The function name to be matched is its user-visible name, such as vector<int> blah(const vector<int> &) , not the internal mangled name
(e.g., _Z4blahRSt6vectorIiSaIiEE ). The match is done on substrings: if the sym parameter is a substring of the function name, it is considered to be a match. For C99 and C ++ extended identifiers, the function name must be given in UTF-8, not using universal character names.
-fpatchable-function-entry= N [, M ]
Generate N NOPs right at the beginning of each function, with the function entry point before the M th NOP. If M is omitted, it defaults to 0 so the function entry points to the address just at the first NOP. The NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time, provided that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_nop . This behavior is target-specific and may also depend on the architecture variant and/or other compilation options.
For run-time identification, the starting addresses of these areas, which correspond to their respective function entries minus M , are additionally collected in the __patchable_function_entries section of the resulting binary.
Note that the value of __attribute__ ((patchable_function_ entry (N,M))) takes precedence
‘ -fpatchable-function-entry=N,M ’.
over
This can command-line be used to option increase the area size or to remove it completely on a single function. If N=0 , no pad location is recorded.
The NOP instructions are inserted at—and maybe before, depending on M —the function entry address, even before the prologue.
The maximum value of N and M is 65535.
3.13 Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the ‘ -E ’ option, nothing is done except preprocessing. Some of these options make sense only together with ‘ -E ’ because they cause the preprocessor output to be unsuitable for actual compilation.
In addition to the options listed here, there are a number of options to control search paths for include files documented in
Section 3.16 [Directory Options], page 262
. Options to control preprocessor diagnostics are listed in
Section 3.8 [Warning Options], page 89 .
-D name Predefine name as a macro, with definition 1 .
Chapter 3: GCC Command Options 249
-D name = definition
The contents of definition are tokenized and processed as if they appeared during translation phase three in a ‘ #define ’ directive. In particular, the definition is truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell’s quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any).
Parentheses are meaningful to most shells, so you should quote the option.
With sh and csh , ‘ -D’ name ( args...
)= definition ’ ’ works.
‘ -D ’ and ‘ -U ’ options are processed in the order they are given on the command line. All ‘ -imacros file ’ and ‘ -include file ’ options are processed after all
‘ -D ’ and ‘ -U ’ options.
-U name Cancel any previous definition of name , either built in or provided with a ‘ -D ’ option.
-include file
Process file as if #include "file" appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor’s working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the #include "..." search chain as normal.
If multiple ‘ -include ’ options are given, the files are included in the order they appear on the command line.
-imacros file
Exactly like ‘ -include ’, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations.
All files specified by ‘ -imacros ’ are processed before all files specified by
‘ -include ’.
-undef Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
-pthread Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking.
This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from ‘ -include ’ or
‘ -imacros ’ command-line options.
Unless specified explicitly (with ‘ -MT ’ or ‘ -MQ ’), the object file name consists of the name of the source file with any suffix replaced with object file suffix and
250 Using the GNU Compiler Collection (GCC)
-MM with any leading directory parts removed. If there are many included files then the rule is split into several lines using ‘ \ ’-newline. The rule has no commands.
This option does not suppress the preprocessor’s debug output, such as ‘ -dM ’.
To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with ‘ -MF ’, or use an environment variable like DEPENDENCIES_OUTPUT (see
Section 3.21 [Environment Variables], page 496
). Debug output is still sent to the regular output stream as normal.
Passing ‘ -M ’ to the driver implies ‘ -E ’, and suppresses warnings with an implicit
‘ -w ’.
Like ‘ -M ’ but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an ‘ #include ’ directive does not in itself determine whether that header appears in ‘ -MM ’ dependency output.
-MF file When used with ‘ -M ’ or ‘ -MM ’, specifies a file to write the dependencies to. If no ‘ -MF ’ switch is given the preprocessor sends the rules to the same place it would send preprocessed output.
When used with the driver options ‘ -MD ’ or ‘ -MMD ’, ‘ -MF ’ overrides the default dependency output file.
If file is ‘ ’, then the dependencies are written to ‘ stdout ’.
-MG In conjunction with an option such as ‘ -M ’ requesting dependency generation,
‘ -MG ’ assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. ‘ -MG ’ also suppresses preprocessed output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-Mno-modules
Disable dependency generation for compiled module interfaces.
-MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the
‘ Makefile ’ to match.
This is typical output: test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default
CPP takes the name of the main input file, deletes any directory components and any file suffix such as ‘ .c
’, and appends the platform’s usual object suffix.
The result is the target.
Chapter 3: GCC Command Options 251
An ‘ -MT ’ option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to ‘ -MT ’, or use multiple ‘ -MT ’ options.
For example, ‘ -MT ’$(objpfx)foo.o’ ’ might give
$(objpfx)foo.o: foo.c
-MQ target
Same as ‘ -MT ’, but it quotes any characters which are special to Make.
‘ -MQ ’$(objpfx)foo.o’ ’ gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with ‘ -MQ ’.
-MD ‘ -MD ’ is equivalent to ‘ -M -MF file ’, except that ‘ -E ’ is not implied. The driver determines file based on whether an ‘ -o ’ option is given. If it is, the driver uses its argument but with a suffix of ‘ .d
’, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a ‘ .d
’ suffix.
If ‘ -MD ’ is used in conjunction with ‘ -E ’, any ‘ -o ’ switch is understood to specify the dependency output file (see
[-MF], page 250 ), but if used without ‘
-E ’, each
‘ -o ’ is understood to specify a target object file.
Since ‘ -E ’ is not implied, ‘ -MD ’ can be used to generate a dependency output file as a side effect of the compilation process.
-MMD Like ‘ -MD ’ except mention only user header files, not system header files.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed.
This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with ‘ -C ’ to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.
‘ -fpreprocessed ’ is implicit if the input file has one of the extensions ‘ .i
’,
‘ .ii
’ or ‘ .mi
’. These are the extensions that GCC uses for preprocessed files created by ‘ -save-temps ’.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option’s behavior depends on the ‘ -E ’ and ‘ -fpreprocessed ’ options.
With ‘ -E ’, preprocessing is limited to the handling of directives such as #define ,
#ifdef , and #error . Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the ‘ -dD ’ option is implicitly enabled.
With ‘ -fpreprocessed ’, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__ , which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only .
With both ‘ -E ’ and ‘ -fpreprocessed ’, the rules for ‘ -fpreprocessed ’ take precedence.
This enables full preprocessing of files previously preprocessed with -E -fdirectives-only .
252 Using the GNU Compiler Collection (GCC)
-fdollars-in-identifiers
Accept ‘ $ ’ in identifiers.
-fextended-identifiers
Accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C ++ .
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with canonicalization.
-fmax-include-depth= depth
Set the maximum depth of the nested #include. The default is 200.
-ftabstop= width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.
-ftrack-macro-expansion [ = level ]
Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value ‘ 0 ’ of level de-activates this option.
Value ‘ 1 ’ tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value ‘ 2 ’ tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is ‘ 2 ’.
Note that -ftrack-macro-expansion=2 is activated by default.
-fmacro-prefix-map= old = new
When preprocessing files residing in directory ‘ old ’, expand the __FILE__ and
__BASE_FILE__ macros as if the files resided in directory ‘ new ’ instead. This can be used to change an absolute path to a relative path by using ‘ .
’ for new which can result in more reproducible builds that are location independent. This option also affects __builtin_FILE() during compilation. See also
‘ -ffile-prefix-map ’.
-fexec-charset= charset
Set the execution character set, used for string and character constants. The default is UTF-8.
charset can be any encoding supported by the system’s iconv library routine.
-fwide-exec-charset= charset
Set the wide execution character set, used for wide string and character constants. The default is UTF-32 or UTF-16, whichever corresponds to the width of wchar_t . As with ‘ -fexec-charset ’, charset can be any encoding supported by the system’s iconv library routine; however, you will have problems with encodings that do not fit exactly in wchar_t .
Chapter 3: GCC Command Options 253
-finput-charset= charset
Set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there’s a conflict.
charset can be any encoding supported by the system’s iconv library routine.
-fpch-deps
When using precompiled headers (see
Section 3.22 [Precompiled Headers], page 500
), this flag causes the dependency-output flags to also list the files from the precompiled header’s dependencies. If not specified, only the precompiled header are listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header (see
) together with ‘ -E ’. It inserts a special #pragma , #pragma
GCC pch_preprocess " filename " in the output to mark the place where the precompiled header was found, and its filename . When ‘ -fpreprocessed ’ is in use, GCC recognizes this #pragma and loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by ‘ -save-temps ’.
You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC’s current directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes.
GCC uses this directory, when it’s present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form ‘ -fno-working-directory ’.
If the ‘ -P ’ flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever.
-A predicate = answer
Make an assertion with the predicate predicate and answer answer . This form is preferred to the older form ‘ -A predicate ( answer ) ’, which is still supported, because it does not use shell special characters.
-A predicate = answer
Cancel an assertion with the predicate predicate and answer answer .
-C Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.
254 Using the GNU Compiler Collection (GCC)
-CC
-P
You should be prepared for side effects when using ‘ -C ’; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a ‘ # ’.
Do not discard comments, including during macro expansion. This is like ‘ -C ’, except that comments contained within macros are also passed through to the output file where the macro is expanded.
In addition to the side effects of the ‘ -C ’ option, the ‘ -CC ’ option causes all
C ++ -style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.
The ‘ -CC ’ option is generally used to support lint comments.
Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO
C preprocessors. See the GNU CPP manual for details.
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the ‘ -E ’ switch, or when invoking CPP explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character sequences, all starting with
‘ ??
’, that are defined by ISO C to stand for single characters. For example, ‘ ??/ ’ stands for ‘ \ ’, so ‘ ’??/n’ ’ is a character constant for a newline.
The nine trigraphs and their replacements are
Trigraph:
Replacement:
??( ??) ??< ??> ??= ??/ ??’ ??!
??-
[ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the ‘ -std ’ and ‘ -ansi ’ options.
-remap
-H
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
Print the name of each header file used, in addition to other normal activities.
Each name is indented to show how deep in the ‘ #include ’ stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ‘ ...x
’ and a valid one with ‘ ...!
’ .
-d letters
Says to make debugging dumps during compilation as specified by letters . The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the
Chapter 3: GCC Command Options 255 result is undefined. See
Section 3.18 [Developer Options], page 275
, for more information.
-dM
-dD
-dN
-dI
-dU
Instead of the normal output, generate a list of ‘ #define ’ directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file ‘ foo.h
’, the command touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use ‘ -dM ’ without the ‘ -E ’ option, ‘ -dM ’ is interpreted as a synonym for ‘ -fdump-rtl-mach ’. See Section “Developer Options” in gcc .
Like ‘ -dM ’ except in two respects: it does not include the predefined macros, and it outputs both the ‘ #define ’ directives and the result of preprocessing. Both kinds of output go to the standard output file.
Like ‘ -dD ’, but emit only the macro names, not their expansions.
Output ‘ #include ’ directives in addition to the result of preprocessing.
Like ‘ -dD ’ except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and ‘ #undef ’ directives are also output for macros tested but undefined at the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used from CPP or with
‘ -E ’, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to.
When used from GCC without ‘ -E ’, this option has no effect.
-Wp, option
You can use ‘ -Wp, option ’ to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and ‘ -Wp ’ forcibly bypasses this phase. The preprocessor’s direct interface is undocumented and subject to change, so whenever possible you should avoid using ‘ -Wp ’ and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor.
You can use this to supply system-specific preprocessor options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use
‘ -Xpreprocessor ’ twice, once for the option and once for the argument.
256 Using the GNU Compiler Collection (GCC)
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing.
If this option is provided, the appropriate language front end ( cc1 , cc1plus , or cc1obj for C, C ++ , and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the ‘ -B ’ or ‘ -wrapper ’ options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation.
-flarge-source-files
Adjust GCC to expect large source files, at the expense of slower compilation and higher memory usage.
Specifically, GCC normally tracks both column numbers and line numbers within source files and it normally prints both of these numbers in diagnostics. However, once it has processed a certain number of source lines, it stops tracking column numbers and only tracks line numbers. This means that diagnostics for later lines do not include column numbers. It also means that options like ‘ -Wmisleading-indentation ’ cease to work at that point, although the compiler prints a note if this happens. Passing ‘ -flarge-source-files ’ significantly increases the number of source lines that GCC can process before it stops tracking columns.
3.14 Passing Options to the Assembler
You can pass options to the assembler.
-Wa, option
Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply systemspecific assembler options that GCC does not recognize.
If you want to pass an option that takes an argument, you must use
‘ -Xassembler ’ twice, once for the option and once for the argument.
3.15 Options for Linking
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.
Chapter 3: GCC Command Options 257
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be used as arguments. See
Section 3.2 [Overall Options], page 33
.
-flinker-output= type
This option controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin. For debugging the compiler and if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually.
If type is ‘ exec ’, code generation produces a static binary. In this case ‘ -fpic ’ and ‘ -fpie ’ are both disabled.
If type is ‘ dyn ’, code generation produces a shared library. In this case ‘ -fpic ’ or ‘ -fPIC ’ is preserved, but not enabled automatically. This allows to build shared libraries without position-independent code on architectures where this is possible, i.e. on x86.
If type is ‘ pie ’, code generation produces an ‘ -fpie ’ executable. This results in similar optimizations as ‘ exec ’ except that ‘ -fpie ’ is not disabled if specified at compilation time.
If type is ‘ rel ’, the compiler assumes that incremental linking is done.
The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if
‘ -ffat-lto-objects ’ is specified, binary code is produced for future non-LTO linking.
The object file produced by incremental linking is smaller than a static library produced from the same object files. At link time the result of incremental linking also loads faster than a static library assuming that the majority of objects in the library are used.
Finally ‘ nolto-rel ’ configures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of benefits of whole program optimizations are lost.
During the incremental link (by ‘ -r ’) the linker plugin defaults to ‘ rel ’. With current interfaces to GNU Binutils it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of object files in incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses ‘ nolto-rel ’. To maintain whole program optimization, it is recommended to link such objects into static library instead. Alternatively it is possible to use H.J. Lu’s binutils with support for mixed objects.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
258 Using the GNU Compiler Collection (GCC)
-fuse-ld=lld
Use the LLVM lld linker instead of the default linker.
-fuse-ld=mold
Use the Modern Linker ( mold ) instead of the default linker.
-l library
-l library
Search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)
The ‘ -l ’ option is passed directly to the linker by GCC. Refer to your linker documentation for exact details. The general description below applies to the
GNU linker.
The linker searches a standard list of directories for the library. The directories searched include several standard system directories plus any that you specify with ‘ -L ’.
Static libraries are archives of object files, and have file names like
‘ lib library .a
’. Some targets also support shared libraries, which typically have names like ‘ lib library .so
’.
If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the
‘ -static ’ option is used.
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘ foo.o -lz bar.o
’ searches library ‘ z ’ after file ‘ foo.o
’ but before
‘ bar.o
’. If ‘ bar.o
’ refers to functions in ‘ z ’, those functions may not be loaded.
-lobjc You need this special case of the ‘ -l ’ option in order to link an Objective-C or
Objective-C ++ program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used normally, unless ‘ -nostdlib ’, ‘ -nolibc ’, or ‘ -nodefaultlibs ’ is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as ‘ -static-libgcc ’ or ‘ -shared-libgcc ’, are ignored. The standard startup files are used normally, unless ‘ -nostartfiles ’ is used.
The compiler may generate calls to memcmp , memset , memcpy and memmove .
These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
-nolibc Do not use the C library or system libraries tightly coupled with it when linking. Still link with the startup files, ‘ libgcc ’ or toolchain provided language support libraries such as ‘ libgnat ’, ‘ libgfortran ’ or ‘ libstdc++ ’ unless options preventing their inclusion are used as well. This typically removes ‘ -lc ’ from the link command line, as well as system libraries that normally go with it
Chapter 3: GCC Command Options 259 and become meaningless when absence of a C library is assumed, for example
‘ -lpthread ’ or ‘ -lm ’ in some configurations. This is intended for bare-board targets when there is indeed no C library available.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as ‘ -static-libgcc ’ or
‘ -shared-libgcc ’, are ignored.
The compiler may generate calls to memcmp , memset , memcpy and memmove .
These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by ‘ -nostdlib ’ and ‘ -nodefaultlibs ’ is ‘ libgcc.a
’, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. (See
Section “Interfacing to GCC Output” in GNU Compiler Collection (GCC) Internals , for more discussion of ‘ libgcc.a
’.) In most cases, you need ‘ libgcc.a
’ even when you want to avoid other standard libraries. In other words, when you specify ‘ -nostdlib ’ or ‘ -nodefaultlibs ’ you should usually specify ‘ -lgcc ’ as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main , used to ensure C ++ constructors are called; see Section “ collect2 ” in GNU
Compiler Collection (GCC) Internals .)
-e entry
--entry= entry
Specify that the program entry point is entry . The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address.
-pie Produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (‘ -fpie ’, ‘ -fPIE ’, or model suboptions) when you specify this linker option.
-no-pie Don’t produce a dynamically linked position independent executable.
-static-pie
Produce a static position independent executable on targets that support it.
A static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (‘ -fpie ’,
‘ -fPIE ’, or model suboptions) when you specify this linker option.
-pthread Link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking.
-r Produce a relocatable object as output. This is also known as partial linking.
260 Using the GNU Compiler Collection (GCC)
-rdynamic
Pass the flag ‘ -export-dynamic ’ to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the executable.
-static On systems that support dynamic linking, this overrides ‘ -pie ’ and prevents linking with the shared libraries. On other systems, this option has no effect.
-shared Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (‘ -fpic ’,
‘ -fPIC ’, or model suboptions) when you specify this linker option.
1
-shared-libgcc
-static-libgcc
On systems that provide ‘ libgcc ’ as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of
‘ libgcc ’ was built when the compiler was configured, these options have no effect.
There are several situations in which an application should use the shared
‘ libgcc ’ instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared ‘ libgcc ’.
Therefore, the G ++ driver automatically adds ‘ -shared-libgcc ’ whenever you build a shared library or a main executable, because C ++ programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared ‘ libgcc ’. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option ‘ --eh-frame-hdr ’, it links the shared version of ‘ libgcc ’ into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of ‘ libgcc ’, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G ++ driver, or using the option
‘ -shared-libgcc ’, such that it is linked with the shared ‘ libgcc ’.
-static-libasan
When the ‘ -fsanitize=address ’ option is used to link a program, the GCC driver automatically links against ‘ libasan ’.
If ‘ libasan ’ is available as a
1
On some systems, ‘ gcc -shared ’ needs to build supplementary stub code for constructors to work. On multi-libbed systems, ‘ gcc -shared ’ must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.
Chapter 3: GCC Command Options 261 shared library, and the ‘ -static ’ option is not used, then this links against the shared version of ‘ libasan ’. The ‘ -static-libasan ’ option directs the GCC driver to link ‘ libasan ’ statically, without necessarily linking other libraries statically.
-static-libtsan
When the ‘ -fsanitize=thread ’ option is used to link a program, the GCC driver automatically links against ‘ libtsan ’.
If ‘ libtsan ’ is available as a shared library, and the ‘ -static ’ option is not used, then this links against the shared version of ‘ libtsan ’. The ‘ -static-libtsan ’ option directs the GCC driver to link ‘ libtsan ’ statically, without necessarily linking other libraries statically.
-static-liblsan
When the ‘ -fsanitize=leak ’ option is used to link a program, the GCC driver automatically links against ‘ liblsan ’. If ‘ liblsan ’ is available as a shared library, and the ‘ -static ’ option is not used, then this links against the shared version of ‘ liblsan ’. The ‘ -static-liblsan ’ option directs the GCC driver to link ‘ liblsan ’ statically, without necessarily linking other libraries statically.
-static-libubsan
When the ‘ -fsanitize=undefined ’ option is used to link a program, the GCC driver automatically links against ‘ libubsan ’. If ‘ libubsan ’ is available as a shared library, and the ‘ -static ’ option is not used, then this links against the shared version of ‘ libubsan ’. The ‘ -static-libubsan ’ option directs the GCC driver to link ‘ libubsan ’ statically, without necessarily linking other libraries statically.
-static-libstdc++
When the g++ program is used to link a C ++ program, it normally automatically links against ‘ libstdc++ ’. If ‘ libstdc++ ’ is available as a shared library, and the ‘ -static ’ option is not used, then this links against the shared version of
‘ libstdc++ ’. That is normally fine. However, it is sometimes useful to freeze the version of ‘ libstdc++ ’ used by the program without going all the way to a fully static link. The ‘ -static-libstdc++ ’ option directs the g++ driver to link ‘ libstdc++ ’ statically, without necessarily linking other libraries statically.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option ‘ -Xlinker
-z -Xlinker defs ’). Only a few systems support this option.
-T script Use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the ‘ -T ’ option may be required when linking to avoid references to undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.
262 Using the GNU Compiler Collection (GCC)
If you want to pass an option that takes a separate argument, you must use
‘ -Xlinker ’ twice, once for the option and once for the argument. For example, to pass ‘ -assert definitions ’, you must write ‘ -Xlinker -assert -Xlinker definitions ’. It does not work to write ‘ -Xlinker "-assert definitions" ’, because this passes the entire string as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass arguments to linker options using the ‘ option = value ’ syntax than as separate arguments. For example, you can specify ‘ -Xlinker -Map=output.map
’ rather than ‘ -Xlinker
-Map -Xlinker output.map
’. Other linkers may not support this syntax for command-line options.
-Wl, option
Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, ‘ -Wl,-Map,output.map
’ passes ‘ -Map output.map
’ to the linker. When using the GNU linker, you can also get the same effect with ‘ -Wl,-Map=output.map
’.
-u symbol Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use ‘ -u ’ multiple times with different symbols to force loading of additional library modules.
-z keyword
‘ -z ’ is passed directly on to the linker along with the keyword keyword . See the section in the documentation of your linker for permitted values and their meanings.
3.16 Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with ‘ = ’ or $SYSROOT , then the ‘ = ’ or $SYSROOT is replaced by the sysroot prefix; see ‘ --sysroot ’ and ‘ -isysroot ’.
Directories specified with ‘ -iquote ’ apply only to the quote form of the directive, #include " file " .
Directories specified with ‘ -I ’, ‘ -isystem ’, or ‘ -idirafter ’ apply to lookup for both the #include " file " and
#include < file > directives.
You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:
1. For the quote form of the include directive, the directory of the current file is searched first.
Chapter 3: GCC Command Options 263
-I-
2. For the quote form of the include directive, the directories specified by
‘ -iquote ’ options are searched in left-to-right order, as they appear on the command line.
3. Directories specified with ‘ -I ’ options are scanned in left-to-right order.
4. Directories specified with ‘ -isystem ’ options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with ‘ -idirafter ’ options are scanned in left-to-right order.
You can use ‘ -I ’ to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use ‘ -isystem ’ for that.
The ‘ -isystem ’ and ‘ -idirafter ’ options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories.
If a standard system include directory, or a directory specified with ‘ -isystem ’, is also specified with ‘ -I ’, the ‘ -I ’ option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC’s procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed.
If you really need to change the search order for system directories, use the
‘ -nostdinc ’ and/or ‘ -isystem ’ options.
Split the include path. This option has been deprecated. Please use ‘ -iquote ’ instead for ‘ -I ’ directories before the ‘ -I’ and remove the ‘ -I’ option.
Any directories specified with ‘ -I ’ options before ‘ -I’ are searched only for headers requested with #include " file " ; they are not searched for
#include < file > . If additional directories are specified with ‘ -I ’ options after the ‘ -I’, those directories are searched for all ‘ #include ’ directives.
In addition, ‘ -I’ inhibits the use of the directory of the current file directory as the first search directory for #include " file " . There is no way to override this effect of ‘ -I’.
-iprefix prefix
Specify prefix as the prefix for subsequent ‘ -iwithprefix ’ options. If the prefix represents a directory, you should include the final ‘ / ’.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with ‘ -iprefix ’, and add the resulting directory to the include search path. ‘ -iwithprefixbefore ’ puts it in the same place ‘ -I ’ would; ‘ -iwithprefix ’ puts it where ‘ -idirafter ’ would.
-isysroot dir
This option is like the ‘ --sysroot ’ option, but applies only to header files
(except for Darwin targets, where it applies to both header files and libraries).
See the ‘ --sysroot ’ option for more information.
264 Using the GNU Compiler Collection (GCC)
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C ++ headers.
-nostdinc
Do not search the standard system directories for header files. Only the directories explicitly specified with ‘ -I ’, ‘ -iquote ’, ‘ -isystem ’, and/or ‘ -idirafter ’ options (and the directory of the current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C ++ -specific standard directories, but do still search the other standard directories. (This option is used when building the C ++ library.)
-iplugindir= dir
Set the directory to search for plugins that are passed by ‘ -fplugin= name ’ instead of ‘ -fplugin= path / name .so
’. This option is not meant to be used by the user, but only passed by the driver.
-L dir Add directory dir to the list of directories to be searched for ‘ -l ’.
-B prefix This option specifies where to find the executables, libraries, include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms cpp , cc1 , as and ld . It tries prefix as a prefix for each program it tries to run, both with and without ‘ machine / version / ’ for the corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries the ‘ -B ’ prefix, if any. If that name is not found, or if ‘ -B ’ is not specified, the driver tries two standard prefixes, ‘ /usr/lib/gcc/ ’ and ‘ /usr/local/lib/gcc/ ’. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.
The compiler checks to see if the path provided by ‘ -B ’ refers to a directory, and if necessary it adds a directory separator character at the end of the path.
‘ -B ’ prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into ‘ -L ’ options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into ‘ -isystem ’ options for the preprocessor.
In this case, the compiler appends ‘ include ’ to the prefix.
The runtime support file ‘ libgcc.a
’ can also be searched for using the ‘ -B ’ prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.
Another way to specify a prefix much like the ‘ -B ’ prefix is to use the environment variable GCC_EXEC_PREFIX . See
Section 3.21 [Environment Variables], page 496
.
As a special kludge, if the path provided by ‘ -B ’ is ‘ [dir/]stage N / ’, where N is a number in the range 0 to 9, then it is replaced by ‘ [dir/]include ’. This is to help with boot-strapping the compiler.
Chapter 3: GCC Command Options 265
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to ‘ /../ ’ or ‘ /./ ’, or make the path absolute when generating a relative prefix.
--sysroot= dir
Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in ‘ /usr/include ’ and libraries in
‘ /usr/lib ’, it instead searches ‘ dir /usr/include ’ and ‘ dir /usr/lib ’.
If you use both this option and the ‘ -isysroot ’ option, then the ‘ --sysroot ’ option applies to libraries, but the ‘ -isysroot ’ option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of ‘ --sysroot ’ still works, but the library aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory specified with
‘ --sysroot ’, depending on the other options used, so that headers may for example be found in ‘ dir / suffix /usr/include ’ instead of ‘ dir /usr/include ’.
This option disables the addition of such a suffix.
3.17 Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of ‘ -ffoo ’ is
‘ -fno-foo ’. In the table below, only one of the forms is listed—the one that is not the default. You can figure out the other form by either removing ‘ no’ or adding it.
-fstack-reuse= reuse-level
This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries.
reuse level can be ‘ all ’, ‘ named_vars ’, or ‘ none ’. ‘ all ’ enables stack reuse for all local variables and temporaries,
‘ named_vars ’ enables the reuse only for user defined local variables with names, and ‘ none ’ disables stack reuse completely. The default value is ‘ all ’. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization.
For example, int *p;
{ int local1; p = &local1; local1 = 10;
....
266 Using the GNU Compiler Collection (GCC)
}
{
} int local2; local2 = 20;
...
if (*p == 10) // out of scope use of local1
{
}
Another example: struct A
{
A(int k) : i(k), j(k) { } int i; int j;
};
A *ap; void foo(const A& ar)
{ ap = &ar;
} void bar()
{ foo(A(10)); // temp object’s lifetime ends when foo returns
}
{
A a(20);
....
} ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
The lifetime of a compiler generated temporary is well defined by the C ++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries’ stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization.
-ftrapv This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options ‘ -ftrapv ’ and ‘ -fwrapv ’ override each other, so using ‘ -ftrapv ’ ‘ -fwrapv ’ on the command-line results in ‘ -fwrapv ’ being effective. Note that only active options override, so using ‘ -ftrapv ’ ‘ -fwrapv ’
‘ -fno-wrapv ’ on the command-line results in ‘ -ftrapv ’ being effective.
Chapter 3: GCC Command Options 267
-fwrapv This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation.
This flag enables some optimizations and disables others.
The options ‘ -ftrapv ’ and ‘ -fwrapv ’ override each other, so using ‘ -ftrapv ’
‘ -fwrapv ’ on the command-line results in ‘ -fwrapv ’ being effective. Note that only active options override, so using ‘ -ftrapv ’ ‘ -fwrapv ’ ‘ -fno-wrapv ’ on the command-line results in ‘ -ftrapv ’ being effective.
-fwrapv-pointer
This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation.
This flag disables some optimizations which assume pointer overflow is invalid.
-fstrict-overflow
This option implies ‘ -fno-wrapv ’ ‘ -fno-wrapv-pointer ’ and when negated implies ‘ -fwrapv ’ ‘ -fwrapv-pointer ’.
-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC enables it by default for languages like C ++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C ++ . You may also wish to disable this option if you are compiling older C ++ programs that don’t use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions.
Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e.
memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as SIGALRM . This enables
‘ -fexceptions ’.
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don’t otherwise contribute to the execution of the program can be optimized away. This does not affect calls to functions except those with the pure or const attributes. This option is enabled by default for the Ada and C ++ compilers, as permitted by the language specifications. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels.
-funwind-tables
Similar to ‘ -fexceptions ’, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf.
268 Using the GNU Compiler Collection (GCC)
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target machine.
The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C ++ compiler uses the STB_GNU_UNIQUE binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of RTLD_LOCAL ; this is necessary to avoid problems with a library used by two different RTLD_LOCAL plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol.
But this causes dlclose to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via dlclose and dlopen , you can use ‘ -fno-gnu-unique ’.
-fpcc-struct-return
Return “short” struct and union values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the ‘ -fpcc-struct-return ’ switch is not binary compatible with code compiled with the ‘ -freg-struct-return ’ switch. Use it to conform to a non-default application binary interface.
-freg-struct-return
Return struct and union values in registers when possible. This is more efficient for small structures than ‘ -fpcc-struct-return ’.
If you specify neither ‘ -fpcc-struct-return ’ nor ‘ -freg-struct-return ’,
GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to ‘ -fpcc-struct-return ’, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.
Warning: code compiled with the ‘ -freg-struct-return ’ switch is not binary compatible with code compiled with the ‘ -fpcc-struct-return ’ switch. Use it to conform to a non-default application binary interface.
-fshort-enums
Allocate to an enum type only as many bytes as it needs for the declared range of possible values. Specifically, the enum type is equivalent to the smallest integer type that has enough room.
Warning: the ‘ -fshort-enums ’ switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
Chapter 3: GCC Command Options 269
-fshort-wchar
Override the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is useful for building programs to run under WINE.
Warning: the ‘ -fshort-wchar ’ switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fcommon In C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with the extern keyword, which do not allocate storage.
The default is ‘ -fno-common ’, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit.
The ‘ -fcommon ’ places uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition.
This behavior is inconsistent with C ++ , and on many targets implies a speed and code size penalty on global variable references. It is mainly useful to enable legacy code to link without errors.
-fno-ident
Ignore the #ident directive.
-finhibit-size-directive
Don’t output a .size
assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling ‘ crtstuff.c
’; you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself).
‘ -fno-verbose-asm ’, the default, causes the extra information to be omitted and is useful when comparing two assembler files.
The added comments include:
• information on the compiler version and command-line options,
• the source code lines associated with the assembly instructions, in the form
FILENAME:LINENUMBER:CONTENT OF LINE,
• hints on which high-level expressions correspond to the various assembly instruction operands.
For example, given this C source file:
270 Using the GNU Compiler Collection (GCC) int test (int n)
{ int i; int total = 0; for (i = 0; i < n; i++) total += i * i; return total;
} compiling to (x86 64) assembly via ‘ -S ’ and emitting the result direct to stdout via ‘ -o ’ ‘ ’ gcc -S test.c -fverbose-asm -Os -o gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0; xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++) xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++) cmpl %edi, %edx # n, i jge .L5 #,
# test.c:7: total += i * i; movl %edx, %ecx # i, tmp92 imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++) incl %edx # i
# test.c:7: total += i * i; addl %ecx, %eax # tmp92, <retval> jmp .L2 #
.L5:
# test.c:10: } ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change.
-frecord-gcc-switches
This switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. This switch is only implemented on
Chapter 3: GCC Command Options 271
-fpic
-fPIC some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the ‘ -fverbose-asm ’ switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also ‘ -grecord-gcc-switches ’ for another way of storing compiler options into the object file.
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that ‘ -fpic ’ does not work; in that case, recompile with ‘ -fPIC ’ instead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the
Sun 386i. Code generated for the IBM RS/6000 is always position-independent.
When this flag is set, the macros __pic__ and __PIC__ are defined to 1.
If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table.
This option makes a difference on AArch64, m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
When this flag is set, the macros __pic__ and __PIC__ are defined to 2.
-fpie
-fPIE These options are similar to ‘ -fpic ’ and ‘ -fPIC ’, but the generated positionindependent code can be only linked into executables. Usually these options are used to compile code that will be linked using the ‘ -pie ’ GCC option.
‘ -fpie ’ and ‘ -fPIE ’ both define the macros __pie__ and __PIE__ . The macros have the value 1 for ‘ -fpie ’ and 2 for ‘ -fPIE ’.
-fno-plt Do not use the PLT for external function calls in position-independent code.
Instead, load the callee address at call sites from the GOT and branch to it.
This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with ‘ -fno-plt ’ all external symbols are resolved at load time.
Alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.
272 Using the GNU Compiler Collection (GCC)
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with ‘ -fpic ’ or ‘ -fPIC ’ for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed.
-fno-bit-tests
Do not use bit tests for switch statements even where it would be more efficient than other code generation strategies.
-ffixedreg
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machinespecific and are defined in the REGISTER_NAMES macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-usedreg
Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg .
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine’s execution model produces disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-savedreg
Treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine’s execution model produces disastrous results.
A different sort of disaster results from the use of this flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[= n ]
Without a value specified, pack all structure members together without holes.
When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location.
Warning: the ‘ -fpack-struct ’ switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally,
Chapter 3: GCC Command Options 273 it makes the code suboptimal. Use it to conform to a non-default application binary interface.
-fleading-underscore
This option and its counterpart, ‘ -fno-leading-underscore ’, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.
Warning: the ‘ -fleading-underscore ’ switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.
-ftls-model= model
Alter the thread-local storage model to be used (see
). The model argument should be one of ‘ global-dynamic ’,
‘ local-dynamic ’, ‘ initial-exec ’ or ‘ local-exec ’. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if ‘ -fpic ’ is not given on the command line.
The default without ‘ -fpic ’ is ‘ initial-exec ’; with ‘ -fpic ’ the default is
‘ global-dynamic ’.
-ftrampolines
For targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that do not need them, like for example HP-PA or IA-64, do nothing.
A trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly.
‘ -fno-trampolines ’ is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing,
‘ -fno-trampolines ’ is enabled by default only for Ada.
Moreover, code compiled with ‘ -ftrampolines ’ and code compiled with
‘ -fno-trampolines ’ are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care.
For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.
-fvisibility= [ default|internal|hidden|protected ]
Set the default ELF image symbol visibility to the specified option—all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent
274 Using the GNU Compiler Collection (GCC) symbol clashes. It is strongly recommended that you use this in any shared objects you distribute.
Despite the nomenclature, ‘ default ’ always means public; i.e., available to be linked against from outside the shared object. ‘ protected ’ and ‘ internal ’ are pretty useless in real-world usage so the only other commonly used option is
‘ hidden ’. The default if ‘ -fvisibility ’ isn’t specified is ‘ default ’, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by “How To Write Shared Libraries” by Ulrich
Drepper (which can be found at https://www.akkadia.org/drepper/ )— however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public.
This is the norm with DLLs on Windows and with
‘ -fvisibility=hidden ’ and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects.
For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop . Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads—making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C ++ specification requirements, operator new and operator delete must always be of default visibility.
Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say
#pragma GCC visibility push(default) before including any such headers.
extern declarations are not affected by ‘ -fvisibility ’, so a lot of code can be recompiled with ‘ -fvisibility=hidden ’ with no modifications. However, this means that calls to extern functions with no explicit visibility use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.
Note that ‘ -fvisibility ’ does affect C ++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the ‘ type_info ’ nodes are unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at https://gcc.gnu.org/wiki/Visibility .
Chapter 3: GCC Command Options 275
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field’s type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient
32-bit access.
If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memorymapped registers unrelated to the one being updated.
In some cases, such as when the packed attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time.
Note: Due to restrictions of the C/C ++ 11 memory model, write accesses are not allowed to touch non bit-field members. It is therefore recommended to define all bits of the field’s type as bit-field members.
The default value of this option is determined by the application binary interface for the target processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the __sync family of functions may be used to implement the C ++ 11 __atomic family of functions.
The default value of this option is enabled, thus the only useful form of the option is ‘ -fno-sync-libcalls ’. This option is used in the implementation of the ‘ libatomic ’ runtime library.
3.18 GCC Developer Options
This section describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC’s configuration, such as where it searches for libraries. You should rarely need to use any of these options for ordinary compilation and linking tasks.
Many developer options that cause GCC to dump output to a file take an optional
‘ = filename ’ suffix.
You can specify ‘ stdout ’ or ‘ ’ to dump to standard output, and
‘ stderr ’ for standard error.
If ‘ = filename ’ is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not
276 Using the GNU Compiler Collection (GCC) an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler’s pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named ‘ final ’, even if they are executed earlier. The phase letter is one of ‘ i ’ (inter-procedural analysis), ‘ l ’
(language-specific), ‘ r ’ (RTL), or ‘ t ’ (tree). The files are created in the directory of the output file.
-fcallgraph-info
-fcallgraph-info= MARKERS
Makes the compiler output callgraph information for the program, on a perobject-file basis. The information is generated in the common VCG format.
It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified. When the su marker is specified, the callgraph is decorated with stack usage information; it is equivalent to ‘ -fstack-usage ’.
When the da marker is specified, the callgraph is decorated with information about dynamically allocated objects.
When compiling with ‘ -flto ’, no callgraph information is output along with the object file. At LTO link time, ‘ -fcallgraph-info ’ may generate multiple callgraph information files next to intermediate LTO output files.
-d letters
-fdump-rtlpass
-fdump-rtlpass = filename
Says to make debugging dumps during compilation at times specified by letters .
This is used for debugging the RTL-based passes of the compiler.
Some ‘ -d letters ’ switches have different meaning when ‘ -E ’ is used for preprocessing. See
Section 3.13 [Preprocessor Options], page 248
, for information about preprocessor-specific dump options.
Debug dumps can be enabled with a ‘ -fdump-rtl ’ switch or some ‘ -d ’ option letters .
Here are the possible letters for use in pass and letters , and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
Chapter 3: GCC Command Options 277
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
‘ -fdump-rtl-btl1 ’ and ‘ -fdump-rtl-btl2 ’ enable dumping after the two branch target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
‘ -fdump-rtl-ce1 ’, ‘ -fdump-rtl-ce2 ’, and ‘ -fdump-rtl-ce3 ’ enable dumping after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
‘ -fdump-rtl-cse1 ’ and ‘ -fdump-rtl-cse2 ’ enable dumping after the two common subexpression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
‘ -fdump-rtl-dce1 ’ and ‘ -fdump-rtl-dce2 ’ enable dumping after the two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
278 Using the GNU Compiler Collection (GCC)
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
‘ -fdump-rtl-fwprop1 ’ and ‘ -fdump-rtl-fwprop2 ’ enable dumping after the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
‘ -fdump-rtl-gcse1 ’ and ‘ -fdump-rtl-gcse2 ’ enable dumping after global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
‘ -fdump-rtl-loop2 ’ enables dumping after the rtl loop optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
‘ -fdump-rtl-sched1 ’ and ‘ -fdump-rtl-sched2 ’ enable dumping after the basic block scheduling passes.
Chapter 3: GCC Command Options 279
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some architectures.
-fdump-rtl-stack
Dump after conversion from GCC’s “flat register file” registers to the x87’s stack-like registers. This pass is only run on x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
‘ -fdump-rtl-subreg1 ’ and ‘ -fdump-rtl-subreg2 ’ enable dumping after the two subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
280 Using the GNU Compiler Collection (GCC)
-dA
-dD
-dH
-dp
-dP
-dx
Annotate the assembler output with miscellaneous debugging information.
Dump all macro definitions, at the end of preprocessing, in addition to normal output.
Produce a core dump whenever an error occurs.
Annotate the assembler output with a comment indicating which pattern and alternative is used. The length and cost of each instruction are also printed.
Dump the RTL in the assembler output as a comment before each instruction. Also turns on ‘ -dp ’ annotation.
Just generate RTL for a function instead of compiling it. Usually used with ‘ -fdump-rtl-expand ’.
-fdump-debug
Dump debugging information generated during the debug generation phase.
-fdump-earlydebug
Dump debugging information generated during the early debug generation phase.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations.
-freport-bug
Collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without ‘ -g ’.
-fdump-unnumbered-links
When doing debugging dumps (see ‘ -d ’ option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.
-fdump-ipaswitch
-fdump-ipaswitch options
Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file.
The following dumps are possible:
‘ all ’ Enables all inter-procedural analysis dumps.
‘ cgraph ’ Dumps information about call-graph optimization, unused function removal, and inlining decisions.
Chapter 3: GCC Command Options 281
‘ inline ’ Dump after function inlining.
Additionally, the options ‘ -optimized ’, ‘ -missed ’, ‘ -note ’, and ‘ -all ’ can be provided, with the same meaning as for ‘ -fopt-info ’, defaulting to
‘ -optimized ’.
For example, ‘ -fdump-ipa-inline-optimized-missed ’ will emit information on callsites that were inlined, along with callsites that were not inlined.
By default, the dump will contain messages about successful optimizations
(equivalent to ‘ -optimized ’) together with low-level details about the analysis.
-fdump-lang
Dump language-specific information.
The file name is made by appending
‘ .lang
’ to the source file name.
-fdump-lang-all
-fdump-langswitch
-fdump-langswitch options
-fdump-langswitch options = filename
Control the dumping of language-specific information. The options and filename portions behave as described in the ‘ -fdump-tree ’ option. The following switch values are accepted:
‘ all ’
‘ class ’
Enable all language-specific dumps.
Dump class hierarchy information.
Virtual table information is emitted unless ’‘ slim ’’ is specified. This option is applicable to
C ++ only.
‘ module ’ Dump module information. Options ‘ lineno ’ (locations), ‘ graph ’
(reachability), ‘ blocks ’ (clusters), ‘ uid ’ (serialization), ‘ alias ’
(mergeable), ‘ asmname ’ (Elrond), ‘ eh ’ (mapper) & ‘ vops ’ (macros) may provide additional information. This option is applicable to
C ++ only.
‘ raw ’ Dump the raw internal tree data. This option is applicable to C ++ only.
-fdump-passes
Print on ‘ stderr ’ the list of optimization passes that are turned on and off by the current command-line options.
-fdump-statisticsoption
Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in ‘ .statistics
’ to the source file name, and the file is created in the same directory as the output file. If the
‘ option ’ form is used, ‘ -stats ’ causes counters to be summed over the whole compilation unit while ‘ -details ’ dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled.
282 Using the GNU Compiler Collection (GCC)
-fdump-tree-all
-fdump-treeswitch
-fdump-treeswitch options
-fdump-treeswitch options = filename
Control the dumping at various stages of processing the intermediate language tree to a file. If the ‘ options ’ form is used, options is a list of ‘ ’ separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available
‘ address ’ Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.
‘ asmname ’ If DECL_ASSEMBLER_NAME has been set for a given decl, use that in the dump instead of DECL_NAME . Its primary use is ease of use working backward from mangled names in the assembly file.
‘ slim ’ When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.
‘ raw ’ Print a raw representation of the tree. By default, trees are prettyprinted into a C-like representation.
‘ details ’ Enable more detailed dumps (not honored by every dump option).
Also include information from the optimization passes.
‘ stats ’ Enable dumping various statistics about the pass (not honored by every dump option).
‘ blocks ’ Enable showing basic block boundaries (disabled in raw dumps).
‘ graph ’ For each of the other indicated dump files (‘ -fdump-rtlpass ’), dump a representation of the control flow graph suitable for viewing with GraphViz to ‘ file .
passid .
pass .dot
’. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is always dumped in slim form.
‘ vops ’ Enable showing virtual operands for every statement.
‘ lineno ’ Enable showing line numbers for statements.
‘ uid ’ Enable showing the unique ID ( DECL_UID ) for each variable.
‘ verbose ’ Enable showing the tree dump for each statement.
Chapter 3: GCC Command Options 283
‘
‘ eh ’ scev ’
Enable showing the EH region number holding each statement.
Enable showing scalar evolution analysis details.
‘ optimized ’
Enable showing optimization information (only available in certain passes).
‘ missed ’ Enable showing missed optimization information (only available in certain passes).
‘ note ’ Enable other detailed optimization information (only available in certain passes).
‘ all ’ Turn on all options, except ‘ raw ’, ‘ slim ’, ‘ verbose ’ and ‘ lineno ’.
‘ optall ’ Turn on all optimization options, i.e., ‘ optimized ’, ‘ missed ’, and
‘ note ’.
To determine what tree dumps are available or find the dump for a pass of interest follow the steps below.
1. Invoke GCC with ‘ -fdump-passes ’ and in the ‘ stderr ’ output look for a code that corresponds to the pass you are interested in. For example, the codes tree-evrp , tree-vrp1 , and tree-vrp2 correspond to the three
Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass.
2. To enable the creation of the dump file, append the pass code to the
‘ -fdump’ option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the ‘ -fdump-tree-evrp ’ option. Optionally, you may specify the name of the dump file. If you don’t specify one, GCC creates as described below.
3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter
‘ t ’ for tree passes (and the letter ‘ r ’ for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named
‘ myfile.c.038t.evrp
’ in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another.
-fopt-info
-fopt-infooptions
-fopt-infooptions = filename
Controls optimization dumps from various optimization passes.
If the
‘ options ’ form is used, options is a list of ‘ ’ separated option keywords to select the dump details and optimizations.
The options can be divided into three groups:
1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.
284 Using the GNU Compiler Collection (GCC)
The options from each group can be freely mixed as they are non-overlapping.
However, in case of any conflicts, the later options override the earlier options on the command line.
The following options control which kinds of messages should be emitted:
‘ optimized ’
Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.
‘ missed ’ Print information about missed optimizations. Individual passes control which information to include in the output.
‘ note ’ Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
This includes ‘ all ’ Print detailed optimization information.
‘ optimized ’, ‘ missed ’, and ‘ note ’.
The following option controls the dump verbosity:
‘ internals ’
By default, only “high-level” messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers.
One or more of the following option keywords can be used to describe a group of optimizations:
‘
‘ ipa ’ loop ’
Enable dumps from all interprocedural optimizations.
Enable dumps from all loop optimizations.
‘ inline ’ Enable dumps from all inlining optimizations.
‘ omp ’ Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
Enable dumps from all vectorization optimizations.
‘ vec ’
‘ optall ’ Enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to ‘ optimized-optall ’, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as “internals”.
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename . Otherwise the dump is output onto
‘ stderr ’. Though multiple ‘ -fopt-info ’ options are accepted, only one of them can include a filename . If other filenames are provided then all but the first such option are ignored.
Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, ‘ stderr ’ should be used instead.
Chapter 3: GCC Command Options 285
In the following example, the optimization info is output to ‘ stderr ’: gcc -O3 -fopt-info
This example: gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into ‘ missed.all
’, and this one: gcc -O2 -ftree-vectorize -fopt-info-vec-missed prints information about missed optimization opportunities from vectorization passes on ‘ stderr ’.
Note that ‘ -fopt-info-vec-missed ’ is equivalent to ‘ -fopt-info-missed-vec ’. The order of the optimization group names and message types listed after ‘ -fopt-info ’ does not matter.
As another example, gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized locations from all the inlining passes into ‘ inline.txt
’.
Finally, consider: gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames ‘ vec.miss
’ and ‘ loop.opt
’ are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only ‘ vec.miss
’ is produced which contains dumps from the vectorizer about missed opportunities.
-fsave-optimization-record
Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support ‘ -fopt-info ’.
This option is experimental and the format of the data within the compressed
JSON file is subject to change.
It is roughly equivalent to a machine-readable version of ‘ -fopt-info-all ’, as a collection of messages with source file, line number and column number, with the following additional data for each message:
• the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
• the function name of the code being optimized, where applicable,
• the “inlining chain” for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
• objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
• the GCC pass that emitted the message, and
• the location in GCC’s own code from which the message was emitted
Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes.
286 Using the GNU Compiler Collection (GCC)
-fsched-verbose= n
On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.
For n greater than zero, ‘ -fsched-verbose ’ outputs the same information as
‘ -fdump-rtl-sched1 ’ and ‘ -fdump-rtl-sched2 ’. For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, ‘ -fsched-verbose ’ also includes dependence info.
-fenablekind pass
-fdisablekind pass = range-list
This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.
-fdisable-ipapass
Disable IPA pass pass .
pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
-fdisable-rtlpass
-fdisable-rtlpass = range-list
Disable RTL pass pass .
pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon.
The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function’s call graph node’s uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option ‘ -fdump-passes ’.
-fdisable-treepass
-fdisable-treepass = range-list
Disable tree pass pass . See ‘ -fdisable-rtl ’ for the description of option arguments.
-fenable-ipapass
Enable IPA pass pass .
pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
-fenable-rtlpass
-fenable-rtlpass = range-list
Enable RTL pass pass . See ‘ -fdisable-rtl ’ for option argument description and examples.
Chapter 3: GCC Command Options 287
-fenable-treepass
-fenable-treepass = range-list
Enable tree pass pass . See ‘ -fdisable-rtl ’ for the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking= n
Enable internal consistency checking. The default depends on the compiler configuration. ‘ -fchecking=2 ’ enables further internal consistency checking that might affect code generation.
-frandom-seed= string
This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data files and the object files that produce them. You can use the ‘ -frandom-seed ’ option to produce reproducibly identical object files.
The string can either be a number (decimal, octal or hex) or an arbitrary string
(in which case it’s converted to a number by computing CRC32).
The string should be different for every file you compile.
-save-temps
Store the usual “temporary” intermediate files permanently; name them as auxiliary output files, as specified described under ‘ -dumpbase ’ and ‘ -dumpdir ’.
When used in combination with the ‘ -x ’ command-line option, ‘ -save-temps ’ is sensible enough to avoid overwriting an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using ‘ -save-temps ’.
-save-temps=cwd
Equivalent to ‘ -save-temps -dumpdir ./ ’.
-save-temps=obj
Equivalent to ‘ -save-temps -dumpdir ‘outdir/’ ’, where ‘ outdir/ ’ is the directory of the output file specified after the ‘ -o ’ option, including any directory
288 Using the GNU Compiler Collection (GCC) separators. If the ‘ -o ’ option is not used, the ‘ -save-temps=obj ’ switch behaves like ‘ -save-temps=cwd ’.
-time [ = file ]
Report the CPU time taken by each subprocess in the compilation sequence.
For C source files, this is the compiler proper and assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the “user time”, that is time spent executing the program itself. The second number is “system time”, time spent executing operating system routines on behalf of the program.
Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like this:
0.12 0.01 cc1 options
0.00 0.01 as options
The “user time” and the “system time” are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.
-fdump-final-insns [ = file ]
Dump the final internal representation (RTL) to file . If the optional argument is omitted (or if file is .
), the name of the dump file is determined by appending
.gkd
to the dump base name, see ‘ -dumpbase ’.
-fcompare-debug [ = opts ]
If no error occurs during compilation, run the compiler a second time, adding opts and ‘ -fcompare-debug-second ’ to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ.
If the equal sign is omitted, the default ‘ -gtoggle ’ is used.
The environment variable GCC_COMPARE_DEBUG , if defined, non-empty and nonzero, implicitly enables ‘ -fcompare-debug ’.
If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts , otherwise the default ‘ -gtoggle ’ is used.
‘ -fcompare-debug= ’, with the equal sign but without opts , is equivalent to
‘ -fno-compare-debug ’, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during ‘ -fcompare-debug ’ testing, set GCC_COMPARE_
DEBUG to say ‘ -fcompare-debug-not-overridden ’, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to
‘ -w%n-fcompare-debug not overridden ’ will do.
Chapter 3: GCC Command Options 289
-fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation requested by ‘ -fcompare-debug ’, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files are renamed so as to contain the .gk
additional extension during the second compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper.
-gtoggle Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with ‘ -fcompare-debug ’.
-fvar-tracking-assignments-toggle
Toggle ‘ -fvar-tracking-assignments ’, in the same way that ‘ -gtoggle ’ toggles ‘ -g ’.
-Q Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
-ftime-report-details
Record the time consumed by infrastructure parts separately for each pass.
-fira-verbose= n
Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.
-flto-report
Prints a report with internal details on the workings of the link-time optimizer.
The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via ‘ -flto ’).
Disabled by default.
-flto-report-wpa
Like ‘ -flto-report ’, but only print for the WPA phase of link-time optimization.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
290 Using the GNU Compiler Collection (GCC)
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.
-fprofile-report
Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.
-fstack-usage
Makes the compiler output stack usage information for the program, on a perfunction basis. The filename for the dump is made by appending ‘ .su
’ to the auxname .
auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:
• The name of the function.
• A number of bytes.
• One or more qualifiers: static , dynamic , bounded .
The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.
The qualifier dynamic means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.
-fstats Emit statistics about front-end processing at the end of the compilation. This option is supported only by the C ++ front end, and the information is generally only useful to the G ++ development team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt= counter-value-list
Set the internal debug counter lower and upper bound.
counter-value-list is a comma-separated list of name : lower bound1 upper bound1 [: lower bound2 upper bound2 ...] tuples which sets the name of the counter and list of closed intervals. The lower bound is optional and is zero initialized if not set. For example, with ‘ -fdbg-cnt=dce:2-4:10-11,tail_call:10 ’, dbg_cnt(dce) returns true only for second, third, fourth, tenth and eleventh invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations.
Chapter 3: GCC Command Options 291
-print-file-name= library
Print the full absolute name of the library file library that would be used when linking—and don’t do anything else. With this option, GCC does not compile or link anything; it just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in
GCC_EXEC_PREFIX .
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ‘ ; ’, and each switch starts with an ‘ @ ’ instead of the ‘ ’, without spaces between multiple switches. This is supposed to ease shell processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some ‘ lib ’ subdirectory. If OS libraries are present in the ‘ lib ’ subdirectory and no multilibs are used, this is usually just ‘ .
’, if OS libraries are present in ‘ lib suffix ’ sibling directories this prints e.g. ‘ ../lib64 ’, ‘ ../lib ’ or ‘ ../lib32 ’, or if
OS libraries are present in ‘ lib/ subdir ’ subdirectories it prints e.g. ‘ amd64 ’,
‘ sparcv9 ’ or ‘ ev6 ’.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some ‘ lib ’ subdirectory.
-print-prog-name= program
Like ‘ -print-file-name ’, but searches for a program such as cpp .
-print-libgcc-file-name
Same as ‘ -print-file-name=libgcc.a
’.
This is useful when you use ‘ -nostdlib ’ or ‘ -nodefaultlibs ’ but you do want to link with ‘ libgcc.a
’. You can do: gcc -nostdlib files ... ‘gcc -print-libgcc-file-name‘
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories gcc searches—and don’t do anything else.
This is useful when gcc prints the error message ‘ installation problem, cannot exec cpp0: No such file or directory ’. To resolve this you either need to put ‘ cpp0 ’ and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don’t forget the trailing ‘ / ’. See
[Environment Variables], page 496 .
-print-sysroot
Print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the ‘ --sysroot ’ option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.
292 Using the GNU Compiler Collection (GCC)
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix—and don’t do anything else.
-dumpmachine
Print the compiler’s target machine (for example, ‘ i686-pc-linux-gnu ’)—and don’t do anything else.
-dumpversion
Print the compiler version (for example, 3.0
, 6.3.0
or 7 )—and don’t do anything else. This is the compiler version used in filesystem paths and specs.
Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version).
-dumpfullversion
Print the full compiler version—and don’t do anything else. The output is always three numbers separated by dots, major, minor and patchlevel version.
-dumpspecs
Print the compiler’s built-in specs—and don’t do anything else. (This is used when GCC itself is being built.) See
Section 3.20 [Spec Files], page 487 .
3.19 Machine-Dependent Options
Each target machine supported by GCC can have its own options—for example, to allow you to compile for a particular processor variant or ABI, or to control optimizations specific to that machine. By convention, the names of machine-specific options start with ‘ -m ’.
Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.
3.19.1 AArch64 Options
These options are defined for AArch64 implementations:
-mabi= name
Generate code for the specified data model. Permissible values are ‘ ilp32 ’ for
SysV-like data model where int, long int and pointers are 32 bits, and ‘ lp64 ’ for SysV-like data model where int is 32 bits, but long int and pointers are 64 bits.
The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.
-mbig-endian
Generate big-endian code. This is the default when GCC is configured for an
‘ aarch64_be-*-* ’ target.
Chapter 3: GCC Command Options 293
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.
-mlittle-endian
Generate little-endian code. This is the default when GCC is configured for an
‘ aarch64-*-* ’ but not an ‘ aarch64_be-*-* ’ target.
-mcmodel=tiny
Generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
-mcmodel=small
Generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model.
-mcmodel=large
Generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. The
‘ -mcmodel=large ’ option is incompatible with ‘ -mabi=ilp32 ’, ‘ -fpic ’ and
‘ -fPIC ’.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former behavior is the default.
-mstack-protector-guard= guard
-mstack-protector-guard-reg= reg
-mstack-protector-guard-offset= offset
Generate stack protection code using canary at guard . Supported locations are
‘ global ’ for a global canary or ‘ sysreg ’ for a canary in an appropriate system register.
With the latter choice the options ‘ -mstack-protector-guard-reg= reg ’ and
‘ -mstack-protector-guard-offset= offset ’ furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
294 Using the GNU Compiler Collection (GCC)
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
-mtls-size= size
Specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum number
835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53 erratum number
843419. This erratum workaround is made at link time and this will only pass the corresponding flag to the linker.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation. This option only has an effect if ‘ -ffast-math ’ or ‘ -funsafe-math-optimizations ’ is used as well. Enabling this reduces precision of reciprocal square root results to about
16 bits for single precision and to 32 bits for double precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option only has an effect if ‘ -ffast-math ’ or ‘ -funsafe-math-optimizations ’ is used as well.
Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision.
If enabled, it implies
‘ -mlow-precision-recip-sqrt ’.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option only has an effect if
‘ -ffast-math ’ or ‘ -funsafe-math-optimizations ’ is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.
-mtrack-speculation
-mno-track-speculation
Enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to __builtin_speculation_safe_copy to permit a more efficient code sequence to be generated.
Chapter 3: GCC Command Options 295
-moutline-atomics
-mno-outline-atomics
Enable or disable calls to out-of-line helpers to implement atomic operations.
These helpers will, at runtime, determine if the LSE instructions from
ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.
This option is only applicable when compiling for the base ARMv8.0
instruction set.
If using a later revision, e.g.
‘ -march=armv8.1-a ’ or
‘ -march=armv8-a+lse ’, the ARMv8.1-Atomics instructions will be used directly. The same applies when using ‘ -mcpu= ’ when the selected cpu supports the ‘ lse ’ feature. This option is on by default.
-march= name
Specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the form ‘ -march= arch { + [ no ] feature } *’.
The table below summarizes the permissible values for arch and the features that they enable by default: arch value
‘ armv8-a ’
‘ armv8.1-a ’
‘ armv8.2-a ’
‘ armv8.3-a ’
‘ armv8.4-a ’
‘ armv8.5-a ’
‘ armv8.6-a ’
‘ armv8.7-a ’
‘ armv8.8-a ’
‘ armv9-a ’
‘ armv8-r ’
Architecture
Armv8-A
Armv8.1-A
Armv8.2-A
Armv8.3-A
Armv8.4-A
Armv8.5-A
Armv8.6-A
Armv8.7-A
Armv8.8-a
Armv9-A
Armv8-R
Includes by default
‘ +fp ’, ‘ +simd ’
‘ armv8-a ’, ‘ +crc ’, ‘ +lse ’, ‘ +rdma ’
‘ armv8.1-a ’
‘ armv8.2-a ’, ‘ +pauth ’
‘ armv8.3-a ’, ‘ +flagm ’, ‘ +fp16fml ’, ‘ +dotprod ’
‘ armv8.4-a ’, ‘ +sb ’, ‘ +ssbs ’, ‘ +predres ’
‘ armv8.5-a ’, ‘ +bf16 ’, ‘ +i8mm ’
‘ armv8.6-a ’, ‘ +ls64 ’
‘ armv8.7-a ’, ‘ +mops ’
‘ armv8.5-a ’, ‘ +sve ’, ‘ +sve2 ’
‘ armv8-r ’
The value ‘ native ’ is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system,
The permissible values for feature are listed in the sub-section on
‘ -mcpu ’ Feature Modifiers], page 298 . Where conflicting feature modifiers are
specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code. If ‘ -march ’ is specified without either of ‘ -mtune ’ or
‘ -mcpu ’ also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture.
-mtune= name
Specify the name of the target processor for which GCC should tune the performance of the code.
Permissible values for this option are:
‘ generic ’, ‘ cortex-a35 ’, ‘ cortex-a53 ’, ‘ cortex-a55 ’, ‘ cortex-a57 ’,
‘ cortex-a72 ’, ‘ cortex-a73 ’, ‘ cortex-a75 ’, ‘ cortex-a76 ’, ‘ cortex-a76ae ’,
‘ cortex-a77 ’, ‘ cortex-a65 ’, ‘ cortex-a65ae ’, ‘ cortex-a34 ’, ‘ cortex-a78 ’,
296 Using the GNU Compiler Collection (GCC)
‘ cortex-a78ae ’, ‘ cortex-a78c ’, ‘ ares ’, ‘ exynos-m1 ’, ‘ emag ’, ‘ falkor ’,
‘ neoverse-512tvb ’, ‘ neoverse-e1 ’, ‘ neoverse-n1 ’, ‘ neoverse-n2 ’,
‘ neoverse-v1 ’, ‘ qdf24xx ’, ‘ saphira ’, ‘ phecda ’, ‘ xgene1 ’, ‘ vulcan ’,
‘ octeontx ’, ‘ octeontx81 ’, ‘ octeontx83 ’, ‘ octeontx2 ’, ‘ octeontx2t98 ’,
‘ octeontx2t96 ’ ‘ octeontx2t93 ’, ‘ octeontx2f95 ’, ‘ octeontx2f95n ’,
‘ octeontx2f95mm ’, ‘ a64fx ’, ‘ thunderx ’, ‘ thunderxt88 ’, ‘ thunderxt88p1 ’,
‘ thunderxt81 ’, ‘ tsv110 ’, ‘ thunderxt83 ’, ‘ thunderx2t99 ’, ‘ thunderx3t110 ’,
‘ zeus ’, ‘ cortex-a57.cortex-a53 ’, ‘ cortex-a72.cortex-a53 ’,
‘ cortex-a73.cortex-a35 ’, ‘ cortex-a73.cortex-a53 ’, ‘ cortex-a75.cortex-a55 ’,
‘ cortex-a76.cortex-a55 ’, ‘ cortex-r82 ’, ‘ cortex-x1 ’, ‘ cortex-x2 ’,
‘ cortex-a510 ’, ‘ cortex-a710 ’, ‘ ampere1 ’, ‘ native ’.
The values ‘ cortex-a57.cortex-a53 ’, ‘ cortex-a72.cortex-a53 ’,
‘ cortex-a73.cortex-a35 ’, ‘ cortex-a73.cortex-a53 ’, ‘ cortex-a75.cortex-a55 ’,
‘ cortex-a76.cortex-a55 ’ specify that GCC should tune for a big.LITTLE
system.
The value ‘ neoverse-512tvb ’ specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for
256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below.
Additionally on native AArch64 GNU/Linux systems the value ‘ native ’ tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system.
Where none of ‘ -mtune= ’, ‘ -mcpu= ’ or ‘ -march= ’ are specified, the code is tuned to perform well across a range of target processors.
This option cannot be suffixed by feature modifiers.
-mcpu= name
Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form ‘ -mcpu= cpu { + [ no ] feature } *’, where the permissible values for cpu are the same as those available for ‘ -mtune ’. The permissible values for feature are documented in the sub-section on
[‘ -march ’ and ‘ -mcpu ’ Feature Modifiers], page 298 . Where conflicting feature modifiers
are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by ‘ -march ’) and to determine the target processor for which to tune for performance (as if by ‘ -mtune ’). Where this option is used in conjunction with ‘ -march ’ or ‘ -mtune ’, those options take precedence over the appropriate part of this option.
‘ -mcpu=neoverse-512tvb ’ is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by ‘ -march ’,
‘ -mcpu=neoverse-512tvb ’ generates code that can run on a Neoverse V1 core,
Chapter 3: GCC Command Options 297 since Neoverse V1 is the first Neoverse core with these properties. Unless overridden by ‘ -mtune ’, ‘ -mcpu=neoverse-512tvb ’ tunes code in the same way as for ‘ -mtune=neoverse-512tvb ’.
-moverride= string
Override tuning decisions made by the back-end in response to a ‘ -mtune= ’ switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases.
This option is only intended to be useful when developing GCC.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler.
-mpc-relative-literal-loads
-mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for
‘ -mcmodel=tiny ’.
-msign-return-address= scope
Select the function scope on which return address signing will be applied. Permissible values are ‘ none ’, which disables return address signing, ‘ non-leaf ’, which enables pointer signing for functions which are not leaf functions, and
‘ all ’, which enables pointer signing for all functions.
The default value is
‘ none ’. This option has been deprecated by -mbranch-protection.
-mbranch-protection= none | standard | pac-ret [+ leaf + b-key ]| bti
Select the branch protection features to use. ‘ none ’ is the default and turns off all types of branch protection. ‘ standard ’ turns on all types of branch protection features. If a feature has additional tuning options, then ‘ standard ’ sets it to its standard level. ‘ pac-ret[+ leaf ] ’ turns on return address signing to its standard level: signing functions that save the return address to memory
(non-leaf functions will practically always do this) using the a-key. The optional argument ‘ leaf ’ can be used to extend the signing to include leaf functions. The optional argument ‘ b-key ’ can be used to sign the functions with the B-key instead of the A-key. ‘ bti ’ turns on branch target identification mechanism.
-mharden-sls= opts
Enable compiler hardening against straight line speculation (SLS).
opts is a comma-separated list of the following options:
‘ retbr ’
‘ blr ’
In addition, ‘ -mharden-sls=all ’ enables all SLS hardening while
‘ -mharden-sls=none ’ disables all SLS hardening.
-msve-vector-bits= bits
Specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.
298 Using the GNU Compiler Collection (GCC)
GCC supports two forms of SVE code generation: “vector-length agnostic” output that works with any size of vector register and “vector-length specific” output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of ‘ bits ’ are: ‘ scalable ’,
‘ 128 ’, ‘ 256 ’, ‘ 512 ’, ‘ 1024 ’ and ‘ 2048 ’.
Specifying ‘ scalable ’ selects vectorlength agnostic output. At present ‘ -msve-vector-bits=128 ’ also generates vector-length agnostic output for big-endian targets. All other values generate vector-length specific code. The behavior of these values may change in future releases and no value except ‘ scalable ’ should be relied on for producing code that is portable across different hardware SVE vector lengths.
The default is ‘ -msve-vector-bits=scalable ’, which produces vector-length agnostic code.
3.19.1.1 ‘
-march
’ and ‘
-mcpu
’ Feature Modifiers
Feature modifiers used with ‘ -march ’ and ‘ -mcpu ’ can be any of the following and their inverses ‘ no feature ’:
‘ crc ’ Enable CRC extension. This is on by default for ‘ -march=armv8.1-a ’.
‘ crypto ’ Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
‘
‘
‘
‘
‘ fp ’ simd sve lse ’
’ rdma ’
’
Enable floating-point instructions. This is on by default for all possible values for options ‘ -march ’ and ‘ -mcpu ’.
Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options ‘ -march ’ and
‘ -mcpu ’.
Enable Scalable Vector Extension instructions. This also enables Advanced
SIMD and floating-point instructions.
Enable Large System Extension instructions.
This is on by default for
‘ -march=armv8.1-a ’.
Enable Round Double Multiply Accumulate instructions. This is on by default for ‘ -march=armv8.1-a ’.
‘ fp16 ’ Enable FP16 extension. This also enables floating-point instructions.
‘ fp16fml ’ Enable FP16 fmla extension. This also enables FP16 extensions and floatingpoint instructions. This option is enabled by default for ‘ -march=armv8.4-a ’.
Use of this option with architectures prior to Armv8.2-A is not supported.
‘ rcpc ’ Enable the RcPc extension. This does not change code generation from GCC, but is passed on to the assembler, enabling inline asm statements to use instructions from the RcPc extension.
‘ dotprod ’ Enable the Dot Product extension. This also enables Advanced SIMD instructions.
‘ aes ’ Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
Chapter 3: GCC Command Options 299
‘
‘
‘ sha2 sha3 sm4 ’
’
’ Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
Enable the sha512 and sha3 crypto extension. This also enables Advanced
SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
‘ profile ’ Enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level and does not affect code generation.
‘ rng ’ Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation.
‘ memtag ’ Enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures prior to Armv8.5-A is not supported.
‘ sb ’ Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler level and does not affect code generation.
This option is enabled by default for ‘ -march=armv8.5-a ’.
‘ ssbs ’ Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for ‘ -march=armv8.5-a ’.
‘ predres ’ Enable the Armv8-a Execution and Data Prediction Restriction instructions.
This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for ‘ -march=armv8.5-a ’.
‘ sve2 ’ Enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.
‘ sve2-bitperm ’
Enable SVE2 bitperm instructions. This also enables SVE2 instructions.
‘ sve2-sm4 ’
Enable SVE2 sm4 instructions. This also enables SVE2 instructions.
‘ sve2-aes ’
Enable SVE2 aes instructions. This also enables SVE2 instructions.
‘
‘ sve2-sha3 ’
Enable SVE2 sha3 instructions. This also enables SVE2 instructions.
tme ’
‘ i8mm ’
Enable the Transactional Memory Extension.
Enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced
SIMD and floating-point instructions. This option is enabled by default for
‘ -march=armv8.6-a ’. Use of this option with architectures prior to Armv8.2-A is not supported.
‘ f32mm ’ Enable 32-bit Floating point Matrix Multiply instructions. This also enables
SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
300 Using the GNU Compiler Collection (GCC)
‘
‘
‘ f64mm bf16 ls64 ’
’
’ Enable 64-bit Floating point Matrix Multiply instructions. This also enables
SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
Enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for ‘ -march=armv8.6-a ’. Use of this option with architectures prior to
Armv8.2-A is not supported.
Enable the 64-byte atomic load and store instructions for accelerators. This option is enabled by default for ‘ -march=armv8.7-a ’.
‘ mops ’
‘ flagm ’
Enable the instructions to accelerate memory operations like memcpy , memmove , memset . This option is enabled by default for ‘ -march=armv8.8-a ’
Enable the Flag Manipulation instructions Extension.
‘ pauth ’ Enable the Pointer Authentication Extension.
Feature ‘ crypto ’ implies ‘ aes ’, ‘ sha2 ’, and ‘ simd ’, which implies ‘ fp ’. Conversely, ‘ nofp ’ implies ‘ nosimd ’, which implies ‘ nocrypto ’, ‘ noaes ’ and ‘ nosha2 ’.
3.19.2 Adapteva Epiphany Options
These ‘ -m ’ options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don’t allocate any register in the range r32 . . .
r63 . That allows code to run on hardware variants that lack these registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size.
-mbranch-cost= num
Set the cost of branches to roughly num “simple” instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
-mcmove Enable the generation of conditional moves.
-mnops= num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an fsub instruction and test the flags. This is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. The default is ‘ -msoft-cmpsf ’, which uses slower, but IEEE-compliant, software comparisons.
-mstack-offset= num
Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the range sp+0...sp+7 can be used by leaf functions without stack allocation. Values other than ‘ 8 ’ or ‘ 16 ’ are untested
Chapter 3: GCC Command Options 301 and unlikely to work. Note also that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate ‘ --with-stack-offset= num ’ option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to truncating.
The default is ‘ -mround-nearest ’.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls are in the range of the b / bl instructions, so use these instructions for direct calls. The default is ‘ -mlong-calls ’.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which ‘ -mlong-calls ’ semantics are in effect.
-mfp-mode= mode
Set the prevailing mode of the floating-point unit. This determines the floatingpoint mode that is provided and expected at function call and return time.
Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
‘ caller ’ Any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
‘ truncate ’
This is the mode used for floating-point calculations with truncating
(i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
‘ round-nearest ’
This is the mode used for floating-point calculations with roundto-nearest-or-even rounding mode.
‘ int ’ This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.
302 Using the GNU Compiler Collection (GCC)
The default is ‘ -mfp-mode=caller ’
-mno-split-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses.
The defaults are ‘ msplit-lohi ’, ‘ -mpost-inc ’, and ‘ -mpost-modify ’.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is ‘ -mvect-double ’, which uses DImode as preferred SIMD mode.
-max-vect-align= num
The maximum alignment for SIMD vector mode types.
num may be 4 or 8.
The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don’t use SIMD vector modes in places that affect size and/or alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case.
-m1regreg
Specify a register to hold the constant − 1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are ‘ r43 ’ and
‘ r63 ’, which specify use of that register as a fixed register, and ‘ none ’, which means that no register is used for this purpose. The default is ‘ -m1reg-none ’.
3.19.3 AMD GCN Options
These options are defined specifically for the AMD GCN port.
-march= gpu
-mtune= gpu
Set architecture type or tuning for gpu . Supported values for gpu are
‘ fiji ’ Compile for GCN3 Fiji devices (gfx803).
‘ gfx900 ’ Compile for GCN5 Vega 10 devices (gfx900).
‘ gfx906 ’ Compile for GCN5 Vega 20 devices (gfx906).
-msram-ecc=on
-msram-ecc=off
-msram-ecc=any
Compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. This feature can be enabled per-process on some devices. The compiled code must match the device mode. The default is ‘ any ’, for devices that support it.
-mstack-size= bytes
Specify how many bytes of stack space will be requested for each GPU thread
(wave-front). Beware that there may be many threads and limited memory
Chapter 3: GCC Command Options 303 available. The size of the stack allocation may also have an impact on run-time performance. The default is 32KB when using OpenACC or OpenMP, and
1MB otherwise.
-mxnack Compile binaries suitable for devices with the XNACK feature enabled. Some devices always require XNACK and some allow the user to configure XNACK.
The compiled code must match the device mode. The default is ‘ -mno-xnack ’.
At present this option is a placeholder for support that is not yet implemented.
3.19.4 ARC Options
The following options control the architecture variant for which code is being compiled:
-mbarrel-shifter
Generate instructions supported by barrel shifter. This is the default unless
‘ -mcpu=ARC601 ’ or ‘ -mcpu=ARCEM ’ is in effect.
-mjli-always
Force to call a function using jli s instruction. This option is valid only for
ARCv2 architecture.
-mcpu= cpu
Set architecture type, register usage, and instruction scheduling parameters for cpu . There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are
‘ arc600 ’ Compile for ARC600. Aliases: ‘ -mA6 ’, ‘ -mARC600 ’.
‘ arc601 ’ Compile for ARC601. Alias: ‘ -mARC601 ’.
‘
‘ arc700 ’ Compile for ARC700.
Aliases: ‘ -mA7 ’, ‘ -mARC700 ’.
This is the default when configured with ‘ --with-cpu=arc700 ’.
‘ arcem ’ archs
‘ em ’
’
Compile for ARC EM.
Compile for ARC HS.
Compile for ARC EM CPU with no hardware extensions.
‘ em4 ’ Compile for ARC EM4 CPU.
‘ em4_dmips ’
Compile for ARC EM4 DMIPS CPU.
‘ em4_fpus ’
Compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
‘ em4_fpuda ’
Compile for ARC EM4 DMIPS CPU with single-precision floatingpoint and double assist instructions.
‘
‘ hs ’ hs34 ’
Compile for ARC HS CPU with no hardware extensions except the atomic instructions.
Compile for ARC HS34 CPU.
304 Using the GNU Compiler Collection (GCC)
‘ hs38 ’ Compile for ARC HS38 CPU.
‘ hs38_linux ’
Compile for ARC HS38 CPU with all hardware extensions on.
‘ arc600_norm ’
Compile for ARC 600 CPU with norm instructions enabled.
‘ arc600_mul32x16 ’
Compile for ARC 600 CPU with norm and 32x16-bit multiply instructions enabled.
‘ arc600_mul64 ’
Compile for ARC 600 CPU with norm and mul64 -family instructions enabled.
‘ arc601_norm ’
Compile for ARC 601 CPU with norm instructions enabled.
‘ arc601_mul32x16 ’
Compile for ARC 601 CPU with norm and 32x16-bit multiply instructions enabled.
‘ arc601_mul64 ’
Compile for ARC 601 CPU with norm and mul64 -family instructions enabled.
‘ nps400 ’ Compile for ARC 700 on NPS400 chip.
‘ em_mini ’ Compile for ARC EM minimalist configuration featuring reduced register set.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the compact implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for the fast implementation.
-mno-dpfp-lrsr
Disable lr and sr instructions from using FPX extension aux registers.
-mea Generate extended arithmetic instructions. Currently only divaw , adds , subs , and sat16 are supported. Only valid for ‘ -mcpu=ARC700 ’.
-mno-mpy Do not generate mpy -family instructions for ARC700. This option is deprecated.
-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate instructions.
-mmul64 Generate mul64 and mulu64 instructions. Only valid for ‘ -mcpu=ARC600 ’.
-mnorm Generate norm instructions. This is the default if ‘ -mcpu=ARC700 ’ is in effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the compact implementation.
Chapter 3: GCC Command Options 305
-mspfp-fast
Generate single-precision FPX instructions, tuned for the fast implementation.
-msimd Enable generation of ARC SIMD instructions via target-specific builtins. Only valid for ‘ -mcpu=ARC700 ’.
-msoft-float
This option ignored; it is provided for compatibility purposes only. Software floating-point code is emitted by default, and this default can overridden by
FPX options; ‘ -mspfp ’, ‘ -mspfp-compact ’, or ‘ -mspfp-fast ’ for single precision, and ‘ -mdpfp ’, ‘ -mdpfp-compact ’, or ‘ -mdpfp-fast ’ for double precision.
-mswap Generate swap instructions.
-matomic This enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores.
-mdiv-rem
Enable div and rem instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option is on by default for
ARC HS.
-mll64 Enable double load/store operations for ARC HS cores.
-mtp-regno= regno
Specify thread pointer register number.
-mmpy-option= multo
Compile ARCv2 code with a multiplier design option. You can specify the option using either a string or numeric value for multo . ‘ wlh1 ’ is the default value. The recognized values are:
‘ 0 ’
‘ none ’
‘ 1 ’
‘ w ’
No multiplier available.
16x16 multiplier, fully pipelined. The following instructions are enabled: mpyw and mpyuw .
‘ 2 ’
‘ wlh1 ’ 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled: mpy , mpyu , mpym , mpymu , and mpy_s .
‘ 3 ’
‘ wlh2 ’ 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled: mpy , mpyu , mpym , mpymu , and mpy_s .
‘ 4 ’
‘ wlh3 ’ Two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled: mpy , mpyu , mpym , mpymu , and mpy_s .
306 Using the GNU Compiler Collection (GCC)
‘
‘ 5 ’ wlh4 ’ One 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled: mpy , mpyu , mpym , mpymu , and mpy_s .
‘
‘ 6 ’ wlh5 ’ One 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled: mpy , mpyu , mpym , mpymu , and mpy_s .
‘ 7 ’
‘ plus_dmpy ’
ARC HS SIMD support.
‘ 8 ’
‘ plus_macd ’
ARC HS SIMD support.
‘ 9 ’
‘ plus_qmacw ’
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu= fpu
Enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are:
‘ fpus ’
‘
‘ fpud ’ fpuda ’
Enables support for single-precision floating-point hardware extensions.
Enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled.
Not available for ARC EM.
Enables support for double-precision floating-point hardware extensions using double-precision assist instructions.
The single-precision floating-point extension is also enabled.
This option is only available for ARC EM.
‘ fpuda_div ’
Enables support for double-precision floating-point hardware extensions using double-precision assist instructions.
The single-precision floating-point, square-root, and divide extensions are also enabled. This option is only available for ARC EM.
‘ fpuda_fma ’
Enables support for double-precision floating-point hardware extensions using double-precision assist instructions.
The single-precision floating-point and fused multiply and add hardware extensions are also enabled. This option is only available for ARC EM.
‘ fpuda_all ’
Enables support for double-precision floating-point hardware extensions using double-precision assist instructions. All single-precision
Chapter 3: GCC Command Options 307
‘ fpus_div ’
Enables support for single-precision floating-point, square-root and divide hardware extensions.
‘ fpud_div ’ floating-point hardware extensions are also enabled. This option is only available for ARC EM.
Enables support for double-precision floating-point, square-root and divide hardware extensions.
This option includes option
‘ fpus_div ’. Not available for ARC EM.
‘ fpus_fma ’
Enables support for single-precision floating-point and fused multiply and add hardware extensions.
‘ fpud_fma ’
Enables support for double-precision floating-point and fused multiply and add hardware extensions. This option includes option
‘ fpus_fma ’. Not available for ARC EM.
‘ fpus_all ’
Enables support for all single-precision floating-point hardware extensions.
‘ fpud_all ’
Enables support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM.
-mirq-ctrl-saved= register-range , blink , lp_count
Specifies general-purposes registers that the processor automatically saves/restores on interrupt entry and exit.
register-range is specified as two registers separated by a dash. The register range always starts with r0 , the upper limit is fp register.
blink and lp count are optional. This option is only valid for ARC EM and ARC HS cores.
-mrgf-banked-regs= number
Specifies the number of registers replicated in second register bank on entry to fast interrupt. Fast interrupts are interrupts with the highest priority level
P0. These interrupts save only PC and STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. Use this option when you are using fast interrupts in an ARC V2 family processor. Permitted values are 4, 8, 16, and 32.
-mlpc-width= width
Specify the width of the lp_count register. Valid values for width are 8, 16, 20,
24, 28 and 32 bits. The default width is fixed to 32 bits. If the width is less than
32, the compiler does not attempt to transform loops in your program to use the zero-delay loop mechanism unless it is known that the lp_count register can hold the required loop-counter value. Depending on the width specified, the compiler and run-time library might continue to use the loop mechanism for various needs. This option defines macro __ARC_LPC_WIDTH__ with the value of width .
308 Using the GNU Compiler Collection (GCC)
-mrf16 This option instructs the compiler to generate code for a 16-entry register file.
This option defines the __ARC_RF16__ preprocessor macro.
-mbranch-index
Enable use of bi or bih instructions to implement jump tables.
The following options are passed through to the assembler, and also define preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol __Xdsp_packa . This option is deprecated.
-mdvbf
-mlock
Passed down to the assembler to enable the dual Viterbi butterfly extension.
Also sets the preprocessor symbol __Xdvbf . This option is deprecated.
Passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol __Xlock .
-mmac-d16
Passed down to the assembler. Also sets the preprocessor symbol __Xxmac_d16 .
This option is deprecated.
-mmac-24 Passed down to the assembler. Also sets the preprocessor symbol __Xxmac_24 .
This option is deprecated.
-mrtsc Passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the preprocessor symbol __Xrtsc . This option is deprecated.
-mswape Passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol __Xswape .
-mtelephony
Passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the preprocessor symbol __Xtelephony . This option is deprecated.
-mxy Passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol __Xxy .
The following options control how the assembly code is annotated:
-misize Annotate assembler instructions with estimated addresses.
-mannotate-align
Explain what alignment considerations lead to the decision to make an instruction short or long.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the arclinux emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested.
Chapter 3: GCC Command Options 309
-marclinux_prof
Passed through to the linker, to specify use of the arclinux_prof emulation.
This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested.
The following options control the semantics of generated code:
-mlong-calls
Generate calls as register indirect calls, thus providing access to the full 32-bit address range.
-mmedium-calls
Don’t use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
-G num Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations.
-mno-sdata
Do not generate sdata references. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile references. This is the default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Does nothing. Preserved for backward compatibility.
-mauto-modify-reg
Enable the use of pre/post modify with register displacement.
-mbbit-peephole
Enable bbit peephole2.
-mno-brcc
This option disables a target-specific pass in ‘ arc_reorg ’ to generate compareand-branch ( br cc ) instructions. It has no effect on generation of these instructions driven by the combiner pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table shortening. This is the default for ‘ -Os ’.
-mcompact-casesi
Enable compact casesi pattern. This is the default for ‘ -Os ’, and only available for ARCv1 cores. This option is deprecated.
310 Using the GNU Compiler Collection (GCC)
-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional execution instructions.
Due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the targetindependent pass to generate conditional execution is often lacking, so the ARC port has kept a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have been done. This pass generally, but not always, improves performance and code size, at the cost of extra compilation time, which is why there is an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using ‘ -mmedium-calls ’ instead.
-mearly-cbranchsi
Enable pre-reload use of the cbranchsi pattern.
-mexpand-adddi
Expand adddi3 and subdi3 at RTL generation time into add.f
, adc etc. This option is deprecated.
-mindexed-loads
Enable the use of indexed loads. This can be problematic because some optimizers then assume that indexed stores exist, which is not the case.
-mlra Enable Local Register Allocation. This is still experimental for ARC, so by default the compiler uses standard reload (i.e. ‘ -mno-lra ’).
-mlra-priority-none
Don’t indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mmillicode
When optimizing for size (using ‘ -Os ’), prologues and epilogues that have to save or restore a large number of registers are often shortened by using call to a special function in libgcc; this is referred to as a millicode call. As these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn on or off millicode call generation.
-mcode-density-frame
This option enable the compiler to emit enter and leave instructions. These instructions are only valid for CPUs with code-density feature.
-mmixed-code
Does nothing. Preserved for backward compatibility.
Chapter 3: GCC Command Options 311
-mq-class
Ths option is deprecated. Enable ‘ q ’ instruction alternatives. This is the default for ‘ -Os ’.
-mRcq
-mRcw
Enable ‘ Rcq ’ constraint handling. Most short code generation depends on this.
This is the default.
Enable ‘ Rcw ’ constraint handling. Most ccfsm condexec mostly depends on this.
This is the default.
-msize-level= level
Fine-tune size optimization with regards to instruction lengths and alignment.
The recognized values for level are:
‘ 0 ’
‘ 1 ’
‘ 2 ’
No size optimization. This level is deprecated and treated like ‘
Short instructions are used opportunistically.
1 ’.
‘ 3 ’
In addition, alignment of loops and of code after barriers are dropped.
In addition, optional data alignment is dropped, and the option
‘ Os ’ is enabled.
This defaults to ‘ 3 ’ when ‘ -Os ’ is in effect. Otherwise, the behavior when this is not set is equivalent to level ‘ 1 ’.
-mtune= cpu
Set instruction scheduling parameters for cpu , overriding any implied by
‘ -mcpu= ’.
Supported values for cpu are
‘ ARC600 ’ Tune for ARC600 CPU.
‘ ARC601 ’ Tune for ARC601 CPU.
‘ ARC700 ’ Tune for ARC700 CPU with standard multiplier block.
‘ ARC700-xmac ’
Tune for ARC700 CPU with XMAC block.
‘ ARC725D ’ Tune for ARC725D CPU.
‘ ARC750D ’ Tune for ARC750D CPU.
-mmultcost= num
Cost to assume for a multiply instruction, with ‘ 4 ’ being equal to a normal instruction.
-munalign-prob-threshold= probability
Does nothing. Preserved for backward compatibility.
The following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release:
-margonaut
Obsolete FPX.
312 Using the GNU Compiler Collection (GCC)
-mbig-endian
-EB Compile code for big-endian targets. Use of these options is now deprecated.
Big-endian code is supported by configuring GCC to build arceb-elf32 and arceb-linux-uclibc targets, for which big endian is the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these options is now deprecated.
Little-endian code is supported by configuring GCC to build arc-elf32 and arc-linux-uclibc targets, for which little endian is the default.
-mbarrel_shifter
Replaced by ‘ -mbarrel-shifter ’.
-mdpfp_compact
Replaced by ‘ -mdpfp-compact ’.
-mdpfp_fast
Replaced by ‘ -mdpfp-fast ’.
-mdsp_packa
Replaced by ‘ -mdsp-packa ’.
-mEA Replaced by ‘ -mea ’.
-mmac_24 Replaced by ‘ -mmac-24 ’.
-mmac_d16
Replaced by ‘ -mmac-d16 ’.
-mspfp_compact
Replaced by ‘ -mspfp-compact ’.
-mspfp_fast
Replaced by ‘ -mspfp-fast ’.
-mtune= cpu
Values ‘ arc600 ’, ‘ arc601 ’, ‘ arc700 ’ and ‘ arc700-xmac ’ for cpu are replaced by
‘ ARC600 ’, ‘ ARC601 ’, ‘ ARC700 ’ and ‘ ARC700-xmac ’ respectively.
-multcost= num
Replaced by ‘ -mmultcost ’.
3.19.5 ARM Options
These ‘ -m ’ options are defined for the ARM port:
-mabi= name
Generate code for the specified ABI.
Permissible values are: ‘ apcs-gnu ’,
‘ atpcs ’, ‘ aapcs ’, ‘ aapcs-linux ’ and ‘ iwmmxt ’.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not strictly necessary for correct execution of the code. Specifying ‘ -fomit-frame-pointer ’ with this option causes the stack frames not to be generated for leaf functions. The default is ‘ -mno-apcs-frame ’.
This option is deprecated.
Chapter 3: GCC Command Options 313
-mapcs This is a synonym for ‘ -mapcs-frame ’ and is deprecated.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb instruction sets.
Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program.
The default is ‘ -mno-thumb-interwork ’, since slightly larger code is generated when
‘ -mthumb-interwork ’ is specified.
In AAPCS configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function’s body. This means that all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this information can be used to locate the start of functions inside an executable piece of code.
The default is ‘ -msched-prolog ’.
-mfloat-abi= name
Specifies which floating-point ABI to use.
Permissible values are: ‘ soft ’,
‘ softfp ’ and ‘ hard ’.
Specifying ‘ soft ’ causes GCC to generate output containing library calls for floating-point operations. ‘ softfp ’ allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.
‘ hard ’ allows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that the hardfloat and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
-mbe8
-mbe32 When linking a big-endian image select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM.
314 Using the GNU Compiler Collection (GCC)
-march= name [ +extension...
]
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the ‘ -mcpu= ’ option.
Permissible names are: ‘ armv4t ’, ‘ armv5t ’, ‘ armv5te ’, ‘ armv6 ’, ‘ armv6j ’,
‘ armv6k ’, ‘ armv6kz ’, ‘ armv6t2 ’, ‘ armv6z ’, ‘ armv6zk ’, ‘ armv7 ’, ‘ armv7-a ’,
‘ armv7ve ’, ‘ armv8-a ’, ‘ armv8.1-a ’, ‘ armv8.2-a ’, ‘ armv8.3-a ’, ‘ armv8.4-a ’,
‘ armv8.5-a ’, ‘ armv8.6-a ’, ‘ armv9-a ’, ‘ armv7-r ’, ‘ armv8-r ’, ‘ armv6-m ’,
‘ armv6s-m ’, ‘ armv7-m ’, ‘ armv7e-m ’, ‘ armv8-m.base
’, ‘ armv8-m.main
’,
‘ armv8.1-m.main
’, ‘ armv9-a ’, ‘ iwmmxt ’ and ‘ iwmmxt2 ’.
Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated: ‘ armv4 ’.
Many of the architectures support extensions. These can be added by appending ‘ + extension ’ to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends. For example, the ‘ +crypto ’ extension will always enable the ‘ +simd ’ extension. The exception to the additive construction is for extensions that are prefixed with ‘ +no...
’: these extensions disable the specified option and any other extensions that may depend on the presence of that extension.
For example, ‘ -march=armv7-a+simd+nofp+vfpv4 ’ is equivalent to writing
‘ -march=armv7-a+vfpv4 ’ since the ‘ +simd ’ option is entirely disabled by the
‘ +nofp ’ option that follows it.
Most extension names are generically named, but have an effect that is dependent upon the architecture to which it is applied. For example, the ‘ +simd ’ option can be applied to both ‘ armv7-a ’ and ‘ armv8-a ’ architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for ‘ armv7-a ’ and the ARMv8-A variant for ‘ armv8-a ’.
The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions.
‘ armv5te ’
‘ armv6 ’
‘ armv6j ’
‘ armv6k ’
‘ armv6kz ’
‘ armv6t2 ’
‘ armv6z ’
‘ armv6zk ’
‘ armv7 ’
‘ +fp ’ The VFPv2 floating-point instructions. The extension
‘ +vfpv2 ’ can be used as an alias for this extension.
Disable the floating-point instructions.
‘ +nofp ’
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
Chapter 3: GCC Command Options 315
‘ +fp ’ The VFPv3 floating-point instructions, with 16 doubleprecision registers. The extension ‘ +vfpv3-d16 ’ can be used as an alias for this extension. Note that floatingpoint is not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and
ARMv7-R architectures.
Disable the floating-point instructions.
‘ armv7-a ’
‘ +nofp ’
‘
‘
‘
+mp
+fp ’
’
+sec ’
The multiprocessing extension.
The security extension.
The VFPv3 floating-point instructions, with 16 doubleprecision registers. The extension ‘ +vfpv3-d16 ’ can be used as an alias for this extension.
‘ +simd ’ The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions.
The extensions ‘ +neon ’ and ‘ +neon-vfpv3 ’ can be used as aliases for this extension.
‘ +vfpv3 ’ The VFPv3 floating-point instructions, with 32 doubleprecision registers.
‘ +vfpv3-d16-fp16 ’
The VFPv3 floating-point instructions, with 16 doubleprecision registers and the half-precision floating-point conversion operations.
‘ +vfpv3-fp16 ’
The VFPv3 floating-point instructions, with 32 doubleprecision registers and the half-precision floating-point conversion operations.
‘ +vfpv4-d16 ’
The VFPv4 floating-point instructions, with 16 doubleprecision registers.
‘ +vfpv4 ’ The VFPv4 floating-point instructions, with 32 doubleprecision registers.
‘ +neon-fp16 ’
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
‘ +neon-vfpv4 ’
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
‘ +nosimd ’ Disable the Advanced SIMD instructions (does not disable floating point).
316 Using the GNU Compiler Collection (GCC)
‘ +nofp ’ Disable the floating-point and Advanced SIMD instructions.
‘ armv7ve ’ The extended version of the ARMv7-A architecture with support for virtualization.
‘ +fp ’ The VFPv4 floating-point instructions, with 16 doubleprecision registers. The extension ‘ +vfpv4-d16 ’ can be used as an alias for this extension.
‘ +simd ’ The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
The extension
‘ +neon-vfpv4 ’ can be used as an alias for this extension.
‘ +vfpv3-d16 ’
The VFPv3 floating-point instructions, with 16 doubleprecision registers.
‘ +vfpv3 ’ The VFPv3 floating-point instructions, with 32 doubleprecision registers.
‘ +vfpv3-d16-fp16 ’
The VFPv3 floating-point instructions, with 16 doubleprecision registers and the half-precision floating-point conversion operations.
‘ +vfpv3-fp16 ’
The VFPv3 floating-point instructions, with 32 doubleprecision registers and the half-precision floating-point conversion operations.
‘ +vfpv4-d16 ’
The VFPv4 floating-point instructions, with 16 doubleprecision registers.
‘ +vfpv4 ’ The VFPv4 floating-point instructions, with 32 doubleprecision registers.
‘ +neon ’ The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions.
The extension
‘ +neon-vfpv3 ’ can be used as an alias for this extension.
‘ +neon-fp16 ’
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
‘ +nosimd ’ Disable the Advanced SIMD instructions (does not disable floating point).
‘ +nofp ’ Disable the floating-point and Advanced SIMD instructions.
Chapter 3: GCC Command Options 317
‘ armv8-a ’
‘
‘
+crc ’
+simd ’
The Cyclic Redundancy Check (CRC) instructions.
The ARMv8-A Advanced SIMD and floating-point instructions.
‘ +crypto ’ The cryptographic instructions.
‘ +nocrypto ’
Disable the cryptographic instructions.
‘ +nofp ’ Disable the floating-point, Advanced SIMD and cryptographic instructions.
‘ +sb ’
‘ +predres ’
Speculation Barrier Instruction.
Execution and Data Prediction Restriction
Instructions.
‘ armv8.1-a ’
‘ +simd ’ The ARMv8.1-A Advanced SIMD and floating-point instructions.
‘ +crypto ’ The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions.
‘ +nocrypto ’
Disable the cryptographic instructions.
‘ +nofp ’ Disable the floating-point, Advanced SIMD and cryptographic instructions.
Speculation Barrier Instruction.
‘ +sb ’
‘ +predres ’
Execution and Data Prediction Restriction
Instructions.
‘ armv8.2-a ’
‘ armv8.3-a ’
‘ +fp16 ’ The half-precision floating-point data processing instructions.
This also enables the Advanced SIMD and floating-point instructions.
‘ +fp16fml ’
The half-precision floating-point fmla extension. This also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.
‘ +simd ’ The ARMv8.1-A Advanced SIMD and floating-point instructions.
‘ +crypto ’ The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions.
318 Using the GNU Compiler Collection (GCC)
‘ +dotprod ’
Enable the Dot Product extension. This also enables
Advanced SIMD instructions.
‘ +nocrypto ’
Disable the cryptographic extension.
‘
‘ +nofp ’ Disable the floating-point, Advanced SIMD and cryptographic instructions.
Speculation Barrier Instruction.
‘ +sb ’
‘ +predres ’
Execution and Data Prediction Restriction
Instructions.
‘ +i8mm ’
+bf16 ’
8-bit Integer Matrix Multiply instructions.
This also enables Advanced SIMD and floating-point instructions.
Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
‘ armv8.4-a ’
‘ +fp16 ’ The half-precision floating-point data processing instructions.
This also enables the Advanced SIMD and floating-point instructions as well as the Dot
Product extension and the half-precision floating-point fmla extension.
‘ +simd ’ The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
‘ +crypto ’ The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as the Dot Product extension.
‘ +nocrypto ’
Disable the cryptographic extension.
‘ +nofp ’ Disable the floating-point, Advanced SIMD and cryptographic instructions.
‘ +sb ’ Speculation Barrier Instruction.
‘ +predres ’
Execution and Data Prediction Restriction
Instructions.
‘ +i8mm ’ 8-bit Integer Matrix Multiply instructions.
This also enables Advanced SIMD and floating-point instructions.
Chapter 3: GCC Command Options 319
‘ +bf16 ’ Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
‘ armv8.5-a ’
‘
‘
+fp16
+simd
’
’
The half-precision floating-point data processing instructions.
This also enables the Advanced SIMD and floating-point instructions as well as the Dot
Product extension and the half-precision floating-point fmla extension.
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
‘
‘ +crypto ’ The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as the Dot Product extension.
‘ +nocrypto ’
Disable the cryptographic extension.
‘ +nofp ’
+i8mm ’
Disable the floating-point, Advanced SIMD and cryptographic instructions.
8-bit Integer Matrix Multiply instructions.
This also enables Advanced SIMD and floating-point instructions.
‘ +bf16 ’ Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
‘ armv8.6-a ’
‘
‘
+fp16
+simd
’
’
The half-precision floating-point data processing instructions.
This also enables the Advanced SIMD and floating-point instructions as well as the Dot
Product extension and the half-precision floating-point fmla extension.
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
‘
‘ +crypto ’ The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as the Dot Product extension.
‘ +nocrypto ’
Disable the cryptographic extension.
‘ +nofp ’
+i8mm ’
Disable the floating-point, Advanced SIMD and cryptographic instructions.
8-bit Integer Matrix Multiply instructions.
This also enables Advanced SIMD and floating-point instructions.
320 Using the GNU Compiler Collection (GCC)
‘ +bf16 ’ Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.
‘ armv7-r ’
‘ +fp.sp
’ The single-precision VFPv3 floating-point instructions.
The extension ‘ +vfpv3xd ’ can be used as an alias for this extension.
‘ +fp ’ The VFPv3 floating-point instructions with 16 doubleprecision registers. The extension + vfpv3-d16 can be used as an alias for this extension.
‘ +vfpv3xd-d16-fp16 ’
The single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
‘ +vfpv3-d16-fp16 ’
The VFPv3 floating-point instructions with 16 doubleprecision registers and the half-precision floating-point conversion operations.
‘ +nofp ’
‘ +idiv ’
Disable the floating-point extension.
The ARM-state integer division instructions.
‘ +noidiv ’ Disable the ARM-state integer division extension.
‘ armv7e-m ’
‘ +fp ’ The single-precision VFPv4 floating-point instructions.
‘ +fpv5 ’ The single-precision FPv5 floating-point instructions.
‘ +fp.dp
’ The single- and double-precision FPv5 floating-point instructions.
Disable the floating-point extensions.
‘ +nofp ’
‘ armv8.1-m.main
’
‘ +dsp ’
‘ +mve ’
The DSP instructions.
The M-Profile Vector Extension (MVE) integer instructions.
‘ +mve.fp
’ The M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.
‘ +fp ’ The single-precision floating-point instructions.
‘ +fp.dp
’ The single- and double-precision floating-point instructions.
‘ +nofp ’ Disable the floating-point extension.
Chapter 3: GCC Command Options 321
‘ +cdecp0, +cdecp1, ... , +cdecp7 ’
Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.
‘ armv8-m.main
’
‘ +dsp ’ The DSP instructions.
‘ +nodsp ’ Disable the DSP extension.
‘ +fp ’ The single-precision floating-point instructions.
‘ +fp.dp
’ The single- and double-precision floating-point instructions.
‘ +nofp ’ Disable the floating-point extension.
‘ +cdecp0, +cdecp1, ... , +cdecp7 ’
Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.
‘ armv8-r ’
‘ +crc ’ The Cyclic Redundancy Check (CRC) instructions.
‘ +fp.sp
’ The single-precision FPv5 floating-point instructions.
‘ +simd ’ The ARMv8-A Advanced SIMD and floating-point instructions.
‘ +crypto ’ The cryptographic instructions.
‘ +nocrypto ’
Disable the cryptographic instructions.
‘ +nofp ’ Disable the floating-point, Advanced SIMD and cryptographic instructions.
‘ -march=native ’ causes the compiler to auto-detect the architecture of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
-mtune= name
This option specifies the name of the target ARM processor for which
GCC should tune the performance of the code.
For some ARM implementations better performance can be obtained by using this option.
Permissible names are: ‘ arm7tdmi ’, ‘ arm7tdmi-s ’, ‘ arm710t ’, ‘ arm720t ’,
‘ arm740t ’, ‘ strongarm ’, ‘ strongarm110 ’, ‘ strongarm1100 ’, ‘ strongarm1110 ’,
‘ arm8 ’, ‘ arm810 ’, ‘ arm9 ’, ‘ arm9e ’, ‘ arm920 ’, ‘ arm920t ’, ‘ arm922t ’,
‘ arm946e-s ’, ‘ arm966e-s ’, ‘ arm968e-s ’, ‘ arm926ej-s ’, ‘ arm940t ’, ‘ arm9tdmi ’,
‘ arm10tdmi ’, ‘ arm1020t ’, ‘ arm1026ej-s ’, ‘ arm10e ’, ‘ arm1020e ’, ‘ arm1022e ’,
‘ arm1136j-s ’, ‘ arm1136jf-s ’, ‘ mpcore ’, ‘ mpcorenovfp ’, ‘ arm1156t2-s ’,
‘ arm1156t2f-s ’, ‘ arm1176jz-s ’, ‘ arm1176jzf-s ’, ‘ generic-armv7-a ’,
322 Using the GNU Compiler Collection (GCC)
‘ cortex-a5 ’, ‘ cortex-a7 ’, ‘ cortex-a8 ’, ‘ cortex-a9 ’, ‘ cortex-a12 ’,
‘ cortex-a15 ’, ‘ cortex-a17 ’, ‘ cortex-a32 ’, ‘ cortex-a35 ’, ‘ cortex-a53 ’,
‘ cortex-a55 ’, ‘ cortex-a57 ’, ‘ cortex-a72 ’, ‘ cortex-a73 ’, ‘ cortex-a75 ’,
‘ cortex-a76 ’, ‘ cortex-a76ae ’, ‘ cortex-a77 ’, ‘ cortex-a78 ’, ‘ cortex-a78ae ’,
‘ cortex-a78c ’, ‘ cortex-a710 ’, ‘ ares ’, ‘ cortex-r4 ’, ‘ cortex-r4f ’,
‘ cortex-r5 ’, ‘ cortex-r7 ’, ‘ cortex-r8 ’, ‘ cortex-r52 ’, ‘ cortex-r52plus ’,
‘ cortex-m0 ’, ‘ cortex-m0plus ’, ‘ cortex-m1 ’, ‘ cortex-m3 ’, ‘ cortex-m4 ’,
‘ cortex-m7 ’, ‘ cortex-m23 ’, ‘ cortex-m33 ’, ‘ cortex-m35p ’, ‘ cortex-m55 ’,
‘ cortex-x1 ’, ‘ cortex-m1.small-multiply ’, ‘ cortex-m0.small-multiply ’,
‘ cortex-m0plus.small-multiply ’, ‘ exynos-m1 ’, ‘ marvell-pj4 ’,
‘ neoverse-n1 ’, ‘ neoverse-n2 ’, ‘ neoverse-v1 ’, ‘ xscale ’, ‘ iwmmxt ’, ‘ iwmmxt2 ’,
‘ ep9312 ’, ‘ fa526 ’, ‘ fa626 ’, ‘ fa606te ’, ‘ fa626te ’, ‘ fmp626 ’, ‘ fa726te ’,
‘ xgene1 ’.
Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE
system.
Permissible names are: ‘ cortex-a15.cortex-a7 ’, ‘ cortex-a17.cortex-a7 ’,
‘ cortex-a57.cortex-a53 ’, ‘ cortex-a72.cortex-a53 ’, ‘ cortex-a72.cortex-a35 ’,
‘ cortex-a73.cortex-a53 ’, ‘ cortex-a75.cortex-a55 ’, ‘ cortex-a76.cortex-a55 ’.
‘ -mtune=genericarch ’ specifies that GCC should tune the performance for a blend of processors within architecture arch . The aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models come and go.
‘ -mtune ’ permits the same extension options as ‘ -mcpu ’, but the extension options do not affect the tuning of the generated code.
‘ -mtune=native ’ causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
-mcpu= name [ +extension...
]
This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by ‘ -march ’) and the ARM processor type for which to tune for performance (as if specified by
‘ -mtune ’). Where this option is used in conjunction with ‘ -march ’ or ‘ -mtune ’, those options take precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural extensions.
Where this is so the architectural extensions are normally enabled by default. If implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted. For floating-point and
Advanced SIMD (Neon) instructions, the settings of the options ‘ -mfloat-abi ’ and ‘ -mfpu ’ must also be considered: floating-point and Advanced SIMD instructions will only be used if ‘ -mfloat-abi ’ is not set to ‘ soft ’; and any setting of ‘ -mfpu ’ other than ‘ auto ’ will override the available floating-point and SIMD extension instructions.
Chapter 3: GCC Command Options 323
For example, ‘ cortex-a9 ’ can be found in three major configurations: integer only, with just a floating-point unit or with floating-point and Advanced SIMD.
The default is to enable all the instructions, but the extensions ‘ +nosimd ’ and
‘ +nofp ’ can be used to disable just the SIMD or both the SIMD and floatingpoint instructions respectively.
Permissible names for this option are the same as those for ‘ -mtune ’.
The following extension options are common to the listed CPUs:
‘ +nodsp ’ Disable the DSP instructions on ‘ cortex-m33 ’, ‘ cortex-m35p ’.
‘ +nofp ’ Disables the floating-point instructions on ‘ arm9e ’, ‘ arm946e-s ’,
‘ arm966e-s ’, ‘ arm968e-s ’, ‘ arm10e ’, ‘ arm1020e ’, ‘ arm1022e ’,
‘ arm926ej-s ’, ‘ arm1026ej-s ’, ‘ cortex-r5 ’, ‘ cortex-r7 ’,
‘ cortex-r8 ’, ‘ cortex-m4 ’, ‘ cortex-m7 ’, ‘ cortex-m33 ’ and
‘ cortex-m35p ’. Disables the floating-point and SIMD instructions on ‘ generic-armv7-a ’, ‘ cortex-a5 ’, ‘ cortex-a7 ’, ‘ cortex-a8 ’,
‘ cortex-a9 ’, ‘ cortex-a12 ’, ‘ cortex-a15 ’, ‘ cortex-a17 ’,
‘ cortex-a15.cortex-a7 ’, ‘ cortex-a17.cortex-a7 ’,
‘ cortex-a32 ’, ‘ cortex-a35 ’, ‘ cortex-a53 ’ and ‘ cortex-a55 ’.
‘ +nofp.dp
’
Disables the double-precision component of the floating-point instructions on ‘ cortex-r5 ’, ‘ cortex-r7 ’, ‘ cortex-r8 ’,
‘ cortex-r52 ’, ‘ cortex-r52plus ’ and ‘ cortex-m7 ’.
‘ +nosimd ’ Disables the SIMD (but not floating-point) instructions on
‘ generic-armv7-a ’, ‘ cortex-a5 ’, ‘ cortex-a7 ’ and ‘ cortex-a9 ’.
‘ +crypto ’ Enables the cryptographic instructions on ‘ cortex-a32 ’,
‘ cortex-a35 ’, ‘ cortex-a53 ’, ‘ cortex-a55 ’, ‘ cortex-a57 ’,
‘ cortex-a72 ’, ‘ cortex-a73 ’, ‘ cortex-a75 ’, ‘ exynos-m1 ’,
‘ xgene1 ’, ‘ cortex-a57.cortex-a53 ’, ‘ cortex-a72.cortex-a53 ’,
‘ cortex-a73.cortex-a35 ’,
‘ cortex-a75.cortex-a55 ’.
‘ cortex-a73.cortex-a53 ’ and
Additionally the ‘ generic-armv7-a ’ pseudo target defaults to VFPv3 with 16 double-precision registers. It supports the following extension options: ‘ mp ’,
‘ sec ’, ‘ vfpv3-d16 ’, ‘ vfpv3 ’, ‘ vfpv3-d16-fp16 ’, ‘ vfpv3-fp16 ’, ‘ vfpv4-d16 ’,
‘ vfpv4 ’, ‘ neon ’, ‘ neon-vfpv3 ’, ‘ neon-fp16 ’, ‘ neon-vfpv4 ’. The meanings are the same as for the extensions to ‘ -march=armv7-a ’.
‘ -mcpu=genericarch ’ is also permissible, and is equivalent to ‘ -march= arch
-mtune=genericarch ’. See ‘ -mtune ’ for more information.
‘ -mcpu=native ’ causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
-mfpu= name
This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: ‘ auto ’, ‘ vfpv2 ’, ‘ vfpv3 ’, ‘ vfpv3-fp16 ’,
324 Using the GNU Compiler Collection (GCC)
‘ vfpv3-d16 ’, ‘ vfpv3-d16-fp16 ’, ‘ vfpv3xd ’, ‘ vfpv3xd-fp16 ’, ‘ neon-vfpv3 ’,
‘ neon-fp16 ’, ‘ vfpv4 ’, ‘ vfpv4-d16 ’, ‘ fpv4-sp-d16 ’, ‘ neon-vfpv4 ’, ‘ fpv5-d16 ’,
‘ fpv5-sp-d16 ’, ‘ fp-armv8 ’, ‘ neon-fp-armv8 ’ and ‘ crypto-neon-fp-armv8 ’.
Note that ‘ neon ’ is an alias for ‘ neon-vfpv3 ’ and ‘ vfp ’ is an alias for ‘ vfpv2 ’.
The setting ‘ auto ’ is the default and is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of
‘ -mcpu ’ and ‘ -march ’.
If the selected floating-point hardware includes the NEON extension (e.g.
‘ -mfpu=neon ’), note that floating-point operations are not generated by
GCC’s auto-vectorization pass unless ‘ -funsafe-math-optimizations ’ is also specified. This is because NEON hardware does not fully implement the
IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision.
You can also set the fpu name at function level by using the target("fpu=") function attributes (see
Section 6.33.5 [ARM Function Attributes], page 579 )
or pragmas (see
Section 6.62.16 [Function Specific Option Pragmas], page 874
).
-mfp16-format= name
Specify the format of the __fp16 half-precision floating-point type. Permissible names are ‘ none ’, ‘ ieee ’, and ‘ alternative ’; the default is ‘ none ’, in which case the __fp16 type is not defined. See
Section 6.13 [Half-Precision], page 533 , for
more information.
-mstructure-size-boundary= n
The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible.
Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.
This option is deprecated.
-mabort-on-noreturn
Generate a call to the function abort at the end of a noreturn function. It is executed if the function tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register.
This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.
Chapter 3: GCC Command Options 325
Even if this switch is enabled, not all function calls are turned into long calls.
The heuristic is that static functions, functions that have the short_call attribute, functions that are inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive are always turned into long calls.
This feature is not enabled by default. Specifying ‘ -mno-long-calls ’ restores the default behavior, as does placing the function calls within the scope of a
#pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.
-mpic-register= reg
Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default is ‘ R9 ’ if target is EABI based or stack-checking is enabled, otherwise the default is ‘ R10 ’.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable
‘ -msingle-pic-base ’ by default.
-mpoke-function-name
Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this: t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0) arm_poke_function_name mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of pc stored at fp + 0 . If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000) .
-mthumb
-marm
326 Using the GNU Compiler Collection (GCC)
Select between generating code that executes in ARM and Thumb states.
The default for most configurations is to generate code that executes in
ARM state, but the default can be changed by configuring GCC with the
‘ --with-mode= ’ state configure option.
You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes (see
[ARM Function Attributes], page 579
) or pragmas (see
Specific Option Pragmas], page 874 ).
-mflip-thumb
Switch ARM/Thumb modes on alternating functions. This option is provided for regression testing of mixed Thumb/ARM code generation, and is not intended for ordinary use in compiling code.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is ‘ -mno-tpcs-frame ’.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is ‘ -mno-apcs-leaf-frame ’.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default.
-mtp= name
Specify the access model for the thread local storage pointer. The valid models are ‘ soft ’, which generates calls to __aeabi_read_tp , ‘ cp15 ’, which fetches the thread pointer from cp15 directly (supported in the arm6k architecture), and
‘ auto ’, which uses the best available method for the selected processor. The default setting is ‘ auto ’.
-mtls-dialect= dialect
Specify the dialect to use for accessing thread local storage. Two dialect s are supported—‘ gnu ’ and ‘ gnu2 ’. The ‘ gnu ’ dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The ‘ gnu2 ’ dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme,
Chapter 3: GCC Command Options 327 but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e. R ARM ABS32).
This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when ‘ -fpic ’ or ‘ -fPIC ’ is specified. This option conflicts with ‘ -mslow-flash-data ’.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used.
This option avoids generating these instructions. This option is enabled by default when
‘ -mcpu=cortex-m3 ’ is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned.
By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed data structures are accessed a byte at a time.
The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined.
-mneon-for-64bits
This option is deprecated and has no effect.
-mslow-flash-data
Assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and off by default.
It conflicts with
‘ -mword-relocations ’.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The default is currently off which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler file. This is an option used only for regression testing of the compiler and not intended for ordinary use in compiling code. This option is disabled by default.
328 Using the GNU Compiler Collection (GCC)
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler.
-mpure-code
Do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format give all text sections the ELF processor-specific section attribute SHF_ARM_PURECODE . This option is only available when generating non-pic code for M-profile targets.
-mcmse Generate secure code as per the " ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification " , which can be found on https://developer.arm.com/documentation/ecm0359818/latest/ .
-mfix-cmse-cve-2021-35465
Mitigate against a potential security issue with the VLLDM instruction in some Mprofile devices when using CMSE (CVE-2021-365465). This option is enabled by default when the option ‘ -mcpu= ’ is used with cortex-m33 , cortex-m35p or cortex-m55 . The option ‘ -mno-fix-cmse-cve-2021-35465 ’ can be used to disable the mitigation.
-mfdpic
-mno-fdpic
Select the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions.
When the compiler is configured for arm-*uclinuxfdpiceabi targets, this option is on by default and implies ‘ -fPIE ’ if none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and the user should explicitly provide the PIC/PIE-related options as needed.
Note that static linking is not supported because it would still involve the dynamic linker when the program self-relocates. If such behavior is acceptable, use -static and -Wl,-dynamic-linker options.
The opposite ‘ -mno-fdpic ’ option is useful (and required) to build the Linux kernel using the same ( arm-*-uclinuxfdpiceabi ) toolchain as the one used to build the userland programs.
3.19.6 AVR Options
These options are defined for AVR implementations:
-mmcu= mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is ‘ avr2 ’.
GCC supports the following AVR devices and ISAs: avr2 “Classic” devices with up to 8 KiB of program memory.
mcu = attiny22 , attiny26 , at90s2313 , at90s2323 , at90s2333 , at90s2343 , at90s4414 , at90s4433 , at90s4434 , at90c8534 , at90s8515 , at90s8535 .
Chapter 3: GCC Command Options 329 avr25 avr3 avr31 avr35 avr4 avr5
“Classic” devices with up to 8 KiB of program memory and with the MOVW instruction.
mcu = attiny13 , attiny13a , attiny24 , attiny24a , attiny25 , attiny261 , attiny261a , attiny2313 , attiny2313a , attiny43u , attiny44 , attiny44a , attiny45 , attiny48 , attiny441 , attiny461 , attiny461a , attiny4313 , attiny84 , attiny84a , attiny85 , attiny87 , attiny88 , attiny828 , attiny841 , attiny861 , attiny861a , ata5272 , ata6616c , at86rf401 .
“Classic” devices with 16 KiB up to 64 KiB of program memory.
mcu = at76c711 , at43usb355 .
“Classic” devices with 128 KiB of program memory.
mcu = atmega103 , at43usb320 .
“Classic” devices with 16 KiB up to 64 KiB of program memory and with the MOVW instruction.
mcu = attiny167 , attiny1634 , atmega8u2 , atmega16u2 , atmega32u2 , ata5505 , ata6617c , ata664251 , at90usb82 , at90usb162 .
“Enhanced” devices with up to 8 KiB of program memory.
mcu = atmega48 , atmega48a , atmega48p , atmega48pa , atmega48pb , atmega8 , atmega8a , atmega8hva , atmega88 , atmega88a , atmega88p , atmega88pa , atmega88pb , atmega8515 , atmega8535 , ata6285 , ata6286 , ata6289 , ata6612c , at90pwm1 , at90pwm2 , at90pwm2b , at90pwm3 , at90pwm3b , at90pwm81 .
“Enhanced” devices with 16 KiB up to 64 KiB of program memory.
mcu = atmega16 , atmega16a , atmega16hva , atmega16hva2 , atmega16hvb , atmega16hvbrevb , atmega16m1 , atmega16u4 , atmega161 , atmega162 , atmega163 , atmega164a , atmega164p , atmega164pa , atmega165pa , atmega165 atmega168
,
, atmega165a atmega168a
,
, atmega165p atmega168p
,
, atmega168pa , atmega168pb , atmega169 , atmega169a , atmega169p , atmega169pa , atmega32 , atmega32a , atmega32c1 , atmega32hvb , atmega32hvbrevb , atmega32m1 , atmega32u4 , atmega32u6 , atmega323 , atmega324a , atmega324p , atmega324pa , atmega324pb , atmega325pa , atmega325 atmega328 ,
, atmega325a atmega328p ,
, atmega325p atmega328pb atmega329 , atmega329a , atmega329p , atmega329pa , atmega3250 ,
,
, atmega3250a , atmega3250p , atmega3250pa , atmega3290 , atmega3290a , atmega3290p , atmega3290pa , atmega406 , atmega64 , atmega64a , atmega64c1 , atmega64hve , atmega64hve2 , atmega64m1 , atmega64rfr2 , atmega640 , atmega644 , atmega644a , atmega644p , atmega644pa , atmega644rfr2 , atmega645 , atmega645a , atmega645p , atmega649 , atmega649a , atmega649p , atmega6450 , atmega6450a , atmega6450p , atmega6490 , atmega6490a , atmega6490p , ata5795 , ata5790 , ata5790n ,
330 Using the GNU Compiler Collection (GCC) avr51 avr6 avrxmega2
“XMEGA” devices with more than 8 KiB and up to 64 KiB of program memory.
mcu = atxmega8e5 , atxmega16a4 , atxmega16a4u , atxmega16c4 , atxmega16d4 , atxmega16e5 , atxmega32a4 , atxmega32a4u , atxmega32c3 , atxmega32c4 , atxmega32d3 , atxmega32d4 , atxmega32e5 .
avrxmega3 ata5791 , ata6613c , ata6614q , ata5782 , ata5831 , ata8210 , ata8510 , ata5702m322 , at90pwm161 , at90pwm216 , at90pwm316 , at90can32 , at90can64 , at90scr100 , at90usb646 , at90usb647 , at94k , m3000 .
“Enhanced” devices with 128 KiB of program memory.
mcu = atmega128 , atmega128a , atmega128rfa1 , atmega128rfr2 , atmega1280 , atmega1281 , atmega1284 , atmega1284p , atmega1284rfr2 , at90can128 , at90usb1286 , at90usb1287 .
“Enhanced” devices with 3-byte PC, i.e. with more than 128 KiB of program memory.
mcu = atmega256rfr2 , atmega2564rfr2 .
atmega2560 , atmega2561 ,
“XMEGA” devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space.
mcu = attiny202 , attiny204 , attiny212 , attiny214 , attiny402 , attiny404 , attiny406 , attiny412 , attiny414 , attiny416 , attiny417 , attiny804 , attiny806 , attiny807 , attiny814 , attiny816 , attiny817 , attiny1604 , attiny1606 , attiny1607 , attiny1614 , attiny1616 , attiny1617 , attiny3214 , attiny3216 , attiny3217 , atmega808 , atmega809 , atmega1608 , atmega1609 , atmega3208 , atmega3209 , atmega4808 , atmega4809 .
avrxmega4
“XMEGA” devices with more than 64 KiB and up to 128 KiB of program memory.
mcu = atxmega64a3 , atxmega64a3u , atxmega64a4u , atxmega64b1 , atxmega64b3 , atxmega64c3 , atxmega64d3 , atxmega64d4 .
avrxmega5
“XMEGA” devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM.
mcu = atxmega64a1 , atxmega64a1u .
avrxmega6
“XMEGA” devices with more than 128 KiB of program memory.
mcu = atxmega128a3 , atxmega128a3u , atxmega128b1 , atxmega128b3 , atxmega128c3 , atxmega128d3 , atxmega128d4 ,
Chapter 3: GCC Command Options 331 atxmega192a3 , atxmega192a3u , atxmega192c3 , atxmega192d3 , atxmega256a3 , atxmega256a3b , atxmega256a3bu , atxmega256a3u , atxmega256c3 , atxmega256d3 , atxmega384c3 , atxmega384d3 .
avrxmega7
“XMEGA” devices with more than 128 KiB of program memory and more than 64 KiB of RAM.
mcu = atxmega128a1 , atxmega128a1u , atxmega128a4u .
avrtiny “TINY” Tiny core devices with 512 B up to 4 KiB of program memory.
mcu = attiny4 , attiny5 , attiny9 , attiny10 , attiny20 , attiny40 .
avr1 This ISA is implemented by the minimal AVR core and supported for assembler only.
mcu = attiny11 , attiny12 , attiny15 , attiny28 , at90s1200 .
-mabsdata
Assume that all data in static storage can be accessed by LDS / STS instructions. This option has only an effect on reduced Tiny devices like ATtiny40.
See also the absdata
Section 6.34.3 [AVR Variable Attributes], page 622
.
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue.
Without this option, outgoing arguments are pushed before calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on AVR so that accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call.
This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack like calls to printf-like functions.
-mbranch-cost= cost
Set the branch costs for conditional branch instructions to cost . Reasonable values for cost are small, non-negative integers. The default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate subroutines.
Code size is smaller.
-mdouble= bits
-mlong-double= bits
Set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Whether or not a specific value for bits is allowed depends on the --with-double= and --with-long-double= configure options , and the same applies for the default values of the options.
332 Using the GNU Compiler Collection (GCC)
-mgas-isr-prologues
Interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the
Section 6.33.6 [ no_gccisr ], page 581
function attribute. This feature is activated per default if optimization is on
(but not with ‘ -Og ’, see
Section 3.11 [Optimize Options], page 159 ), and if GNU
Binutils support PR21683 .
-mint8 Assume int to be 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size.
-mmain-is-OS_task
Do not save registers in main . The effect is the same like attaching attribute
Section 6.33.6 [ OS_task ], page 581
to main . It is activated per default if optimization is on.
-mn-flash= num
Assume that the flash memory has a size of num times 64 KiB.
-mno-interrupts
Generated code is not compatible with hardware interrupts.
Code size is smaller.
-mrelax Try to replace CALL resp.
JMP instruction by the shorter RCALL resp.
RJMP instruction if applicable. Setting ‘ -mrelax ’ just adds the ‘ --mlink-relax ’ option to the assembler’s command line and the ‘ --relax ’ option to the linker’s command line.
Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below.
-mrmw Assume that the device supports the Read-Modify-Write instructions XCH , LAC ,
LAS and LAT .
-mshort-calls
Assume that RJMP and RCALL can target the whole program memory.
This option is used internally for multilib selection. It is not an optimization option, and you don’t need to set it by hand.
-msp8 Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don’t need to set this option by hand.
This option is used internally by the compiler to select and build multilibs for architectures avr2 and avr25 . These architectures mix devices with and without SPH . For any setting other than ‘ -mmcu=avr2 ’ or ‘ -mmcu=avr25 ’ the
Chapter 3: GCC Command Options 333 compiler driver adds or removes this option from the compiler proper’s command line, because the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no SPH register or not.
-mstrict-X
Use address register X in a way proposed by the hardware. This means that X is only used in indirect, post-increment or pre-decrement addressing.
Without this option, the X register may be used in the same way as Y or Z which then is emulated by additional instructions. For example, loading a value with
X+const addressing with a small non-negative const < 64 to a register Rn is performed as adiw r26, const ; X += const ld Rn , X ; Rn = *X sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8 bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for fractional fixedpoint types.
-nodevicelib
Don’t link against AVR-LibC’s device specific library lib<mcu>.a
.
-nodevicespecs
Don’t add ‘ -specs=device-specs/specsmcu ’ to the compiler driver’s command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options.
This means that the user has to supply her private device specs file by means of
‘ -specs= path-to-specs-file ’. There is no more need for option ‘ -mmcu= mcu ’.
This option can also serve as a replacement for the older way of specifying custom device-specs files that needed ‘ -B some-path ’ to point to a directory which contains a folder named device-specs which contains a specs file named specsmcu , where mcu was specified by ‘ -mmcu= mcu ’.
-Waddr-space-convert
Warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without vector prefix. Enabled by default.
3.19.6.1
EIND
and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.
In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used.
334 Using the GNU Compiler Collection (GCC)
Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:
• The compiler never sets EIND .
• The compiler uses EIND implicitly in EICALL / EIJMP instructions or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.
• The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue.
• For indirect calls to functions and computed goto, the linker generates stubs . Stubs are jump pads sometimes also called trampolines . Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
• Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option ‘ -mrelax ’ and the linker option ‘ --relax ’.
There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message.
• The default linker script is arranged for code with EIND = 0 . If code is supposed to work for a setup with EIND != 0 , a custom linker script has to be used in order to place the sections whose name start with .trampolines
into the segment where EIND points to.
• The startup code from libgcc never sets EIND . Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND , see the
AVR-LibC user manual .
• It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3
. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from
AVR-LibC that sets EIND to the segment where the vector table is located.
#include <avr/io.h> static void
__attribute__((section(".init3"),naked,used,no_instrument_function)) init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The __trampolines_start symbol is defined in the linker script.
• Stubs are generated automatically by the linker if the following two conditions are met:
− The address of a label is taken by means of the gs modifier (short for generate stubs ) like so:
LDI r24, lo8(gs( func ))
LDI r25, hi8(gs( func ))
− The final location of that label is in a code segment outside the segment where the stubs are located.
• The compiler emits such gs modifiers for code labels in the following situations:
Chapter 3: GCC Command Options 335
− Taking address of a function or code label.
− Computed goto.
− If prologue-save function is used, see ‘ -mcall-prologues ’ command-line option.
− Switch/case dispatch tables. If you do not want such dispatch tables you can specify the ‘ -fno-jump-tables ’ command-line option.
− C and C ++ constructors/destructors called during startup/shutdown.
− If the tools hit a gs() modifier explained above.
• Jumping to non-symbolic addresses like so is not supported: int main (void)
{
/* Call function at word address 0x2 */ return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be called through a symbol
( func_4 in the example): int main (void)
{ extern int func_4 (void);
/* Call function at byte address 0x4 */ return func_4();
} and the application be linked with ‘ -Wl,--defsym,func_4=0x4 ’. Alternatively, func_4 can be defined in the linker script.
3.19.6.2 Handling of the
RAMPD
,
RAMPX
,
RAMPY
and
RAMPZ
Special
Function Registers
Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a
RAMP register is used as high part of the address: The X , Y , Z address register is concatenated with the RAMPX , RAMPY , RAMPZ special function register, respectively, to get a wide address.
Similarly, RAMPD is used together with direct addressing.
• The startup code initializes the RAMP special function registers with zero.
• If a
[AVR Named Address Spaces], page 537
other than generic or __flash is used, then RAMPZ is set as needed before the operation.
• If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation.
• If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might
(implicitly) use it.
• RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access.
336 Using the GNU Compiler Collection (GCC)
3.19.6.3 AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the ‘ -mmcu= ’ command-line option.
For even more AVR-specific built-in macros see
[AVR Named Address Spaces], page 537
and
Section 6.60.10 [AVR Built-in Functions], page 751 .
__AVR_ARCH__
Build-in macro that resolves to a decimal number that identifies the architecture and depends on the ‘ -mmcu= mcu ’ option. Possible values are:
2 , 25 , 3 , 31 , 35 , 4 , 5 , 51 , 6 for mcu = avr2 , avr25 , avr3 , avr31 , avr35 , avr4 , avr5 , avr51 , avr6 , respectively and
100 , 102 , 103 , 104 , 105 , 106 , 107 for mcu = avrtiny , avrxmega2 , avrxmega3 , avrxmega4 , avrxmega5 , avrxmega6 , avrxmega7 , respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with ‘ -mmcu=atmega8 ’ the macro is defined to 4 .
__AVR_ Device __
Setting ‘ -mmcu= device ’ defines this built-in macro which reflects the device’s name.
For example, ‘ -mmcu=atmega8 ’ defines the built-in macro
__AVR_ATmega8__ , ‘ -mmcu=attiny261a ’ defines __AVR_ATtiny261A__ , etc.
The built-in macros’ names follow the scheme __AVR_ Device __ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in ‘ -mmcu= device ’ is that the latter is always lowercase.
If device is not a device but only a core architecture like ‘ avr51 ’, this macro is not defined.
__AVR_DEVICE_NAME__
Setting ‘ -mmcu= device ’ defines this built-in macro to the device’s name. For example, with ‘ -mmcu=atmega8 ’ the macro is defined to atmega8 .
If device is not a device but only a core architecture like ‘ avr51 ’, this macro is not defined.
__AVR_XMEGA__
The device / architecture belongs to the XMEGA family of devices.
__AVR_HAVE_ELPM__
The device has the ELPM instruction.
__AVR_HAVE_ELPMX__
The device has the ELPM R n ,Z and ELPM R n ,Z+ instructions.
__AVR_HAVE_MOVW__
The device has the MOVW instruction to perform 16-bit register-register moves.
__AVR_HAVE_LPMX__
The device has the LPM R n ,Z and LPM R n ,Z+ instructions.
Chapter 3: GCC Command Options 337
__AVR_HAVE_MUL__
The device has a hardware multiplier.
__AVR_HAVE_JMP_CALL__
The device has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory.
__AVR_HAVE_EIJMP_EICALL__
__AVR_3_BYTE_PC__
The device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.
__AVR_2_BYTE_PC__
The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.
__AVR_HAVE_8BIT_SP__
__AVR_HAVE_16BIT_SP__
The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by ‘ -mtiny-stack ’.
__AVR_HAVE_SPH__
__AVR_SP8__
The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by ‘ -mmcu= ’ and in the cases of ‘ -mmcu=avr2 ’ and ‘ -mmcu=avr25 ’ also by ‘ -msp8 ’.
__AVR_HAVE_RAMPD__
__AVR_HAVE_RAMPX__
__AVR_HAVE_RAMPY__
__AVR_HAVE_RAMPZ__
The device has the RAMPD , RAMPX , RAMPY , RAMPZ special function register, respectively.
__NO_INTERRUPTS__
This macro reflects the ‘ -mno-interrupts ’ command-line option.
__AVR_ERRATA_SKIP__
__AVR_ERRATA_SKIP_JMP_CALL__
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are SBRS , SBRC , SBIS , SBIC and CPSE . The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.
__AVR_ISA_RMW__
The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
__AVR_SFR_OFFSET__= offset
Instructions that can address I/O special function registers directly like IN , OUT ,
SBI , etc. may use a different address as if addressed by an instruction to access
338 Using the GNU Compiler Collection (GCC)
RAM like LD or STS . This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.
__AVR_SHORT_CALLS__
The ‘ -mshort-calls ’ command line option is set.
__AVR_PM_BASE_ADDRESS__= addr
Some devices support reading from flash memory by means of LD* instructions.
The flash memory is seen in the data address space at an offset of __AVR_PM_
BASE_ADDRESS__ . If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put .rodata
into
RAM. This is handled by the default linker description file, and is currently available for avrtiny and avrxmega3 . Even more convenient, there is no need to use address spaces like __flash or features like attribute progmem and pgm_ read_* .
__WITH_AVRLIBC__
The compiler is configured to be used together with AVR-Libc.
See the
‘ --with-avrlibc ’ configure option.
__HAVE_DOUBLE_MULTILIB__
Defined if ‘ -mdouble= ’ acts as a multilib option.
__HAVE_DOUBLE32__
__HAVE_DOUBLE64__
Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option ‘ -mdouble= ’.
__DEFAULT_DOUBLE__
The size in bits of double if ‘ -mdouble= ’ is not set. To test the layout of double in a program, use the built-in macro __SIZEOF_DOUBLE__ .
__HAVE_LONG_DOUBLE32__
__HAVE_LONG_DOUBLE64__
__HAVE_LONG_DOUBLE_MULTILIB__
__DEFAULT_LONG_DOUBLE__
Same as above, but for long double instead of double .
__WITH_DOUBLE_COMPARISON__
Reflects the --with-double-comparison={tristate|bool|libf7} configure option and is defined to 2 or 3 .
__WITH_LIBF7_LIBGCC__
__WITH_LIBF7_MATH__
__WITH_LIBF7_MATH_SYMBOLS__
Reflects the --with-libf7={libgcc|math|math-symbols} configure option .
3.19.7 Blackfin Options
-mcpu= cpu [ sirevision ]
Specifies the name of the target Blackfin processor. Currently, cpu can be one of ‘ bf512 ’, ‘ bf514 ’, ‘ bf516 ’, ‘ bf518 ’, ‘ bf522 ’, ‘ bf523 ’, ‘ bf524 ’, ‘ bf525 ’,
‘ bf526 ’, ‘ bf527 ’, ‘ bf531 ’, ‘ bf532 ’, ‘ bf533 ’, ‘ bf534 ’, ‘ bf536 ’, ‘ bf537 ’, ‘ bf538 ’,
Chapter 3: GCC Command Options 339
-msim
‘ bf539 ’, ‘ bf542 ’, ‘ bf544 ’, ‘ bf547 ’, ‘ bf548 ’, ‘ bf549 ’, ‘ bf542m ’, ‘ bf544m ’,
‘ bf547m ’, ‘ bf548m ’, ‘ bf549m ’, ‘ bf561 ’, ‘ bf592 ’.
The optional sirevision specifies the silicon revision of the target Blackfin processor.
Any workarounds available for the targeted silicon revision are enabled.
If sirevision is ‘ none ’, no workarounds are enabled.
If sirevision is
‘ any ’, all workarounds for the targeted processor are enabled. The __SILICON_
REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is ‘ none ’, the __SILICON_
REVISION__ is not defined. If sirevision is ‘ any ’, the __SILICON_REVISION__ is defined to be 0xffff . If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu . For the ‘ bfin-elf ’ toolchain, this option causes the hardware BSP provided by libgloss to be linked in if ‘ -msim ’ is not given.
Without this option, ‘ bf532 ’ is used as the processor by default.
Note that support for ‘ bf561 ’ is incomplete. For ‘ bf561 ’, only the preprocessor macro is defined.
Specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only for
‘ bfin-elf ’ toolchain. Certain other options, such as ‘ -mid-shared-library ’ and ‘ -mfdpic ’, imply ‘ -msim ’.
-momit-leaf-frame-pointer
Don’t keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions.
-mspecld-anomaly
When enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, __WORKAROUND_
SPECULATIVE_LOADS is defined.
-mno-specld-anomaly
Don’t generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated code does not contain
CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, __WORKAROUND_SPECULATIVE_SYNCS is defined.
-mno-csync-anomaly
Don’t generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
-mlow64k When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory.
-mno-low64k
Assume that the program is arbitrarily large. This is the default.
340 Using the GNU Compiler Collection (GCC)
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies ‘ -fPIC ’. With a ‘ bfin-elf ’ target, this option implies ‘ -msim ’.
-mno-id-shared-library
Generate code that doesn’t assume ID-based shared libraries are being used.
This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID method, but assumes that this library or executable won’t link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won’t link against any ID shared libraries. Slower code is generated for jump and call insns.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment.
This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register.
This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying ‘ -mno-long-calls ’ restores the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.
-mfast-fp
Link with the fast floating-point library.
This library relaxes some of the
IEEE floating-point standard’s rules for checking inputs against Not-a-Number
(NAN), in the interest of performance.
Chapter 3: GCC Command Options 341
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without ‘ -mfdpic ’.
-mmulticore
Build a standalone application for multicore Blackfin processors.
This option causes proper start files and link scripts supporting multicore to be used, and defines the macro __BFIN_MULTICORE .
It can only be used with
‘ -mcpu=bf561 [ sirevision ]’.
This option can be used with ‘ -mcorea ’ or ‘ -mcoreb ’, which selects the oneapplication-per-core programming model. Without ‘ -mcorea ’ or ‘ -mcoreb ’, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as coreb_main .
If this option is not used, the single-core application programming model is used.
-mcorea Build a standalone application for Core A of BF561 when using the oneapplication-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro __BFIN_COREA is defined. This option can only be used in conjunction with ‘ -mmulticore ’.
-mcoreb Build a standalone application for Core B of BF561 when using the one-application-per-core programming model.
Proper start files and link scripts are used to support Core B, and the macro __BFIN_COREB is defined.
When this option is used, coreb_main should be used instead of main . This option can only be used in conjunction with ‘ -mmulticore ’.
-msdram Build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. The loader should initialize SDRAM before loading the application.
-micplb Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
3.19.8 C6X Options
-march= name
This specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code.
Permissible names are: ‘ c62x ’, ‘ c64x ’, ‘ c64x+ ’, ‘ c67x ’, ‘ c67x+ ’, ‘ c674x ’.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim Choose startup files and linker script suitable for the simulator.
-msdata=default
Put small global and static data in the .neardata
section, which is pointed to by register B14 . Put small uninitialized global and static data in the .bss
342 Using the GNU Compiler Collection (GCC) section, which is adjacent to the .neardata
section. Put small read-only data into the .rodata
section. The corresponding sections used for large pieces of data are .fardata
, .far
and .const
.
-msdata=all
Put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the B14 register to access them.
-msdata=none
Make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the .fardata
section, and all uninitialized data in the .far
section. Put all constant data into the .const
section.
3.19.9 CRIS Options
These options are defined specifically for the CRIS ports.
-march= architecture-type
-mcpu= architecture-type
Generate code for the specified architecture.
The choices for architecturetype are ‘ v3 ’, ‘ v8 ’ and ‘ v10 ’ for respectively ETRAX 4, ETRAX 100, and
ETRAX 100 LX. Default is ‘ v0 ’ except for cris-axis-linux-gnu, where the default is ‘ v10 ’.
-mtune= architecture-type
Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions.
The choices for architecture-type are the same as for ‘ -march= architecture-type ’.
-mmax-stack-frame= n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options ‘ -metrax4 ’ and ‘ -metrax100 ’ are synonyms for ‘ -march=v3 ’ and
‘ -march=v8 ’ respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the muls and mulu instructions for CPU models where it applies. This option is active by default.
-mpdebug Enable CRIS-specific verbose debug-related information in the assembly code.
This option also has the effect of turning off the ‘ #NO_APP ’ formatted-code indicator to the assembler at the beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.
-mno-side-effects
Do not emit instructions with side effects in addressing modes other than postincrement.
Chapter 3: GCC Command Options 343
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (‘ no’ options) arrange (eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32bit alignment. ABI details such as structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit Similar to the stack- data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With ‘ -mno-prologue-epilogue ’, the normal function prologue and epilogue which set up the stack frame are omitted and no return instructions or return sequences are generated in the code. Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated.
-mno-gotplt
-mgotplt With ‘ -fpic ’ and ‘ -fPIC ’, don’t generate (do generate) instruction sequences that load addresses for functions from the PLT part of the GOT rather than
(traditional on other architectures) calls to the PLT. The default is ‘ -mgotplt ’.
-melf Legacy no-op option only recognized with the cris-axis-elf and cris-axis-linuxgnu targets.
-mlinux Legacy no-op option only recognized with the cris-axis-linux-gnu target.
-sim
-sim2
This option, recognized for the cris-axis-elf, arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively.
Like ‘ -sim ’, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.
3.19.10 CR16 Options
These options are defined specifically for the CR16 ports.
-mmac Enable the use of multiply-accumulate instructions. Disabled by default.
-mcr16cplus
-mcr16c Generate code for CR16C or CR16C + architecture. CR16C + architecture is default.
344 Using the GNU Compiler Collection (GCC)
-msim Links the library libsim.a which is in compatible with simulator. Applicable to
ELF compiler only.
-mint32 Choose integer type as 32-bit wide.
-mbit-ops
Generates sbit / cbit instructions for bit manipulations.
-mdata-model= model
Choose a data model. The choices for model are ‘ near ’, ‘ far ’ or ‘ medium ’.
‘ medium ’ is default. However, ‘ far ’ is not valid with ‘ -mcr16c ’, as the CR16C architecture does not support the far data model.
3.19.11 C-SKY Options
GCC supports these options when compiling for C-SKY V2 processors.
-march= arch
Specify the C-SKY target architecture.
Valid values for arch are: ‘ ck801 ’,
‘ ck802 ’, ‘ ck803 ’, ‘ ck807 ’, and ‘ ck810 ’. The default is ‘ ck810 ’.
-mcpu= cpu
Specify the C-SKY target processor.
Valid values for cpu are: ‘ ck801 ’,
‘ ck801t ’, ‘ ck802 ’, ‘ ck802t ’, ‘ ck802j ’, ‘ ck803 ’, ‘ ck803h ’, ‘ ck803t ’, ‘ ck803ht ’,
‘ ck803f ’, ‘ ck803fh ’, ‘ ck803e ’, ‘ ck803eh ’, ‘ ck803et ’, ‘ ck803eht ’, ‘ ck803ef ’,
‘ ck803efh ’, ‘ ck803ft ’, ‘ ck803eft ’, ‘ ck803efht ’, ‘ ck803r1 ’, ‘ ck803hr1 ’,
‘ ck803tr1 ’, ‘ ck803htr1 ’, ‘ ck803fr1 ’, ‘ ck803fhr1 ’, ‘ ck803er1 ’, ‘ ck803ehr1 ’,
‘ ck803etr1 ’, ‘ ck803ehtr1 ’, ‘ ck803efr1 ’, ‘ ck803efhr1 ’, ‘ ck803ftr1 ’,
‘ ck803eftr1 ’, ‘ ck803efhtr1 ’, ‘ ck803s ’, ‘ ck803st ’, ‘ ck803se ’, ‘ ck803sf ’,
‘ ck803sef ’, ‘ ck803seft ’, ‘ ck807e ’, ‘ ck807ef ’, ‘ ck807 ’, ‘ ck807f ’, ‘ ck810e ’,
‘ ck810et ’, ‘ ck810ef ’, ‘ ck810eft ’, ‘ ck810 ’, ‘ ck810v ’, ‘ ck810f ’, ‘ ck810t ’,
‘ ck810fv ’, ‘ ck810tv ’, ‘ ck810ft ’, and ‘ ck810ftv ’.
-mbig-endian
-EB
-mlittle-endian
-EL
Select big- or little-endian code. The default is little-endian.
-mfloat-abi= name
Specifies which floating-point ABI to use.
Permissible values are: ‘ soft ’,
‘ softfp ’ and ‘ hard ’.
Specifying ‘ soft ’ causes GCC to generate output containing library calls for floating-point operations. ‘ softfp ’ allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.
‘ hard ’ allows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that the hardfloat and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.
Chapter 3: GCC Command Options 345
-mhard-float
-msoft-float
Select hardware or software floating-point implementations. The default is soft float.
-mdouble-float
-mno-double-float
When ‘ -mhard-float ’ is in effect, enable generation of double-precision float instructions. This is the default except when compiling for CK803.
-mfdivdu
-mno-fdivdu
When ‘ -mhard-float ’ is in effect, enable generation of frecipd , fsqrtd , and fdivd instructions. This is the default except when compiling for CK803.
-mfpu= fpu
Select the floating-point processor.
This option can only be used with ‘ -mhard-float ’.
Values for fpu are ‘ fpv2_sf ’ (equivalent to
‘ -mno-double-float -mno-fdivdu ’), ‘ fpv2 ’ (‘ -mdouble-float -mno-divdu ’), and ‘ fpv2_divd ’ (‘ -mdouble-float -mdivdu ’).
-melrw
-mno-elrw
Enable the extended lrw instruction. This option defaults to on for CK801 and off otherwise.
-mistack
-mno-istack
Enable interrupt stack instructions; the default is off.
The ‘ -mistack ’ option is required to handle the interrupt and isr function attributes (see
Section 6.33.10 [C-SKY Function Attributes], page 584
).
-mmp Enable multiprocessor instructions; the default is off.
-mcp Enable coprocessor instructions; the default is off.
-mcache Enable coprocessor instructions; the default is off.
-msecurity
Enable C-SKY security instructions; the default is off.
-mtrust Enable C-SKY trust instructions; the default is off.
-mdsp
-medsp
-mvdsp Enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively.
All of these options default to off.
-mdiv
-mno-div Generate divide instructions. Default is off.
346 Using the GNU Compiler Collection (GCC)
-msmart
-mno-smart
Generate code for Smart Mode, using only registers numbered 0-7 to allow use of
16-bit instructions. This option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off.
-mhigh-registers
-mno-high-registers
Generate code using the high registers numbered 16-31. This option is not supported on CK801, CK802, or CK803, and is enabled by default for other processors.
-manchor
-mno-anchor
Generate code using global anchor symbol addresses.
-mpushpop
-mno-pushpop
Generate code using push and pop instructions. This option defaults to on.
-mmultiple-stld
-mstm
-mno-multiple-stld
-mno-stm Generate code using stm and ldm instructions. This option isn’t supported on
CK801 but is enabled by default on other processors.
-mconstpool
-mno-constpool
Create constant pools in the compiler instead of deferring it to the assembler.
This option is the default and required for correct code generation on CK801 and CK802, and is optional on other processors.
-mstack-size
-mno-stack-size
Emit .stack_size
directives for each function in the assembly output. This option defaults to off.
-mccrt
-mno-ccrt
Generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off.
-mbranch-cost= n
Set the branch costs to roughly n instructions. The default is 1.
-msched-prolog
-mno-sched-prolog
Permit scheduling of function prologue and epilogue sequences.
Using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled by default.
Chapter 3: GCC Command Options 347
-msim Links the library libsemi.a which is in compatible with simulator. Applicable to ELF compiler only.
3.19.12 Darwin Options
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create “fat” object files; it creates an object file for the single architecture that GCC was built to target. Apple’s GCC on Darwin does create
“fat” files if multiple ‘ -arch ’ options are used; it does so by running the compiler or linker multiple times and joining the results together with ‘ lipo ’.
The subtype of the file created (like ‘ ppc7400 ’ or ‘ ppc970 ’ or ‘ i686 ’) is determined by the flags that specify the ISA that GCC is targeting, like ‘ -mcpu ’ or ‘ -march ’. The
‘ -force_cpusubtype_ALL ’ option can be used to override this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, ‘ as ’, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ‘ ppc750 ’ object file. The linker for shared libraries, ‘ /usr/bin/libtool ’, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ‘ ppc970 ’ object file in a ‘ ppc7400 ’ library). The linker for executables, ld , quietly gives the executable the most restrictive subtype of any of its input files.
-F dir Add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by ‘ -I ’ options and are scanned in a left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a directory with a ‘ Headers ’ and/or ‘ PrivateHeaders ’ directory contained directly in it that ends in ‘ .framework
’.
The name of a framework is the name of this directory excluding the ‘ .framework
’. Headers associated with the framework are found in one of those two directories, with ‘ Headers ’ being searched first. A subframework is a framework directory that is in a framework’s ‘ Frameworks ’ directory.
Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header.
Two subframeworks are siblings if they occur in the same framework.
A subframework should not have the same name as a framework; a warning is issued if this is violated.
Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in ‘ /System/Library/Frameworks ’ and ‘ /Library/Frameworks ’.
An example include looks like #include <Framework/header.h> , where
‘ Framework ’ denotes the name of the framework and ‘ header.h
’ is found in the ‘ PrivateHeaders ’ or ‘ Headers ’ directory.
-iframework dir
Like ‘ -F ’ except the directory is a treated as a system directory. The main difference between this ‘ -iframework ’ and ‘ -F ’ is that with ‘ -iframework ’ the compiler does not warn about constructs contained within header files found via dir . This option is valid only for the C family of languages.
348 Using the GNU Compiler Collection (GCC)
-gused Emit debugging information for symbols that are used. For stabs debugging format, this enables ‘ -feliminate-unused-debug-symbols ’. This is by default
ON.
Emit debugging information for all symbols and types.
-gfull
-mmacosx-version-min= version
The earliest version of MacOS X that this executable will run on is version .
Typical values of version include 10.1
, 10.2
, and 10.3.9
.
If the compiler was built to use the system’s headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible.
-mkernel Enable kernel development mode.
The ‘ -mkernel ’ option sets
‘ -static ’, ‘ -fno-common ’, ‘ -fno-use-cxa-atexit ’, ‘ -fno-exceptions ’,
‘ -fno-non-call-exceptions ’, ‘ -fapple-kext ’, ‘ -fno-weak ’ and ‘ -fno-rtti ’ where applicable.
This mode also sets ‘ -mno-altivec ’, ‘ -msoft-float ’,
‘ -fno-builtin ’ and ‘ -mlong-branch ’ for PowerPC targets.
-mone-byte-bool
Override the defaults for bool so that sizeof(bool)==1 .
By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.
Warning: The ‘ -mone-byte-bool ’ switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load ‘ .o
’ files into already-running programs.
‘ -findirect-data ’ and ‘ -ffix-and-continue ’ are provided for backwards compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
-bundle Produce a Mach-o bundle format file. See man ld(1) for more information.
Chapter 3: GCC Command Options 349
-bundle_loader executable
This option specifies the executable that will load the build output file being linked. See man ld(1) for more information.
-dynamiclib
When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin ‘ libtool ’ command.
-force_cpusubtype_ALL
This causes GCC’s output file to have the ‘ ALL ’ subtype, instead of one controlled by the ‘ -mcpu ’ or ‘ -march ’ option.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
350 Using the GNU Compiler Collection (GCC)
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
3.19.13 DEC Alpha Options
These ‘ -m ’ options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations. When ‘ -msoft-float ’ is specified, functions in ‘ libgcc.a
’ are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point registers.
Chapter 3: GCC Command Options 351
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set.
‘ -mno-fp-regs ’ implies ‘ -msoft-float ’. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0 . This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with ‘ -mno-fp-regs ’ must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.
-mieee The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro
_IEEE_FP is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option ‘ -ieee_with_no_inexact ’.
-mieee-with-inexact
This is like ‘ -mieee ’ except the generated code also maintains the IEEE inexactflag . Turning on this option causes the generated code to implement fullycompliant IEEE math. In addition to _IEEE_FP , _IEEE_FP_EXACT is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag , you should normally not specify this option. Other Alpha compilers call this option ‘ -ieee_with_inexact ’.
-mfp-trap-mode= trap-mode
This option controls what floating-point related traps are enabled. Other Alpha compilers call this option ‘ -fptm trap-mode ’. The trap mode can be set to one of four values:
‘ n ’ This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
‘ u ’
‘
‘ su ’ sui ’
In addition to the traps enabled by ‘ n ’, underflow traps are enabled as well.
Like ‘ u ’, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
Like ‘ su ’, but inexact traps are enabled as well.
-mfp-rounding-mode= rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option ‘ -fprm rounding-mode ’. The rounding-mode can be one of:
352 Using the GNU Compiler Collection (GCC)
‘
‘
‘
‘ n m c d
’
’
’
’ Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
Round towards minus infinity.
Chopped rounding mode. Floating-point numbers are rounded towards zero.
Dynamic rounding mode. A field in the floating-point control register ( fpcr , see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr , ‘ d ’ corresponds to round towards plus infinity.
-mtrap-precision= trap-precision
In the Alpha architecture, floating-point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on the requirements of an application, different levels of precisions can be selected:
‘ p ’
‘ f ’
Program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
Function precision. The trap handler can determine the function that caused a floating-point exception.
‘ i ’ Instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called ‘ -scope_safe ’ and
‘ -resumption_safe ’.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this option unless you also specify ‘ -mtrap-precision=i ’ and either
‘ -mfp-trap-mode=su ’ or ‘ -mfp-trap-mode=sui ’. Its only effect is to emit the line ‘ .eflag 48 ’ in the function prologue of the generated assembly file.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, it outputs the constant as a literal and generates code to load it from the data segment at run time.
Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).
You typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.
Chapter 3: GCC Command Options 353
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via ‘ -mcpu= ’ option or that of the CPU on which
GCC was built if none is specified.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.
-msmall-data
-mlarge-data
When ‘ -mexplicit-relocs ’ is in effect, static data is accessed via gp-relative relocations. When ‘ -msmall-data ’ is used, objects 8 bytes long or smaller are placed in a small data area (the .sdata
and .sbss
sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.
The default is ‘ -mlarge-data ’. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program’s data segment.
When generating code for shared libraries, ‘ -fpic ’ implies ‘ -msmall-data ’ and
‘ -fPIC ’ implies ‘ -mlarge-data ’.
-msmall-text
-mlarge-text
When ‘ -msmall-text ’ is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When ‘ -msmall-data ’ is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1.
The default is ‘ -mlarge-text ’.
354 Using the GNU Compiler Collection (GCC)
-mcpu= cpu_type
Set the instruction set and instruction scheduling parameters for machine type cpu type . You can specify either the ‘ EV ’ style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC defaults to the processor on which the compiler was built.
Supported values for cpu type are
‘ ev4 ’
‘ ev45 ’
‘ 21064 ’ Schedules as an EV4 and has no instruction set extensions.
‘ ev5 ’
‘ 21164 ’ Schedules as an EV5 and has no instruction set extensions.
‘ ev56 ’
‘ 21164a ’ Schedules as an EV5 and supports the BWX extension.
‘ pca56 ’
‘ 21164pc ’
‘ 21164PC ’ Schedules as an EV5 and supports the BWX and MAX extensions.
‘
‘ ev6 ’
21264 ’ Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
‘ ev67 ’
‘ 21264a ’ Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value ‘ native ’, which selects the best architecture option for the host processor. ‘ -mcpu=native ’ has no effect if GCC does not recognize the processor.
-mtune= cpu_type
Set only the instruction scheduling parameters for machine type cpu type . The instruction set is not changed.
Native toolchains also support the value ‘ native ’, which selects the best architecture option for the host processor. ‘ -mtune=native ’ has no effect if GCC does not recognize the processor.
-mmemory-latency= time
Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.
Valid options for time are
‘ number ’ A decimal number representing clock cycles.
Chapter 3: GCC Command Options 355
‘ L1 ’
‘ L2 ’
‘ L3 ’
‘ main ’ The compiler contains estimates of the number of clock cycles for
“typical” EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory.
Note that L3 is only valid for EV5.
3.19.14 eBPF Options
-mframe-limit= bytes
This specifies the hard limit for frame sizes, in bytes. Currently, the value that can be specified should be less than or equal to ‘ 32767 ’. Defaults to whatever limit is imposed by the version of the Linux kernel targeted.
-mkernel= version
This specifies the minimum version of the kernel that will run the compiled program. GCC uses this version to determine which instructions to use, what kernel helpers to allow, etc. Currently, version can be one of ‘ 4.0
’, ‘ 4.1
’, ‘ 4.2
’,
‘ 4.3
’, ‘ 4.4
’, ‘ 4.5
’, ‘ 4.6
’, ‘ 4.7
’, ‘ 4.8
’, ‘ 4.9
’, ‘ 4.10
’, ‘ 4.11
’, ‘ 4.12
’, ‘ 4.13
’, ‘ 4.14
’,
‘ 4.15
’, ‘ 4.16
’, ‘ 4.17
’, ‘ 4.18
’, ‘ 4.19
’, ‘ 4.20
’, ‘ 5.0
’, ‘ 5.1
’, ‘ 5.2
’, ‘ latest ’ and
‘ native ’.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-mjmpext Enable generation of extra conditional-branch instructions. Enabled for CPU v2 and above.
-mjmp32 Enable 32-bit jump instructions. Enabled for CPU v3 and above.
-malu32 Enable 32-bit ALU instructions. Enabled for CPU v3 and above.
-mcpu= version
This specifies which version of the eBPF ISA to target. Newer versions may not be supported by all kernels. The default is ‘ v3 ’.
Supported values for version are:
‘ v1 ’
‘ v2 ’
The first stable eBPF ISA with no special features or extensions.
Supports the jump extensions, as in ‘ -mjmpext ’.
‘ v3 ’ All features of v2, plus:
− 32-bit jump operations, as in ‘ -mjmp32 ’
− 32-bit ALU operations, as in ‘ -malu32 ’
-mco-re Enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires and is implied by ‘ -gbtf ’.
356 Using the GNU Compiler Collection (GCC)
-mno-co-re
Disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when generating BTF debug information for the
BPF target.
-mxbpf Generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
− Save and restore callee-saved registers at function entry and exit, respectively.
3.19.15 FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is no need to include the simulator library (‘ libsim.a
’) on the linker command line.
3.19.16 FT32 Options
These options are defined specifically for the FT32 port.
-msim
-mlra
Specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.
Enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload.
-mnodiv Do not use div and mod instructions.
-mft32b Enable use of the extended instructions of the FT32B processor.
-mcompress
Compress all code using the Ft32B code compression scheme.
-mnopm Do not generate code that reads program memory.
3.19.17 FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
Chapter 3: GCC Command Options 357
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use icc0 and fcc0 .
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies ‘ -fPIE ’. With
‘ -fpic ’ or ‘ -fpie ’, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with ‘ -fPIC ’ or ‘ -fPIE ’, GOT offsets are computed with 32 bits. With a ‘ bfin-elf ’ target, this option implies ‘ -msim ’.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without ‘ -mfdpic ’. It’s enabled by default if optimizing for speed and compiling for shared libraries (i.e., ‘ -fPIC ’ or ‘ -fpic ’), or when an optimization option such as ‘ -O3 ’ or above is present in the command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
358 Using the GNU Compiler Collection (GCC)
-mtls
Do not assume a large TLS segment when generating thread-local code.
-mgprel-ro
Enable the use of GPREL relocations in the FDPIC ABI for data that is known to be in read-only sections. It’s enabled by default, except for ‘ -fpic ’ or ‘ -fpie ’: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With ‘ -fPIC ’ or ‘ -fPIE ’, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it’s more likely to be a win. If it is not,
‘ -mno-gprel-ro ’ can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It’s implied by ‘ -mlibrary-pic ’, as well as by ‘ -fPIC ’ and ‘ -fpic ’ without ‘ -mfdpic ’. You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with ‘ -mno-linked-fp ’.
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit.
This allows the functions to be placed anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn’t create new packets; it merely adds NOPs to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
Chapter 3: GCC Command Options 359
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of && and || in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of && and || in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
360 Using the GNU Compiler Collection (GCC)
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-moptimize-membar
This switch removes redundant membar instructions from the compilergenerated code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant membar instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu= cpu
Select the processor type for which to generate code. Possible values are ‘ frv ’,
‘ fr550 ’, ‘ tomcat ’, ‘ fr500 ’, ‘ fr450 ’, ‘ fr405 ’, ‘ fr400 ’, ‘ fr300 ’ and ‘ simple ’.
3.19.18 GNU/Linux Options
These ‘ -m ’ options are defined for GNU/Linux targets:
-mglibc Use the GNU C library. This is the default except on ‘ *-*-linux-*uclibc* ’,
‘ *-*-linux-*musl* ’ and ‘ *-*-linux-*android* ’ targets.
-muclibc Use uClibc C library. This is the default on ‘ *-*-linux-*uclibc* ’ targets.
-mmusl Use the musl C library. This is the default on ‘ *-*-linux-*musl* ’ targets.
-mbionic Use Bionic C library. This is the default on ‘ *-*-linux-*android* ’ targets.
-mandroid
Compile code compatible with Android platform.
This is the default on
‘ *-*-linux-*android* ’ targets.
When compiling, this option enables ‘ -mbionic ’, ‘ -fPIC ’, ‘ -fno-exceptions ’ and ‘ -fno-rtti ’ by default. When linking, this option makes the GCC driver pass Android-specific options to the linker.
Finally, this option causes the preprocessor macro __ANDROID__ to be defined.
-tno-android-cc
Disable compilation effects of ‘ -mandroid ’, i.e., do not enable ‘ -mbionic ’,
‘ -fPIC ’, ‘ -fno-exceptions ’ and ‘ -fno-rtti ’ by default.
-tno-android-ld
Disable linking effects of ‘ -mandroid ’, i.e., pass standard Linux linking options to the linker.
3.19.19 H8/300 Options
These ‘ -m ’ options are defined for the H8/300 implementations:
-mrelax Shorten some address references at link time, when possible; uses the linker option ‘ -relax ’. See Section “ ld and the H8/300” in Using ld , for a fuller description.
Chapter 3: GCC Command Options 361
-mh Generate code for the H8/300H.
-ms
-mn
Generate code for the H8S.
Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with ‘ -mh ’ or ‘ -ms ’.
-ms2600 Generate code for the H8S/2600. This switch must be used with ‘ -ms ’.
-mexr Extended registers are stored on stack before execution of function with monitor attribute. Default option is ‘ -mexr ’. This option is valid only for H8S targets.
-mno-exr Extended registers are not stored on stack before execution of function with monitor attribute. Default option is ‘ -mno-exr ’. This option is valid only for
H8S targets.
-mint32 Make int data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300.
The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. ‘ -malign-300 ’ causes them to be aligned on 2-byte boundaries.
This option has no effect on the H8/300.
3.19.20 HPPA Options
These ‘ -m ’ options are defined for the HPPA family of computers:
-march= architecture-type
Generate code for the specified architecture. The choices for architecture-type are ‘ 1.0
’ for PA 1.0, ‘ 1.1
’ for PA 1.1, and ‘ 2.0
’ for PA 2.0 processors. Refer to ‘ /usr/lib/sched.models
’ on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for ‘ -march=1.0
’, ‘ -march=1.1
’, and ‘ -march=2.0
’ respectively.
-mcaller-copies
The caller copies function arguments passed by hidden reference. This option should be used with care as it is not compatible with the default 32-bit runtime.
However, only aggregates larger than eight bytes are passed by hidden reference and the option provides better compatibility with OpenMP.
-mjump-in-delay
This option is ignored and provided for compatibility purposes only.
-mdisable-fpregs
Prevent floating-point registers from being used in any manner. This is necessary for compiling kernels that perform lazy context switching of floating-point registers. If you use this option and attempt to perform floating-point operations, the compiler aborts.
362 Using the GNU Compiler Collection (GCC)
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows
GCC to emit code that performs faster indirect calls.
This option does not work in the presence of shared libraries or nested functions.
-mfixed-range= register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a comma.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the
HP-UX 10 linker. This is equivalent to the ‘ +k ’ option to the HP compilers.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mgas Enable the use of assembler directives only GAS understands.
-mschedule= cpu-type
Schedule code according to the constraints for the machine type cpu-type . The choices for cpu-type are ‘ 700 ’ ‘ 7100 ’, ‘ 7100LC ’, ‘ 7200 ’, ‘ 7300 ’ and ‘ 8000 ’. Refer to ‘ /usr/lib/sched.models
’ on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is ‘ 8000 ’.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs.
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine’s usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
‘ -msoft-float ’ changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile ‘ libgcc.a
’, the library that comes with GCC, with
‘ -msoft-float ’ in order for this to work.
Chapter 3: GCC Command Options 363
-msio Generate the predefine, _SIO , for server IO. The default is ‘ -mwsio ’. This generates the predefines, __hp9000s700 , __hp9000s700__ and _WSIO , for workstation IO. These options are available under HP-UX and HI-UX.
-mgnu-ld Use options specific to GNU ld . This passes ‘ -shared ’ to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld . The ld that is called is determined by the ‘ --with-ld ’ configure option, GCC’s program search path, and finally by the user’s PATH . The linker used by GCC can be printed using ‘ which ‘gcc -print-prog-name=ld‘ ’. This option is only available on the
64-bit HP-UX GCC, i.e. configured with ‘ hppa*64*-*-hpux* ’.
-mhp-ld Use options specific to HP ld . This passes ‘ -b ’ to ld when building a shared library and passes ‘ +Accept TypeMismatch ’ to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld . The ld that is called is determined by the ‘ --with-ld ’ configure option, GCC’s program search path, and finally by the user’s PATH . The linker used by GCC can be printed using ‘ which ‘gcc -print-prog-name=ld‘ ’.
This option is only available on the 64-bit HP-UX GCC, i.e. configured with
‘ hppa*64*-*-hpux* ’.
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs.
The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the ‘ -ffunction-sections ’ option, or when using the ‘ -mgas ’ and
‘ -mno-portable-runtime ’ options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long.
-munix= unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are ‘ 93 ’, ‘ 95 ’ and ‘ 98 ’. ‘ 93 ’ is supported on all
HP-UX versions. ‘ 95 ’ is available on HP-UX 10.10 and later. ‘ 98 ’ is available
364 Using the GNU Compiler Collection (GCC) on HP-UX 11.11 and later. The default values are ‘ 93 ’ for HP-UX 10.00, ‘ 95 ’ for HP-UX 10.10 though to 11.00, and ‘ 98 ’ for HP-UX 11.11 and later.
‘ -munix=93 ’ provides the same predefines as GCC 3.3 and 3.4. ‘ -munix=95 ’ provides additional predefines for XOPEN_UNIX and _XOPEN_SOURCE_EXTENDED , and the startfile ‘ unix95.o
’. ‘ -munix=98 ’ provides additional predefines for
_XOPEN_UNIX , _XOPEN_SOURCE_EXTENDED , _INCLUDE__STDC_A1_SOURCE and _
INCLUDE_XOPEN_SOURCE_500 , and the startfile ‘ unix98.o
’.
It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable __xpg4_extended_mask as appropriate.
Most GNU software doesn’t provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the ‘ -static ’ option is specified on HP-UX 10 and later.
-static The HP-UX implementation of setlocale in libc has a dependency on libdld.sl.
There isn’t an archive version of libdld.sl. Thus, when the ‘ -static ’ option is specified, special link options are needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the ‘ -static ’ option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The ‘ -nolibdld ’ option can be used to prevent the GCC driver from adding these link options.
-threads Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker.
3.19.21 IA-64 Options
These are the ‘ -m ’ options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for HP-UX.
-mlittle-endian
Generate code for a little-endian target.
This is the default for AIX5 and
GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don’t) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don’t) code for the GNU linker. This is the default.
-mno-pic Generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI.
Chapter 3: GCC Command Options 365
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don’t) a stop bit immediately before and after volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don’t) ‘ in ’, ‘ loc ’, and ‘ out ’ register names for the stacked registers.
This may make assembler output more readable.
-mno-sdata
-msdata Disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies ‘ -mconstant-gp ’. This is useful when compiling firmware code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using the maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
-mno-inline-sqrt
Do not generate inline code for sqrt .
366 Using the GNU Compiler Collection (GCC)
-mfused-madd
-mno-fused-madd
Do (don’t) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don’t (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so.
-mfixed-range= register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a comma.
-mtls-size= tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
-mtune= cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are
‘ itanium ’, ‘ itanium1 ’, ‘ merced ’, ‘ itanium2 ’, and ‘ mckinley ’.
-milp32
-mlp64 Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This results in generation of ld.a
instructions and the corresponding check instructions ( ld.c
/ chk.a
). The default setting is disabled.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This results in generation of ld.a
instructions and the corresponding check instructions ( ld.c
/ chk.a
). The default setting is enabled.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload).
This results in generation of the ld.s
instructions and the corresponding check instructions chk.s
. The default setting is disabled.
Chapter 3: GCC Command Options 367
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload.
This is effective only with
‘ -msched-br-data-spec ’ enabled. The default setting is enabled.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload.
This is effective only with
‘ -msched-ar-data-spec ’ enabled. The default setting is enabled.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative loads.
This is effective only with
‘ -msched-control-spec ’ enabled. The default setting is enabled.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the data speculation much more conservative. The default setting is disabled.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the control speculation much more conservative. The default setting is disabled.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled.
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling. This flag is disabled by default.
368 Using the GNU Compiler Collection (GCC)
-msched-max-memory-insns= max-insns
Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is
1.
-msched-max-memory-insns-hard-limit
Makes the limit specified by ‘ msched-max-memory-insns ’ a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is “soft”, meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.
3.19.22 LM32 Options
These ‘ -m ’ options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
3.19.23 M32C Options
-mcpu= name
Select the CPU for which code is generated.
name may be one of ‘ r8c ’ for the R8C/Tiny series, ‘ m16c ’ for the M16C (up to /60) series, ‘ m32cm ’ for the
M16C/80 series, or ‘ m32c ’ for the M32C/80 series.
-msim Specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.
-memregs= number
Specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers are used like real registers, so there is a tradeoff between GCC’s ability to fit the code into available registers, and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC’s default runtime libraries.
Chapter 3: GCC Command Options 369
3.19.24 M32R/D Options
These ‘ -m ’ options are defined for Renesas M32R/D architectures:
-m32r2
-m32rx
Generate code for the M32R/2.
Generate code for the M32R/X.
Generate code for the M32R. This is the default.
-m32r
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.
The addressability of a particular object can be set with the model attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler generates the much slower seth/add3/jl instruction sequence).
-msdata=none
Disable use of the small data area. Variables are put into one of .data
, .bss
, or
.rodata
(unless the section attribute has been specified). This is the default.
The small data area consists of sections .sdata
and .sbss
. Objects may be explicitly put in the small data area with the section attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and generate special instructions to reference them.
-G num Put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8. The ‘ -msdata ’ option must be set to one of ‘ sdata ’ or ‘ use ’ for this option to have any effect.
All modules should be compiled with the same ‘ -G num ’ value. Compiling with different values of num may or may not work; if it doesn’t the linker gives an error message—incorrect code is not generated.
-mdebug Makes the M32R-specific code in the compiler display some statistics that might help in debugging programs.
370 Using the GNU Compiler Collection (GCC)
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate= number
Issue number instructions per cycle.
number can only be 1 or 2.
-mbranch-cost= number number can only be 1 or 2. If it is 1 then branches are preferred over conditional code, if it is 2, then the opposite applies.
-mflush-trap= number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func= name
Specifies the name of the operating system function to call to flush the cache.
The default is ‘ _flush_cache ’, but a function call is only used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
3.19.25 M680x0 Options
These are the ‘ -m ’ options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.
-march= arch
Generate code for a specific M680x0 or ColdFire instruction set architecture.
Permissible values of arch for M680x0 architectures are: ‘ 68000 ’, ‘ 68010 ’,
‘ 68020 ’, ‘ 68030 ’, ‘ 68040 ’, ‘ 68060 ’ and ‘ cpu32 ’.
ColdFire architectures are selected according to Freescale’s ISA classification and the permissible values are: ‘ isaa ’, ‘ isaaplus ’, ‘ isab ’ and ‘ isac ’.
GCC defines a macro __mcf arch __ whenever it is generating code for a ColdFire target. The arch in this macro is one of the ‘ -march ’ arguments given above.
When used together, ‘ -march ’ and ‘ -mtune ’ select code that runs on a family of similar processors but that is optimized for a particular microarchitecture.
-mcpu= cpu
Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpu s are: ‘ 68000 ’, ‘ 68010 ’, ‘ 68020 ’, ‘ 68030 ’, ‘ 68040 ’, ‘ 68060 ’, ‘ 68302 ’, ‘ 68332 ’ and
‘ cpu32 ’. The ColdFire cpu s are given by the table below, which also classifies the CPUs into families:
Family ‘ -mcpu ’ arguments
Chapter 3: GCC Command Options 371
‘ 5250 ’
‘ 5271 ’
‘ 5272 ’
‘ 5275 ’
‘ 5282 ’
‘ 53017 ’
‘ 5307 ’
‘ 5329 ’
‘ 5373 ’
‘ 5407 ’
‘ 5475 ’
‘ 5206 ’
‘ 5206e ’
‘ 5208 ’
‘ 5211a ’
‘ 5213 ’
‘ 5216 ’
‘ 52235 ’
‘ 5225 ’
‘ 52259 ’
‘ 5235 ’
‘ 5249 ’
‘ 51 ’ ‘ 51 ’ ‘ 51ac ’ ‘ 51ag ’ ‘ 51cn ’ ‘ 51em ’ ‘ 51je ’ ‘ 51jf ’ ‘ 51jg ’ ‘ 51jm ’ ‘ 51mm ’ ‘ 51qe ’
‘ 51qm ’
‘ 5202 ’ ‘ 5204 ’ ‘ 5206 ’
‘ 5206e ’
‘ 5207 ’ ‘ 5208 ’
‘ 5210a ’ ‘ 5211a ’
‘ 5211 ’ ‘ 5212 ’ ‘ 5213 ’
‘ 5214 ’ ‘ 5216 ’
‘ 52230 ’ ‘ 52231 ’ ‘ 52232 ’ ‘ 52233 ’ ‘ 52234 ’ ‘ 52235 ’
‘ 5224 ’ ‘ 5225 ’
‘ 52252 ’ ‘ 52254 ’ ‘ 52255 ’ ‘ 52256 ’ ‘ 52258 ’ ‘ 52259 ’
‘ 5232 ’ ‘ 5233 ’ ‘ 5234 ’ ‘ 5235 ’ ‘ 523x ’
‘ 5249 ’
‘ 5250 ’
‘ 5270 ’ ‘ 5271 ’
‘ 5272 ’
‘ 5274 ’ ‘ 5275 ’
‘ 5280 ’ ‘ 5281 ’ ‘ 5282 ’ ‘ 528x ’
‘ 53011 ’ ‘ 53012 ’ ‘ 53013 ’ ‘ 53014 ’ ‘ 53015 ’ ‘ 53016 ’ ‘ 53017 ’
‘ 5307 ’
‘ 5327 ’ ‘ 5328 ’ ‘ 5329 ’ ‘ 532x ’
‘ 5372 ’ ‘ 5373 ’ ‘ 537x ’
‘ 5407 ’
‘ 5470 ’ ‘ 5471 ’ ‘ 5472 ’ ‘ 5473 ’ ‘ 5474 ’ ‘ 5475 ’ ‘ 547x ’ ‘ 5480 ’ ‘ 5481 ’ ‘ 5482 ’
‘ 5483 ’ ‘ 5484 ’ ‘ 5485 ’
‘ -mcpu= cpu ’ overrides ‘ -march= arch ’ if arch is compatible with cpu . Other combinations of ‘ -mcpu ’ and ‘ -march ’ are rejected.
GCC defines the macro __mcf_cpu_ cpu when ColdFire target cpu is selected.
It also defines __mcf_family_ family , where the value of family is given by the table above.
-mtune= tune
Tune the code for a particular microarchitecture within the constraints set by
‘ -march ’ and ‘ -mcpu ’. The M680x0 microarchitectures are: ‘ 68000 ’, ‘ 68010 ’,
‘ 68020 ’, ‘ 68030 ’, ‘ 68040 ’, ‘ 68060 ’ and ‘ cpu32 ’. The ColdFire microarchitectures are: ‘ cfv1 ’, ‘ cfv2 ’, ‘ cfv3 ’, ‘ cfv4 ’ and ‘ cfv4e ’.
You can also use ‘ -mtune=68020-40 ’ for code that needs to run relatively well on 68020, 68030 and 68040 targets. ‘ -mtune=68020-60 ’ is similar but includes
68060 targets as well. These two options select the same tuning decisions as
‘ -m68020-40 ’ and ‘ -m68020-60 ’ respectively.
GCC defines the macros __mc arch and __mc arch __ when tuning for 680x0 architecture arch . It also defines mc arch unless either ‘ -ansi ’ or a non-GNU
‘ -std ’ option is used. If GCC is tuning for a range of architectures, as selected by ‘ -mtune=68020-40 ’ or ‘ -mtune=68020-60 ’, it defines the macros for every architecture in the range.
372 Using the GNU Compiler Collection (GCC)
GCC also defines the macro __m uarch __ when tuning for ColdFire microarchitecture uarch , where uarch is one of the arguments given above.
-m68000
-mc68000 Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is equivalent to ‘ -march=68000 ’.
Use this option for microcontrollers with a 68000 or EC000 core, including the
68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68010 Generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is equivalent to ‘ -march=68010 ’.
-m68020
-mc68020 Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to ‘ -march=68020 ’.
-m68030 Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to ‘ -march=68030 ’.
-m68040 Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to ‘ -march=68040 ’.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions.
-m68060 Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to ‘ -march=68060 ’.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.
-mcpu32 Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is equivalent to ‘ -march=cpu32 ’.
Use this option for microcontrollers with a CPU32 or CPU32 + core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.
-m5200 Generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It is equivalent to ‘ -mcpu=5206 ’, and is now deprecated in favor of that option.
Use this option for microcontroller with a 5200 core, including the MCF5202,
MCF5203, MCF5204 and MCF5206.
-m5206e Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent ‘ -mcpu=5206e ’.
-m528x
-m5307
-m5407
Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent ‘ -mcpu=528x ’.
Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent ‘ -mcpu=5307 ’.
Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent ‘ -mcpu=5407 ’.
Chapter 3: GCC Command Options 373
-mcfv4e Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to
‘ -mcpu=547x ’, and is now deprecated in favor of that option.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a
68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.
The option is equivalent to ‘ -march=68020 ’ ‘ -mtune=68020-40 ’.
-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a
68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.
The option is equivalent to ‘ -march=68020 ’ ‘ -mtune=68020-60 ’.
-mhard-float
-m68881 Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro __HAVE_
68881__ on M680x0 targets and __mcffpu__ on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU.
-mdiv
-mno-div Generate (do not generate) ColdFire hardware divide and remainder instructions. If ‘ -march ’ is used without ‘ -mcpu ’, the default is “on” for ColdFire architectures and “off” for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by ‘ -mcpu ’).
For example, the default is “off” for ‘ -mcpu=5206 ’ and “on” for ‘ -mcpu=5206e ’.
GCC defines the macro __mcfhwdiv__ when this option is enabled.
-mshort Consider type int to be 16 bits wide, like short int . Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose
API mandates promotion to 32-bit.
-mno-short
Do not consider type int to be 16 bits wide. This is the default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The ‘ -m68000 ’, ‘ -mcpu32 ’ and ‘ -m5200 ’ options imply ‘ -mnobitfield ’.
-mbitfield
Do use the bit-field instructions. The ‘ -m68020 ’ option implies ‘ -mbitfield ’.
This is the default if you use a configuration designed for a 68020.
374 Using the GNU Compiler Collection (GCC)
-mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf ); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and
CPU32 processors, but not by the 68000 or 5200.
The default is ‘ -mno-rtd ’.
-malign-int
-mno-align-int
Control whether GCC aligns int , long , long long , float , double , and long double variables on a 32-bit boundary (‘ -malign-int ’) or a 16-bit boundary
(‘ -mno-align-int ’). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the ‘ -malign-int ’ switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k.
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies ‘ -fpic ’, allowing at most a
16-bit offset for pc-relative addressing. ‘ -fPIC ’ is not presently supported with
‘ -mpcrel ’, though this could be supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are handled by the system.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies ‘ -fPIC ’.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment.
This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies ‘ -fPIC ’.
Chapter 3: GCC Command Options 375
-mno-id-shared-library
Generate code that doesn’t assume ID-based shared libraries are being used.
This is the default.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed; ‘ -fPIC ’ suffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k.
Anything larger causes the linker to report an error such as: relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with ‘ -mxgot ’. It should then work with very large GOTs. However, code generated with ‘ -mxgot ’ is less efficient, since it takes 4 instructions to fetch the value of a global symbol.
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use ‘ -mxgot ’ when compiling a single object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code.
-mlong-jump-table-offsets
Use 32-bit offsets in switch tables. The default is to use 16-bit offsets.
3.19.26 MCore Options
These are the ‘ -m ’ options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or less.
-mdiv
-mno-div Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as int -sized.
376 Using the GNU Compiler Collection (GCC)
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340 Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the simulator library (‘ libsim.a) ’ from the linker command line.
-mstack-increment= size
Set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much. The default value is 0x1000.
3.19.27 MeP Options
-mabsdiff
Enables the abs instruction, which is the absolute difference between two registers.
-mall-opts
Enables all the optional instructions—average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation.
-maverage
Enables the ave instruction, which computes the average of two registers.
-mbased= n
-mclip
Variables of size n bytes or smaller are placed in the .based
section by default.
Based variables use the $tp register as a base register, and there is a 128-byte limit to the .based
section.
-mbitops Enables the bit operation instructions—bit test ( btstm ), set ( bsetm ), clear
( bclrm ), invert ( bnotm ), and test-and-set ( tas ).
-mc= name Selects which section constant data is placed in.
name may be ‘ tiny ’, ‘ near ’, or ‘ far ’.
Enables the clip instruction. Note that ‘ -mclip ’ is not useful unless you also provide ‘ -mminmax ’.
Chapter 3: GCC Command Options 377
-mconfig= name
Selects one of the built-in core configurations. Each MeP chip has one or more modules in it; each module has a core CPU and a variety of coprocessors, optional instructions, and peripherals. The MeP-Integrator tool, not part of
GCC, provides these configurations through this option; using this option is the same as using all the corresponding command-line options. The default configuration is ‘ default ’.
-mcop Enables the coprocessor instructions. By default, this is a 32-bit coprocessor.
Note that the coprocessor is normally enabled via the ‘ -mconfig= ’ option.
-mcop32 Enables the 32-bit coprocessor’s instructions.
-mcop64 Enables the 64-bit coprocessor’s instructions.
-mivc2
-mdc
Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor.
Causes constant variables to be placed in the .near
section.
-mdiv
-meb
-mel
Enables the div and divu instructions.
Generate big-endian code.
Generate little-endian code.
-mio-volatile
Tells the compiler that any variable marked with the io attribute is to be considered volatile.
-ml Causes variables to be assigned to the .far
section by default.
-mleadz Enables the leadz (leading zero) instruction.
-mm Causes variables to be assigned to the .near
section by default.
-mminmax Enables the min and max instructions.
-mmult Enables the multiplication and multiply-accumulate instructions.
-mno-opts
Disables all the optional instructions enabled by ‘ -mall-opts ’.
-mrepeat Enables the repeat and erepeat instructions, used for low-overhead looping.
-ms Causes all variables to default to the .tiny
section. Note that there is a 65536byte limit to this section. Accesses to these variables use the %gp base register.
-msatur Enables the saturation instructions. Note that the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like as .
-msdram Link the SDRAM-based runtime instead of the default ROM-based runtime.
-msim Link the simulator run-time libraries.
-msimnovec
Link the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables.
378 Using the GNU Compiler Collection (GCC)
-mtf Causes all functions to default to the .far
section. Without this option, functions default to the .near
section.
-mtiny= n Variables that are n bytes or smaller are allocated to the .tiny
section. These variables use the $gp base register. The default for this option is 4, but note that there’s a 65536-byte limit to the .tiny
section.
3.19.28 MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy Do not optimize block moves, use memcpy .
-mno-clearbss
This option is deprecated. Use ‘ -fno-zero-initialized-in-bss ’ instead.
-mcpu= cpu-type
Use features of, and schedule code for, the given CPU. Supported values are in the format ‘ v X .
YY .
Z ’, where X is a major version, YY is the minor version, and
Z is compatibility code. Example values are ‘ v3.00.a
’, ‘ v4.00.b
’, ‘ v5.00.a
’,
‘ v5.00.b
’, ‘ v6.00.a
’.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use ‘ -fstack-check ’ instead.
-mxl-gp-opt
Use GP-relative .sdata
/ .sbss
sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
Chapter 3: GCC Command Options 379
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target.
-mxl-reorder
Use reorder instructions (swap and byte reversed load/store).
-mxl-modeapp-model
Select application model app-model . Valid models are
‘ executable ’ normal executable (default), uses startup code ‘ crt0.o
’.
‘ xmdstub ’ for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file
‘ crt1.o
’ and sets the start address of the program to 0x800.
‘ bootstrap ’ for applications that are loaded using a bootloader. This model uses startup file ‘ crt2.o
’ which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
‘ novectors ’ for applications that do not require any of the MicroBlaze vectors.
This option may be useful for applications running within a monitoring application. This model uses ‘ crt3.o
’ as a startup file.
Option ‘ -xl-modeapp-model ’ is a deprecated alias for ‘ -mxl-modeappmodel ’.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at static link time. This allows data to be referenced by offset from start of text address instead of GOT since PC-relative addressing is not supported.
3.19.29 MIPS Options
-EB
-EL
Generate big-endian code.
Generate little-endian code. This is the default for ‘ mips*el-*-* ’ configurations.
-march= arch
Generate code that runs on arch , which can be the name of a generic MIPS
ISA, or the name of a particular processor.
The ISA names are: ‘ mips1 ’,
‘ mips2 ’, ‘ mips3 ’, ‘ mips4 ’, ‘ mips32 ’, ‘ mips32r2 ’, ‘ mips32r3 ’, ‘ mips32r5 ’,
‘ mips32r6 ’, ‘ mips64 ’, ‘ mips64r2 ’, ‘ mips64r3 ’, ‘ mips64r5 ’ and ‘ mips64r6 ’.
The processor names are: ‘ 4kc ’, ‘ 4km ’, ‘ 4kp ’, ‘ 4ksc ’, ‘ 4kec ’, ‘ 4kem ’, ‘ 4kep ’,
‘ 4ksd ’, ‘ 5kc ’, ‘ 5kf ’, ‘ 20kc ’, ‘ 24kc ’, ‘ 24kf2_1 ’, ‘ 24kf1_1 ’, ‘ 24kec ’, ‘ 24kef2_1 ’,
‘ 24kef1_1 ’, ‘ 34kc ’, ‘ 34kf2_1 ’, ‘ 34kf1_1 ’, ‘ 34kn ’, ‘ 74kc ’, ‘ 74kf2_1 ’, ‘ 74kf1_1 ’,
‘ 74kf3_2 ’, ‘ 1004kc ’, ‘ 1004kf2_1 ’, ‘ 1004kf1_1 ’, ‘ i6400 ’, ‘ i6500 ’, ‘ interaptiv ’,
380 Using the GNU Compiler Collection (GCC)
‘ loongson2e ’, ‘ loongson2f ’, ‘ loongson3a ’, ‘ gs464 ’, ‘ gs464e ’, ‘ gs264e ’, ‘ m4k ’,
‘ m14k ’, ‘ m14kc ’, ‘ m14ke ’, ‘ m14kec ’, ‘ m5100 ’, ‘ m5101 ’, ‘ octeon ’, ‘ octeon+ ’,
‘ octeon2 ’, ‘ octeon3 ’, ‘ orion ’, ‘ p5600 ’, ‘ p6600 ’, ‘ r2000 ’, ‘ r3000 ’, ‘ r3900 ’,
‘ r4000 ’, ‘ r4400 ’, ‘ r4600 ’, ‘ r4650 ’, ‘ r4700 ’, ‘ r5900 ’, ‘ r6000 ’, ‘ r8000 ’, ‘ rm7000 ’,
‘ rm9000 ’, ‘ r10000 ’, ‘ r12000 ’, ‘ r14000 ’, ‘ r16000 ’, ‘ sb1 ’, ‘ sr71000 ’, ‘ vr4100 ’,
‘ vr4111 ’, ‘ vr4120 ’, ‘ vr4130 ’, ‘ vr4300 ’, ‘ vr5000 ’, ‘ vr5400 ’, ‘ vr5500 ’, ‘ xlr ’ and
‘ xlp ’. The special value ‘ from-abi ’ selects the most compatible architecture for the selected ABI (that is, ‘ mips1 ’ for 32-bit ABIs and ‘ mips3 ’ for 64-bit
ABIs).
The native Linux/GNU toolchain also supports the value ‘ native ’, which selects the best architecture option for the host processor. ‘ -march=native ’ has no effect if GCC does not recognize the processor.
In processor names, a final ‘ 000 ’ can be abbreviated as ‘ k ’ (for example,
‘ -march=r2k ’). Prefixes are optional, and ‘ vr ’ may be written ‘ r ’.
Names of the form ‘ n f2_1 ’ refer to processors with FPUs clocked at half the rate of the core, names of the form ‘ n f1_1 ’ refer to processors with FPUs clocked at the same rate as the core, and names of the form ‘ n f3_2 ’ refer to processors with
FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons,
‘ n f ’ is accepted as a synonym for ‘ n f2_1 ’ while ‘ n x ’ and ‘ b fx ’ are accepted as synonyms for ‘ n f1_1 ’.
GCC defines two macros based on the value of this option. The first is _MIPS_
ARCH , which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_ foo , where foo is the capitalized value of _MIPS_
ARCH . For example, ‘ -march=r2000 ’ sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000 .
Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate ‘ 000 ’ as ‘ k ’. In the case of ‘ from-abi ’, the macro names the resolved architecture (either "mips1" or "mips3" ). It names the default architecture when no ‘ -march ’ option is given.
-mtune= arch
Optimize for arch . Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for ‘ -march ’.
When this option is not used, GCC optimizes for the processor specified by
‘ -march ’. By using ‘ -march ’ and ‘ -mtune ’ together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family.
‘ -mtune ’ defines the macros _MIPS_TUNE and _MIPS_TUNE_ foo , which work in the same way as the ‘ -march ’ ones described above.
-mips1
-mips2
-mips3
Equivalent to ‘ -march=mips1 ’.
Equivalent to ‘ -march=mips2 ’.
Equivalent to ‘ -march=mips3 ’.
Chapter 3: GCC Command Options 381
-mips4 Equivalent to ‘ -march=mips4 ’.
-mips32 Equivalent to ‘ -march=mips32 ’.
-mips32r3
Equivalent to ‘ -march=mips32r3 ’.
-mips32r5
Equivalent to ‘ -march=mips32r5 ’.
-mips32r6
Equivalent to ‘ -march=mips32r6 ’.
-mips64 Equivalent to ‘ -march=mips64 ’.
-mips64r2
Equivalent to ‘ -march=mips64r2 ’.
-mips64r3
Equivalent to ‘ -march=mips64r3 ’.
-mips64r5
Equivalent to ‘ -march=mips64r5 ’.
-mips64r6
Equivalent to ‘ -march=mips64r6 ’.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or
MIPS64 architecture, it makes use of the MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means of mips16 and nomips16 attributes. See
Section 6.33 [Function Attributes], page 548
, for more information.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code.
-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard (uncompressed) MIPS
ISA be link-compatible with MIPS16 and microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot jump directly to
MIPS16 or microMIPS code; it must either use a call or an indirect jump.
‘ -minterlink-compressed ’ therefore disables direct jumps unless GCC knows that the target of the jump is not compressed.
-minterlink-mips16
-mno-interlink-mips16
Aliases of ‘ -minterlink-compressed ’ and ‘ -mno-interlink-compressed ’.
These options predate the microMIPS ASE and are retained for backwards compatibility.
382 Using the GNU Compiler Collection (GCC)
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates
64-bit code when you select a 64-bit architecture, but you can use ‘ -mgp32 ’ to get 32-bit code instead.
For information about the O64 ABI, see https://gcc.gnu.org/projects/ mipso64-abi.html
.
GCC supports a variant of the o32 ABI in which floating-point registers are
64 rather than 32 bits wide. You can select this combination with ‘ -mabi=32 ’
‘ -mfp64 ’. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in ‘ $f0 ’ only, not a ‘ $f0 ’/‘ $f1 ’ pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved.
Two additional variants of the o32 ABI are supported to enable a transition from
32-bit to 64-bit registers. These are FPXX (‘ -mfpxx ’) and FP64A (‘ -mfp64 ’
‘ -mno-odd-spreg ’). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the
FP64 extension but forbids the use of odd-numbered single-precision registers.
This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style dynamic objects. ‘ -mabicalls ’ is the default for SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries. This option only affects ‘ -mabicalls ’.
All ‘ -mabicalls ’ code has traditionally been position-independent, regardless of options like ‘ -fPIC ’ and ‘ -fpic ’. However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locallydefined functions. This mode is selected by ‘ -mno-shared ’.
‘ -mno-shared ’ depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. However, the option does not affect the
Chapter 3: GCC Command Options 383
ABI of the final executable; it only affects the ABI of relocatable objects. Using
‘ -mno-shared ’ generally makes executables both smaller and quicker.
‘ -mshared ’ is the default.
-mplt
-mno-plt Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects ‘ -mno-shared -mabicalls ’. For the n64 ABI, this option has no effect without ‘ -msym32 ’.
You can make ‘ -mplt ’ the default by configuring GCC with ‘ --with-mips-plt ’.
The default is ‘ -mno-plt ’ otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k.
Anything larger causes the linker to report an error such as: relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with ‘ -mxgot ’. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use ‘ -mxgot ’ when a single object file accesses more than 64k’s worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
Assume that general-purpose registers are 32 bits wide.
-mgp32
-mgp64
-mfp32
Assume that general-purpose registers are 64 bits wide.
Assume that floating-point registers are 32 bits wide.
Assume that floating-point registers are 64 bits wide.
-mfp64
-mfpxx Do not assume the width of floating-point registers.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
-mno-float
Equivalent to ‘ -msoft-float ’, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point printf formats). If code compiled with
‘ -mno-float ’ accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure.
384 Using the GNU Compiler Collection (GCC)
-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is the default.
-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI, ‘ -mno-odd-spreg ’ is set by default.
-mabs=2008
-mabs=legacy
These options control the treatment of the special not-a-number (NaN) IEEE
754 floating-point data with the abs.
fmt and neg.
fmt machine instructions.
By default or when ‘ -mabs=legacy ’ is used the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead unless the ‘ -ffinite-math-only ’ option has also been specified.
The ‘ -mabs=2008 ’ option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN.
These instructions are therefore always used for the respective operations.
-mnan=2008
-mnan=legacy
These options control the encoding of the special not-a-number (NaN) IEEE
754 floating-point data.
The ‘ -mnan=legacy ’ option selects the legacy encoding. In this case quiet NaNs
(qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.
The ‘ -mnan=2008 ’ option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0.
The default is ‘ -mnan=legacy ’ unless GCC has been configured with
‘ --with-nan=2008 ’.
-mllsc
-mno-llsc
Use (do not use) ‘ ll ’, ‘ sc ’, and ‘ sync ’ instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.
Chapter 3: GCC Command Options 385
‘ -mllsc ’ is useful if the runtime environment can emulate the instructions and
‘ -mno-llsc ’ can be useful when compiling for nonstandard ISAs.
You can make either option the default by configuring GCC with ‘ --with-llsc ’ and
‘ --without-llsc ’ respectively. ‘ --with-llsc ’ is the default for some configurations; see the installation documentation for details.
-mdsp
-mno-dsp Use (do not use) revision 1 of the MIPS DSP ASE. See
DSP Built-in Functions], page 757
. This option defines the preprocessor macro
__mips_dsp . It also defines __mips_dsp_rev to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE. See
DSP Built-in Functions], page 757 . This option defines the preprocessor macros
__mips_dsp and __mips_dspr2 . It also defines __mips_dsp_rev to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions. See
[MIPS Paired-Single Support], page 761
.
This option requires hardware floating-point support to be enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. See
Section 6.60.16.3 [MIPS-3D Built-in
. The option ‘ -mips3d ’ implies ‘ -mpaired-single ’.
-mmicromips
-mno-micromips
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by means of micromips and nomicromips attributes. See
, for more information.
-mmt
-mno-mt Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu Use (do not use) the MIPS MCU ASE instructions.
-meva
-mno-eva Use (do not use) the MIPS Enhanced Virtual Addressing instructions.
386 Using the GNU Compiler Collection (GCC)
-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ) instructions.
-mxpa
-mno-xpa Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
-mcrc
-mno-crc Use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.
-mginv
-mno-ginv
Use (do not use) the MIPS Global INValidate (GINV) instructions.
-mloongson-mmi
-mno-loongson-mmi
Use (do not use) the MIPS Loongson MultiMedia extensions Instructions
(MMI).
-mloongson-ext
-mno-loongson-ext
Use (do not use) the MIPS Loongson EXTensions (EXT) instructions.
-mloongson-ext2
-mno-loongson-ext2
Use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
-mlong64 Force long types to be 64 bits wide. See ‘ -mlong32 ’ for an explanation of the default and the way that the pointer size is determined.
-mlong32 Force long , int , and pointer types to be 32 bits wide.
The default size of int s, long s and pointers depends on the ABI. All the supported ABIs use 32-bit int s. The n64 ABI uses 64-bit long s, as does the
64-bit EABI; the others use 32-bit long s. Pointers are the same size as long s, or the same size as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with ‘ -mabi=64 ’ and
‘ -mno-abicalls ’ because it allows GCC to generate shorter and faster references to symbolic addresses.
-G num Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see ‘ -mgpopt ’ for details.
The default ‘ -G ’ option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the ‘ -G ’ behavior to local data too, such as to static variables in C. ‘ -mlocal-sdata ’ is the default for all configurations.
Chapter 3: GCC Command Options 387
If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with
‘ -mno-local-sdata ’.
You might also want to build large libraries with
‘ -mno-local-sdata ’, so that the libraries leave more room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the ‘ -G ’ limit. ‘ -mextern-sdata ’ is the default for all configurations.
If you compile a module Mod with ‘ -mextern-sdata ’ ‘ -G num ’ ‘ -mgpopt ’, and
Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough ‘ -G ’ setting or attach a section attribute to Var ’s definition. If Var is common, you must link the application with a high-enough ‘ -G ’ setting.
The easiest way of satisfying these restrictions is to compile and link every module with the same ‘ -G ’ option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported ‘ -G ’ setting and additionally using ‘ -mno-extern-sdata ’ to stop the library from making assumptions about externally-defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see ‘ -G ’, ‘ -mlocal-sdata ’ and ‘ -mextern-sdata ’. ‘ -mgpopt ’ is the default for all configurations.
‘ -mno-gpopt ’ is useful for cases where the $gp register might not hold the value of _gp . For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp . (In such situations, the boot monitor itself is usually compiled with
‘ -G0 ’.)
‘ -mno-gpopt ’ implies ‘ -mno-local-sdata ’ and ‘ -mno-extern-sdata ’.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized const variables in the read-only data section. This option is only meaningful in conjunction with ‘ -membedded-data ’.
388 Using the GNU Compiler Collection (GCC)
-mcode-readable= setting
Specify whether GCC may generate code that reads from executable sections.
There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections. This is the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read
Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the
M4K, automatically redirect PC-relative loads to the instruction
RAM.
-mcode-readable=no
Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the %hi() and %lo() assembler relocation operators.
This option has been superseded by ‘ -mexplicit-relocs ’ but is retained for backwards compatibility.
-mexplicit-relocs
-mno-explicit-relocs
Use (do not use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by ‘ -mno-explicit-relocs ’, is to use assembler macros instead.
‘ -mexplicit-relocs ’ is the default if GCC was configured to use an assembler that supports relocation operators.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is ‘ -mcheck-zero-division ’.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ( SIGFPE ).
Use
‘ -mdivide-traps ’ to allow conditional traps on architectures that support them and ‘ -mdivide-breaks ’ to force the use of breaks.
Chapter 3: GCC Command Options 389
The default is usually ‘ -mdivide-traps ’, but this can be overridden at configure time using ‘ --with-divide=breaks ’. Divide-by-zero checks can be completely disabled using ‘ -mno-check-zero-division ’.
-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding.
-munaligned-access
-mno-unaligned-access
Enable (disable) direct unaligned access for MIPS Release 6.
MIPSr6 requires load/store unaligned-access support, by hardware or trap&emulate. So
‘ -mno-unaligned-access ’ may be needed by kernel.
-mmemcpy
-mno-memcpy
Force (do not force) the use of memcpy for non-trivial block moves. The default is ‘ -mno-memcpy ’, which allows GCC to inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the jal instruction. Calling functions using jal is more efficient but requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is ‘ -mno-long-calls ’.
-mmad
-mno-mad Enable (disable) use of the mad , madu and mul instructions, as provided by the
R4650 ISA.
-mimadd
-mno-imadd
Enable (disable) use of the madd and msub integer instructions. The default is ‘ -mimadd ’ on architectures that support madd and msub except for the 74k architecture where it was found to generate slower code.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is ‘ -mfused-madd ’.
On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the
FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.
-nocpp Tell the MIPS assembler to not run its preprocessor over user assembler files
(with a ‘ .s
’ suffix) when assembling them.
390 Using the GNU Compiler Collection (GCC)
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata.
The workarounds are implemented by the assembler rather than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
− A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
− A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
− An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
− A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
− ll / sc sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier.
This option can only be used if the target architecture supports branch-likely instructions.
‘ -mfix-r10000 ’ is the default when ‘ -march=r10000 ’ is used;
‘ -mno-fix-r10000 ’ is the default otherwise.
-mfix-r5900
-mno-fix-r5900
Do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer and always schedule a nop instruction there instead. The short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC.
-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 dmult / dmultu errata. The workarounds are implemented by the assembler rather than by GCC.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
− dmultu does not always produce the correct result.
Chapter 3: GCC Command Options 391
− div and ddiv do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in ‘ libgcc.a
’.
At present, these functions are only provided by the mips64vr*-elf configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 mflo / mfhi errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using mflo and mfhi if the VR4130 macc , macchi , dmacc and dmacchi instructions are available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 “F1” and “F2” floating-point errata.)
-mr10k-cache-barrier= setting
Specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the “taken” branch. It later aborts these instructions if the predicted outcome is wrong.
However, on the R10K, even aborted instructions can have side effects.
This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the
“dirty” line is flushed, the cached data overwrites the DMA-ed data. See the
R10K processor manual for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. ‘ -mr10k-cache-barrier= setting ’ controls GCC’s implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:
1. the memory occupied by the current function’s stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.
It is the kernel’s responsibility to ensure that speculative accesses to these regions are indeed safe.
If the input program contains a function declaration such as: void foo (void);
392 Using the GNU Compiler Collection (GCC) then the implementation of foo must allow j foo and jal foo to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default setting.
-mflush-func= func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function must take the same arguments as the common _flush_func , that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but commonly is either _flush_func or __cpu_flush .
mbranch-cost= num
Set the cost of branches to roughly num “simple” instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
A zero cost redundantly selects the default, which is based on the ‘ -mtune ’ setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use.
-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be generated. The default is
‘ -mcompact-branches=optimal ’.
The ‘ -mcompact-branches=never ’ option ensures that compact branch instructions will never be generated.
Chapter 3: GCC Command Options 393
The ‘ -mcompact-branches=always ’ option ensures that a compact branch instruction will be generated if available. If a compact branch instruction is not available, a delay slot form of the branch will be used instead.
This option is supported from MIPS Release 6 onwards.
The ‘ -mcompact-branches=optimal ’ option will cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting
64-bit code, then we can use both FP pipes. Otherwise, we can only use one
FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level ‘ -O3 ’.
-msynci
-mno-synci
Enable (disable) generation of synci instructions on architectures that support it. The synci instructions (if enabled) are generated when __builtin__
_clear_cache is compiled.
This option defaults to ‘ -mno-synci ’, but the default can be overridden by configuring GCC with ‘ --with-synci ’.
When compiling code for single processor systems, it is generally safe to use synci . However, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call.
‘ -mrelax-pic-calls ’ is the default if GCC was configured to use an assembler and a linker that support the .reloc
assembly directive and
‘ -mexplicit-relocs ’ is in effect.
With ‘ -mno-explicit-relocs ’, this optimization can be performed by the assembler and the linker alone without help from the compiler.
394 Using the GNU Compiler Collection (GCC)
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows _mcount to modify the calling function’s return address. When enabled, this option extends the usual _mcount interface with a new ra-address parameter, which has type intptr_t * and is passed in register $12 .
_mcount can then modify the return address by doing both of the following:
• Returning the new address in register $31 .
• Storing the new address in * ra-address , if ra-address is nonnull.
The default is ‘ -mno-mcount-ra-address ’.
-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32 ABI. When using the o32
ABI, calling functions will allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization will suppress the allocation of the frame header if it can be determined that it is unused.
This optimization is off by default at all optimization levels.
-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of lwxc1 , swxc1 , ldxc1 , sdxc1 instructions. Enabled by default.
-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of 4-operand madd.s
, madd.d
and related instructions. Enabled by default.
3.19.30 MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the rE epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
Chapter 3: GCC Command Options 395
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor. With the default, ‘ -mno-knuthdiv ’, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a ‘ : ’ to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
-melf Generate an executable in the ELF format, rather than the default ‘ mmo ’ format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses . Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require ‘ -mno-base-addresses ’.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.
3.19.31 MN10300 Options
These ‘ -m ’ options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
-mam33 Generate code using features specific to the AM33 processor.
396 Using the GNU Compiler Collection (GCC)
-mno-am33
Do not generate code using features specific to the AM33 processor. This is the default.
-mam33-2 Generate code using features specific to the AM33/2.0 processor.
-mam34 Generate code using features specific to the AM34 processor.
-mtune= cpu-type
Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type must be one of ‘ mn10300 ’, ‘ am33 ’, ‘ am33-2 ’ or ‘ am34 ’.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer in both a0 and d0 . Otherwise, the pointer is returned only in a0 , and attempts to call such functions without a prototype result in errors. Note that this option is on by default; use ‘ -mno-return-pointer-on-d0 ’ to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.
This option makes symbolic debugging impossible.
-mliw Allow the compiler to generate Long Instruction Word instructions if the target is the ‘ AM33 ’ or later. This is the default. This option defines the preprocessor macro __LIW__ .
-mno-liw Do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro __NO_LIW__ .
-msetlb Allow the compiler to generate the SETLB and Lcc instructions if the target is the ‘ AM33 ’ or later. This is the default. This option defines the preprocessor macro __SETLB__ .
-mno-setlb
Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro __NO_SETLB__ .
3.19.32 Moxie Options
-meb Generate big-endian code. This is the default for ‘ moxie-*-* ’ configurations.
Generate little-endian code.
-mel
-mmul.x
Generate mul.x and umul.x instructions. This is the default for ‘ moxiebox-*-* ’ configurations.
-mno-crt0
Do not link in the C run-time initialization object file.
Chapter 3: GCC Command Options 397
3.19.33 MSP430 Options
These options are defined for the MSP430:
-masm-hex
-mmcu=
Force assembly output to always use hex constants. Normally such constants are signed decimals, but this option is available for testsuite and/or aesthetic purposes.
Select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with ‘ __ ’. This in turn is used by the ‘ msp430.h
’ header file to select an MCUspecific supplementary header file.
The option also sets the ISA to use. If the MCU name is one that is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name of ‘ msp430 ’ can also be used to select the 430
ISA. Similarly the generic ‘ msp430x ’ MCU name selects the 430X ISA.
In addition an MCU-specific linker script is added to the linker command line.
The script’s name is the name of the MCU with ‘ .ld
’ appended. Thus specifying
‘ -mmcu=xxx ’ on the gcc command line defines the C preprocessor symbol __XXX_
_ and cause the linker to search for a script called ‘ xxx.ld
’.
The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external ‘ devices.csv
’ file can be used to extend device support beyond those that have been hard-coded.
GCC searches for the ‘ devices.csv
’ file using the following methods in the given precedence order, where the first method takes precendence over the second which takes precedence over the third.
Include path specified with -I and -L
‘ devices.csv
’ will be searched for in each of the directories specified by include paths and linker library search paths.
Path specified by the environment variable ‘ MSP430_GCC_INCLUDE_DIR ’
Define the value of the global environment variable
‘ MSP430_GCC_INCLUDE_DIR ’ to the full path to the directory containing devices.csv, and GCC will search this directory for devices.csv.
If devices.csv is found, this directory will also be registered as an include path, and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying -I and -L on the command line.
The ‘ msp430-elf{,bare}/include/devices ’ directory
Finally, GCC will examine ‘ msp430-elf{,bare}/include/devices ’ from the toolchain root directory. This directory does not exist in a default installation, but if the user has created it and copied
‘ devices.csv
’ there, then the MCU data will be read. As above, this directory will also be registered as an include path, and linker library path.
If none of the above search methods find ‘ devices.csv
’, then the hard-coded
MCU data is used.
398 Using the GNU Compiler Collection (GCC)
-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts between the MCU name specified by the ‘ -mmcu ’ option and the ISA set by the ‘ -mcpu ’ option and/or the hardware multiply support set by the ‘ -mhwmult ’ option. It also toggles warnings about unrecognized MCU names. This option is on by default.
-mcpu=
-msim
Specifies the ISA to use.
Accepted values are ‘ msp430 ’, ‘ msp430x ’ and
‘ msp430xv2 ’. This option is deprecated. The ‘ -mmcu= ’ option should be used to select the ISA.
Link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the ‘ -mmcu= ’ option.
-mlarge Use large-model addressing (20-bit pointers, 20-bit size_t ).
-msmall Use small-model addressing (16-bit pointers, 16-bit size_t ).
-mrelax This option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link.
mhwmult= Describes the type of hardware multiply supported by the target. Accepted values are ‘ none ’ for no hardware multiply, ‘ 16bit ’ for the original 16-bit-only multiply supported by early MCUs. ‘ 32bit ’ for the 16/32-bit multiply supported by later MCUs and ‘ f5series ’ for the 16/32-bit multiply supported by
F5-series MCUs. A value of ‘ auto ’ can also be given. This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the
‘ -mmcu ’ option. If no ‘ -mmcu ’ option is specified or if the MCU name is not recognized then no hardware multiply support is assumed.
auto is the default setting.
Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at ‘ -O3 ’ or higher however the hardware multiplier is invoked inline. This makes for bigger, but faster code.
The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code.
-minrt Enable the use of a minimum runtime environment - no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.
-mtiny-printf
Enable reduced code size printf and puts library functions. The ‘ tiny ’ implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications.
Support for streams has been removed and the string to be printed will always be sent to stdout via the write syscall. The string is not buffered before it is sent to write.
Chapter 3: GCC Command Options 399
This option requires Newlib Nano IO, so GCC must be configured with
‘ --enable-newlib-nano-formatted-io ’.
-mmax-inline-shift=
This option takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an mspabi helper function is used instead. The default value is 4.
This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts > 1 on the 430 ISA).
Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead.
-mcode-region=
-mdata-region=
These options tell the compiler where to place functions and data that do not have one of the lower , upper , either or section attributes. Possible values are lower , upper , either or any . The first three behave like the corresponding attribute. The fourth possible value any - is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections ( .text
,
.data
, etc) to the memory regions.
-msilicon-errata=
This option passes on a request to assembler to enable the fixes for the named silicon errata.
-msilicon-errata-warn=
This option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied.
-mwarn-devices-csv
-mno-warn-devices-csv
Warn if ‘ devices.csv
’ is not found or there are problem parsing it (default: on).
3.19.34 NDS32 Options
These options are defined for NDS32 implementations:
-mbig-endian
Generate code in big-endian mode.
-mlittle-endian
Generate code in little-endian mode.
-mreduced-regs
Use reduced-set registers for register allocation.
-mfull-regs
Use full-set registers for register allocation.
-mcmov Generate conditional move instructions.
400 Using the GNU Compiler Collection (GCC)
-mno-cmov
Do not generate conditional move instructions.
-mext-perf
Generate performance extension instructions.
-mno-ext-perf
Do not generate performance extension instructions.
-mext-perf2
Generate performance extension 2 instructions.
-mno-ext-perf2
Do not generate performance extension 2 instructions.
-mext-string
Generate string extension instructions.
-mno-ext-string
Do not generate string extension instructions.
-mv3push Generate v3 push25/pop25 instructions.
-mno-v3push
Do not generate v3 push25/pop25 instructions.
-m16-bit Generate 16-bit instructions.
-mno-16-bit
Do not generate 16-bit instructions.
-misr-vector-size= num
Specify the size of each interrupt vector, which must be 4 or 16.
-mcache-block-size= num
Specify the size of each cache block, which must be a power of 2 between 4 and
512.
-march= arch
Specify the name of the target architecture.
-mcmodel= code-model
Set the code model to one of
‘ small ’ All the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space.
‘ medium ’ The data segment must be within 512KB while the read-only data segment can be within 4GB addressing space. The text segment should be still within 16MB addressing space.
‘ large ’ All the text and data segments can be within 4GB addressing space.
-mctor-dtor
Enable constructor/destructor feature.
-mrelax Guide linker to relax instructions.
Chapter 3: GCC Command Options 401
3.19.35 Nios II Options
These are the options defined for the Altera Nios II processor.
-G num Put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8.
-mgpopt= option
-mgpopt
-mno-gpopt
Generate (do not generate) GP-relative accesses. The following option names are recognized:
‘ none ’ Do not generate GP-relative accesses.
‘ local ’
‘ global ’ As for ‘ local ’, but also generate GP-relative accesses for small data objects that are external, weak, or common. If you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same ‘ -G ’ setting.
‘ data ’
Generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. Also use GPrelative addressing for objects that have been explicitly placed in a small data section via a section attribute.
Generate GP-relative accesses for all data objects in the program.
If you use this option, the entire data and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.
‘ all ’ Generate GP-relative addresses for function pointers as well as data pointers. If you use this option, the entire text, data, and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.
‘ -mgpopt ’ is equivalent to ‘ -mgpopt=local ’, and ‘ -mno-gpopt ’ is equivalent to
‘ -mgpopt=none ’.
The default is ‘ -mgpopt ’ except when ‘ -fpic ’ or ‘ -fPIC ’ is specified to generate position-independent code. Note that the Nios II ABI does not permit GPrelative accesses from shared libraries.
You may need to specify ‘ -mno-gpopt ’ explicitly when building programs that include large amounts of small data, including large GOT data sections. In this case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section.
-mgprel-sec= regexp
This option specifies additional section names that can be accessed via GPrelative addressing. It is most useful in conjunction with section attributes on variable declarations (see
Section 6.34.1 [Common Variable Attributes],
402 Using the GNU Compiler Collection (GCC) page 614 ) and a custom linker script. The
regexp is a POSIX Extended Regular
Expression.
This option does not affect the behavior of the ‘ -G ’ option, and the specified sections are in addition to the standard .sdata
and .sbss
small-data sections that are recognized by ‘ -mgpopt ’.
-mr0rel-sec= regexp
This option specifies names of sections that can be accessed via a 16-bit offset from r0 ; that is, in the low 32K or high 32K of the 32-bit address space. It is most useful in conjunction with section attributes on variable declarations (see
Section 6.34.1 [Common Variable Attributes], page 614
) and a custom linker script. The regexp is a POSIX Extended Regular Expression.
In contrast to the use of GP-relative addressing for small data, zero-based addressing is never generated by default and there are no conventional section names used in standard linker scripts for sections in the low or high areas of memory.
-mel
-meb Generate little-endian (default) or big-endian (experimental) code, respectively.
-march= arch
This specifies the name of the target Nios II architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: ‘ r1 ’, ‘ r2 ’.
The preprocessor macro __nios2_arch__ is available to programs, with value
1 or 2, indicating the targeted ISA level.
-mbypass-cache
-mno-bypass-cache
Force all load and store instructions to always bypass cache by using I/O variants of the instructions. The default is not to bypass the cache.
-mno-cache-volatile
-mcache-volatile
Volatile memory access bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.
-mno-fast-sw-div
-mfast-sw-div
Do not use table-based fast divide for small numbers. The default is to use the fast divide at ‘ -O3 ’ and above.
-mno-hw-mul
-mhw-mul
-mno-hw-mulx
-mhw-mulx
-mno-hw-div
-mhw-div Enable or disable emitting mul , mulx and div family of instructions by the compiler. The default is to emit mul and not emit div and mulx .
Chapter 3: GCC Command Options 403
-mbmx
-mno-bmx
-mcdx
-mno-cdx Enable or disable generation of Nios II R2 BMX (bit manipulation) and
CDX (code density) instructions.
Enabling these instructions also requires
‘ -march=r2 ’.
Since these instructions are optional extensions to the R2 architecture, the default is not to emit them.
-mcustominsn = N
-mno-custominsn
Each ‘ -mcustominsn = N ’ option enables use of a custom instruction with encoding N when generating code that uses insn .
For example,
‘ -mcustom-fadds=253 ’ generates custom instruction 253 for single-precision floating-point add operations instead of the default behavior of using a library call.
The following values of insn are supported. Except as otherwise noted, floatingpoint operations are expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions (see
Section 6.59 [Other Builtins], page 722 ).
Single-precision floating point:
‘ fadds ’, ‘ fsubs ’, ‘ fdivs ’, ‘ fmuls ’
Binary arithmetic operations.
‘ fnegs ’ Unary negation.
‘ fabss ’ Unary absolute value.
‘ fcmpeqs ’, ‘ fcmpges ’, ‘ fcmpgts ’, ‘ fcmples ’, ‘ fcmplts ’, ‘ fcmpnes ’
Comparison operations.
‘ fmins ’, ‘ fmaxs ’
Floating-point minimum and maximum.
These instructions are only generated if ‘ -ffinite-math-only ’ is specified.
‘ fsqrts ’ Unary square root operation.
‘ fcoss ’, ‘ fsins ’, ‘ ftans ’, ‘ fatans ’, ‘ fexps ’, ‘ flogs ’
Floating-point trigonometric and exponential functions. These instructions are only generated if ‘ -funsafe-math-optimizations ’ is also specified.
Double-precision floating point:
‘ faddd ’, ‘ fsubd ’, ‘ fdivd ’, ‘ fmuld ’
Binary arithmetic operations.
‘ fnegd ’ Unary negation.
‘ fabsd ’ Unary absolute value.
‘ fcmpeqd ’, ‘ fcmpged ’, ‘ fcmpgtd ’, ‘ fcmpled ’, ‘ fcmpltd ’, ‘ fcmpned ’
Comparison operations.
404 Using the GNU Compiler Collection (GCC)
‘ fmind ’, ‘ fmaxd ’
Double-precision minimum and maximum. These instructions are only generated if ‘ -ffinite-math-only ’ is specified.
‘ fsqrtd ’ Unary square root operation.
‘ fcosd ’, ‘ fsind ’, ‘ ftand ’, ‘ fatand ’, ‘ fexpd ’, ‘ flogd ’
Double-precision trigonometric and exponential functions. These instructions are only generated if ‘ -funsafe-math-optimizations ’ is also specified.
Conversions:
‘ fextsd ’ Conversion from single precision to double precision.
‘ ftruncds ’
Conversion from double precision to single precision.
‘ fixsi ’, ‘ fixsu ’, ‘ fixdi ’, ‘ fixdu ’
Conversion from floating point to signed or unsigned integer types, with truncation towards zero.
‘ round ’ Conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero.
This corresponds to the __builtin_lroundf function when
‘ -fno-math-errno ’ is used.
‘ floatis ’, ‘ floatus ’, ‘ floatid ’, ‘ floatud ’
Conversion from signed or unsigned integer types to floating-point types.
In addition, all of the following transfer instructions for internal registers X and
Y must be provided to use any of the double-precision floating-point instructions. Custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X. The other operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2 .
A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register
Y. GCC automatically generates the necessary code sequences to write register
X and/or read register Y when double-precision floating-point instructions are used.
‘ fwrx ’ Write src1 into the least significant half of X and src2 into the most significant half of X.
‘ fwry ’ Write src1 into Y.
‘ frdxhi ’, ‘ frdxlo ’
Read the most or least (respectively) significant half of X and store it in dest .
‘ frdy ’ Read the value of Y and store it into dest .
Chapter 3: GCC Command Options 405
Note that you can gain more local control over generation of Nios II custom instructions by using the target("custominsn = N ") and target("no-custominsn ") function attributes (see
Section 6.33 [Function Attributes], page 548 )
or pragmas (see
Section 6.62.16 [Function Specific Option Pragmas], page 874
).
-mcustom-fpu-cfg= name
This option enables a predefined, named set of custom instruction encodings
(see ‘ -mcustominsn ’ above). Currently, the following sets are defined:
‘ -mcustom-fpu-cfg=60-1 ’ is equivalent to:
-mcustom-fmuls=252
-mcustom-fadds=253
-mcustom-fsubs=254
-fsingle-precision-constant
‘ -mcustom-fpu-cfg=60-2 ’ is equivalent to:
-mcustom-fmuls=252
-mcustom-fadds=253
-mcustom-fsubs=254
-mcustom-fdivs=255
-fsingle-precision-constant
‘ -mcustom-fpu-cfg=72-3 ’ is equivalent to:
-mcustom-floatus=243
-mcustom-fixsi=244
-mcustom-floatis=245
-mcustom-fcmpgts=246
-mcustom-fcmples=249
-mcustom-fcmpeqs=250
-mcustom-fcmpnes=251
-mcustom-fmuls=252
-mcustom-fadds=253
-mcustom-fsubs=254
-mcustom-fdivs=255
-fsingle-precision-constant
‘ -mcustom-fpu-cfg=fph2 ’ is equivalent to:
-mcustom-fabss=224
-mcustom-fnegs=225
-mcustom-fcmpnes=226
-mcustom-fcmpeqs=227
-mcustom-fcmpges=228
-mcustom-fcmpgts=229
-mcustom-fcmples=230
-mcustom-fcmplts=231
-mcustom-fmaxs=232
-mcustom-fmins=233
-mcustom-round=248
-mcustom-fixsi=249
-mcustom-floatis=250
-mcustom-fsqrts=251
-mcustom-fmuls=252
-mcustom-fadds=253
-mcustom-fsubs=254
-mcustom-fdivs=255
406 Using the GNU Compiler Collection (GCC)
Custom instruction assignments given by individual ‘ -mcustominsn = ’ options override those given by ‘ -mcustom-fpu-cfg= ’, regardless of the order of the options on the command line.
Note that you can gain more local control over selection of a FPU configuration by using the target("custom-fpu-cfg= name ") function attribute (see
Section 6.33 [Function Attributes], page 548 ) or pragma (see
[Function Specific Option Pragmas], page 874 ).
The name fph2 is an abbreviation for Nios II Floating Point Hardware
2 Component .
Please note that the custom instructions enabled by
‘ -mcustom-fmins=233 ’ and ‘ -mcustom-fmaxs=234 ’ are only generated if
‘ -ffinite-math-only ’ is specified.
The custom instruction enabled by
‘ -mcustom-round=248 ’ is only generated if ‘ -fno-math-errno ’ is specified. In contrast to the other configurations, ‘ -fsingle-precision-constant ’ is not set.
These additional ‘ -m ’ options are available for the Altera Nios II ELF (bare-metal) target:
-mhal Link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with
‘ -msys-crt0= ’ to specify the location of the alternate startup code provided by the HAL BSP.
-msmallc Link with a limited version of the C library, ‘ -lsmallc ’, rather than Newlib.
-msys-crt0= startfile startfile is the file name of the startfile (crt0) to use when linking. This option is only useful in conjunction with ‘ -mhal ’.
-msys-lib= systemlib systemlib is the library name of the library that provides low-level system calls required by the C library, e.g.
read and write . This option is typically used to link with a library provided by a HAL BSP.
3.19.36 Nvidia PTX Options
These options are defined for Nvidia PTX:
-m64 Ignored, but preserved for backward compatibility. Only 64-bit ABI is supported.
-misa= ISA-string
Generate code for given the specified PTX ISA (e.g. ‘ sm_35 ’). ISA strings must be lower-case. Valid ISA strings include ‘ sm_30 ’ and ‘ sm_35 ’. The default ISA is sm 35.
-mptx= version-string
Generate code for given the specified PTX version (e.g. ‘ 6.3
’). Valid version strings include ‘ 3.1
’ and ‘ 6.3
’. The default PTX version is 3.1.
-mmainkernel
Link in code for a main kernel. This is for stand-alone instead of offloading execution.
Chapter 3: GCC Command Options 407
-moptimize
Apply partitioned execution optimizations. This is the default when any level of optimization is selected.
-msoft-stack
Generate code that does not use .local
memory directly for stack storage.
Instead, a per-warp stack pointer is maintained explicitly. This enables variablelength stack allocation (with variable-length arrays or alloca ), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option ‘ -mgomp ’.
-muniform-simt
Switch to code generation variant that allows to execute all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master.
Shared memory array int __nvptx_uni[] stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position tid.y
with current lane index to compute the master lane index.
-mgomp Generate code for use in OpenMP offloading: enables ‘ -msoft-stack ’ and
‘ -muniform-simt ’ options, and selects corresponding multilib variant.
3.19.37 OpenRISC Options
These options are defined for OpenRISC:
-mboard= name
Configure a board specific runtime. This will be passed to the linker for newlib board library linking. The default is or1ksim .
-mnewlib This option is ignored; it is for compatibility purposes only. This used to select linker and preprocessor options for use with newlib.
-msoft-div
-mhard-div
Select software or hardware divide ( l.div
, l.divu
) instructions. This default is hardware divide.
-msoft-mul
-mhard-mul
Select software or hardware multiply ( l.mul
, l.muli
) instructions. This default is hardware multiply.
408 Using the GNU Compiler Collection (GCC)
-msoft-float
-mhard-float
Select software or hardware for floating point operations. The default is software.
-mdouble-float
When ‘ -mhard-float ’ is selected, enables generation of double-precision floating point instructions. By default functions from ‘ libgcc ’ are used to perform double-precision floating point operations.
-munordered-float
When ‘ -mhard-float ’ is selected, enables generation of unordered floating point compare and set flag ( lf.sfun* ) instructions. By default functions from
‘ libgcc ’ are used to perform unordered floating point compare and set flag operations.
-mcmov
-mror
-mrori
Enable generation of conditional move ( l.cmov
) instructions. By default the equivalent will be generated using set and branch.
Enable generation of rotate right ( l.ror
) instructions. By default functions from ‘ libgcc ’ are used to perform rotate right operations.
Enable generation of rotate right with immediate ( l.rori
) instructions. By default functions from ‘ libgcc ’ are used to perform rotate right with immediate operations.
-msext Enable generation of sign extension ( l.ext* ) instructions. By default memory loads are used to perform sign extension.
-msfimm Enable generation of compare and set flag with immediate ( l.sf*i ) instructions. By default extra instructions will be generated to store the immediate to a register first.
-mshftimm
Enable generation of shift with immediate ( l.srai
, l.srli
, l.slli
) instructions. By default extra instructions will be generated to store the immediate to a register first.
-mcmodel=small
Generate OpenRISC code for the small model: The GOT is limited to 64k.
This is the default model.
-mcmodel=large
Generate OpenRISC code for the large model: The GOT may grow up to 4G in size.
3.19.38 PDP-11 Options
These options are defined for the PDP-11:
-mfpu Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies -m45.
-msoft-float
Do not use hardware floating point.
Chapter 3: GCC Command Options 409
-mac0 Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0 Return floating-point results in memory. This is the default.
-m40 Generate code for a PDP-11/40. Implies -msoft-float -mno-split.
-m45
-m10
Generate code for a PDP-11/45. This is the default.
Generate code for a PDP-11/10. Implies -msoft-float -mno-split.
-mint16
-mno-int32
Use 16-bit int . This is the default.
-mint32
-mno-int16
Use 32-bit int .
-msplit Target has split instruction and data space. Implies -m45.
-munix-asm
Use Unix assembler syntax.
-mdec-asm
Use DEC assembler syntax.
-mgnu-asm
-mlra
Use GNU assembler syntax. This is the default.
Use the new LRA register allocator. By default, the old “reload” allocator is used.
3.19.39 picoChip Options
These ‘ -m ’ options are defined for picoChip implementations:
-mae= ae_type
Set the instruction set, register set, and instruction scheduling parameters for array element type ae type . Supported values for ae type are ‘ ANY ’, ‘ MUL ’, and
‘ MAC ’.
‘ -mae=ANY ’ selects a completely generic AE type. Code generated with this option runs on any of the other AE types. The code is not as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g., multiplication) do not work properly on all types of AE.
‘ -mae=MUL ’ selects a MUL AE type. This is the most useful AE type for compiled code, and is the default.
‘ -mae=MAC ’ selects a DSP-style MAC AE. Code compiled with this option may suffer from poor performance of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores.
-msymbol-as-address
Enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it into a register. Typically, the use of this option generates larger programs, which run faster than when the option isn’t used. However, the results vary from program to program, so it is left as a user option, rather than being permanently enabled.
410 Using the GNU Compiler Collection (GCC)
-mno-inefficient-warnings
Disables warnings about the generation of inefficient code. These warnings can be generated, for example, when compiling code that performs byte-level memory operations on the MAC AE type. The MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations. This is inefficient and a warning is generated to indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support. This option disables these warnings.
3.19.40 PowerPC Options
These are listed under See
Section 3.19.44 [RS/6000 and PowerPC Options], page 415
.
3.19.41 PRU Options
These command-line options are defined for PRU target:
-minrt Link with a minimum runtime environment, with no support for static initializers and constructors. Using this option can significantly reduce the size of the final ELF binary. Beware that the compiler could still generate code with static initializers and constructors. It is up to the programmer to ensure that the source program will not use those features.
-mmcu= mcu
Specify the PRU MCU variant to use.
Check Newlib for the exact list of supported MCUs.
-mno-relax
Make GCC pass the ‘ --no-relax ’ command-line option to the linker instead of the ‘ --relax ’ option.
-mloop Allow (or do not allow) GCC to use the LOOP instruction.
-mabi= variant
Specify the ABI variant to output code for. ‘ -mabi=ti ’ selects the unmodified
TI ABI while ‘ -mabi=gnu ’ selects a GNU variant that copes more naturally with certain GCC assumptions. These are the differences:
‘ Function Pointer Size ’
TI ABI specifies that function (code) pointers are 16-bit, whereas
GNU supports only 32-bit data and code pointers.
‘ Optional Return Value Pointer ’
Function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer.
The current ‘ -mabi=ti ’ implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with ‘ -mabi=ti ’.
Chapter 3: GCC Command Options 411
Relaxation is a GNU feature and for safety reasons is disabled when using
‘ -mabi=ti ’. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions.
3.19.42 RISC-V Options
These command-line options are defined for RISC-V targets:
-mbranch-cost= n
Set the cost of branches to roughly n instructions.
-mplt
-mno-plt When generating PIC code, do or don’t allow the use of PLTs. Ignored for non-PIC. The default is ‘ -mplt ’.
-mabi= ABI-string
Specify integer and floating-point calling convention.
ABI-string contains two parts: the size of integer types and the registers used for floating-point types.
For example ‘ -march=rv64ifd -mabi=lp64d ’ means that ‘ long ’ and pointers are 64-bit (implicitly defining ‘ int ’ to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with ‘ -march=rv64ifd
-mabi=lp64f ’, which still allows the compiler to generate code that uses the
F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or ‘ -march=rv64ifd -mabi=lp64 ’, in which no floatingpoint arguments will be passed in registers.
The default for this argument is system dependent, users who want a specific calling convention should specify one explicitly. The valid calling conventions are: ‘ ilp32 ’, ‘ ilp32f ’, ‘ ilp32d ’, ‘ lp64 ’, ‘ lp64f ’, and ‘ lp64d ’. Some calling conventions are impossible to implement on some ISAs: for example,
‘ -march=rv32if -mabi=ilp32d ’ is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There is also the ‘ ilp32e ’ ABI that can only be used with the ‘ rv32e ’ architecture. This
ABI is not well specified at present, and is subject to change.
-mfdiv
-mno-fdiv
Do or don’t use hardware floating-point divide and square root instructions.
This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions.
-mdiv
-mno-div Do or don’t use hardware instructions for integer division. This requires the
M extension. The default is to use them if the specified architecture has these instructions.
-march= ISA-string
Generate code for given RISC-V ISA (e.g. ‘ rv64im ’).
ISA strings must be lower-case. Examples include ‘ rv64i ’, ‘ rv32g ’, ‘ rv32e ’, and ‘ rv32imaf ’.
When ‘ -march= ’ is not specified, use the setting from ‘ -mcpu ’.
412 Using the GNU Compiler Collection (GCC)
If both ‘ -march ’ and ‘ -mcpu= ’ are not specified, the default for this argument is system dependent, users who want a specific architecture extensions should specify one explicitly.
-mcpu= processor-string
Use architecture of and optimize the output for the given processor, specified by particular CPU name. Permissible values for this option are: ‘ sifive-e20 ’,
‘ sifive-e21 ’, ‘ sifive-e24 ’, ‘ sifive-e31 ’, ‘ sifive-e34 ’, ‘ sifive-e76 ’,
‘ sifive-s21 ’, ‘ sifive-s51 ’, ‘ sifive-s54 ’, ‘ sifive-s76 ’, ‘ sifive-u54 ’, and
‘ sifive-u74 ’.
-mtune= processor-string
Optimize the output for the given processor, specified by microarchitecture or particular CPU name.
Permissible values for this option are: ‘ rocket ’,
‘ sifive-3-series ’, ‘ sifive-5-series ’, ‘ sifive-7-series ’, ‘ size ’, and all valid options for ‘ -mcpu= ’.
When ‘ -mtune= ’ is not specified, use the setting from ‘ -mcpu ’, the default is
‘ rocket ’ if both are not specified.
The ‘ size ’ choice is not intended for use by end-users. This is used when ‘ -Os ’ is specified. It overrides the instruction cost info provided by ‘ -mtune= ’, but does not override the pipeline info. This helps reduce code size while still giving good performance.
-mpreferred-stack-boundary= num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary.
If ‘ -mpreferred-stack-boundary ’ is not specified, the default is 4 (16 bytes or
128-bits).
Warning: If you use this switch, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.
-msmall-data-limit= n
Put global and static data smaller than n bytes into a special section (on some targets).
-msave-restore
-mno-save-restore
Do or don’t use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues.
-mshorten-memrefs
-mno-shorten-memrefs
Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of ’base register + large offset’ with a new load/store of ’new base + small offset’. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores only.
Chapter 3: GCC Command Options 413
-mstrict-align
-mno-strict-align
Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not.
-mcmodel=medlow
Generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses − 2 GiB and + 2 GiB. Programs can be statically or dynamically linked. This is the default code model.
-mcmodel=medany
Generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked.
-mexplicit-relocs
-mno-exlicit-relocs
Use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization.
-mrelax
-mno-relax
Take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. The default is to take advantage of linker relaxations.
-memit-attribute
-mno-emit-attribute
Emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature requires at least binutils 2.32.
-malign-data= type
Control how GCC aligns variables and constants of array, structure, or union types. Supported values for type are ‘ xlen ’ which uses x register width as the alignment value, and ‘ natural ’ which uses natural alignment. ‘ xlen ’ is the default.
-mbig-endian
Generate big-endian code. This is the default when GCC is configured for a
‘ riscv64be-*-* ’ or ‘ riscv32be-*-* ’ target.
-mlittle-endian
Generate little-endian code.
This is the default when GCC is configured for a ‘ riscv64-*-* ’ or ‘ riscv32-*-* ’ but not a ‘ riscv64be-*-* ’ or
‘ riscv32be-*-* ’ target.
414 Using the GNU Compiler Collection (GCC)
-mstack-protector-guard= guard
-mstack-protector-guard-reg= reg
-mstack-protector-guard-offset= offset
Generate stack protection code using canary at guard . Supported locations are
‘ global ’ for a global canary or ‘ tls ’ for per-thread canary in the TLS block.
With the latter choice the options ‘ -mstack-protector-guard-reg= reg ’ and
‘ -mstack-protector-guard-offset= offset ’ furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel.
3.19.43 RL78 Options
-msim Links in additional target libraries to support operation within a simulator.
-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and division support to be used.
The simplest is none , which uses software for both multiplication and division.
This is the default. The g13 value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The g14 value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value rl78 is an alias for g14 and the value mg10 is an alias for none .
In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: __RL78_MUL_NONE__ , __RL78_MUL_G13__ or __
RL78_MUL_G14__ .
-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these operations.
The G10 or S1 core does not have register banks, so it uses a different calling convention.
If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit ‘ -mmul=none ’ option on the command line. Thus specifying ‘ -mcpu=g13 ’ enables the use of the G13 hardware multiply peripheral and specifying ‘ -mcpu=g10 ’ disables the use of hardware multiplications altogether.
Note, although the RL78/G14 core is the default target, specifying ‘ -mcpu=g14 ’ or ‘ -mcpu=rl78 ’ on the command line does change the behavior of the toolchain since it also enables G14 hardware multiply support. If these options are not
Chapter 3: GCC Command Options 415 specified on the command line then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support.
In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: __RL78_G10__ , __RL78_G13__ or __RL78_G14__ .
-mg10
-mg13
-mg14
-mrl78 These are aliases for the corresponding ‘ -mcpu= ’ option. They are provided for backwards compatibility.
-mallregs
Allow the compiler to use all of the available registers. By default registers r24..r31
are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well.
-m64bit-doubles
-m32bit-doubles
Make the double data type be 64 bits (‘ -m64bit-doubles ’) or 32 bits
(‘ -m32bit-doubles ’) in size. The default is ‘ -m32bit-doubles ’.
-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should preserve the MDUC registers.
This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers as this makes the interrupt handlers faster. The target option -mg13 needs to be passed for this to work as this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls another function.
3.19.44 IBM RS/6000 and PowerPC Options
These ‘ -m ’ options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
416 Using the GNU Compiler Collection (GCC)
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring
GCC.
Specifying the ‘ -mcpu= cpu_type ’ overrides the specification of these options. We recommend you use the ‘ -mcpu= cpu_type ’ option rather than the options listed above.
Specifying ‘ -mpowerpc-gpopt ’ allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying ‘ -mpowerpc-gfxopt ’ allows GCC to use the optional
PowerPC architecture instructions in the Graphics group, including floatingpoint select.
The ‘ -mmfcrf ’ option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The ‘ -mpopcntb ’ option allows
GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The ‘ -mpopcntd ’ option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The
‘ -mfprnd ’ option allows GCC to generate the FP round to integer instructions implemented on the POWER5 + processor and other processors that support the
PowerPC V2.03 architecture. The ‘ -mcmpb ’ option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The ‘ -mhard-dfp ’ option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors.
The ‘ -mpowerpc64 ’ option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as
64-bit, doubleword quantities. GCC defaults to ‘ -mno-powerpc64 ’.
-mcpu= cpu_type
Set architecture type, register usage, and instruction scheduling parameters for machine type cpu type . Supported values for cpu type are ‘ 401 ’, ‘ 403 ’, ‘ 405 ’,
‘ 405fp ’, ‘ 440 ’, ‘ 440fp ’, ‘ 464 ’, ‘ 464fp ’, ‘ 476 ’, ‘ 476fp ’, ‘ 505 ’, ‘ 601 ’, ‘ 602 ’, ‘ 603 ’,
Chapter 3: GCC Command Options 417
‘ 603e ’, ‘ 604 ’, ‘ 604e ’, ‘ 620 ’, ‘ 630 ’, ‘ 740 ’, ‘ 7400 ’, ‘ 7450 ’, ‘ 750 ’, ‘ 801 ’, ‘ 821 ’, ‘ 823 ’,
‘ 860 ’, ‘ 970 ’, ‘ 8540 ’, ‘ a2 ’, ‘ e300c2 ’, ‘ e300c3 ’, ‘ e500mc ’, ‘ e500mc64 ’, ‘ e5500 ’,
‘ e6500 ’, ‘ ec603e ’, ‘ G3 ’, ‘ G4 ’, ‘ G5 ’, ‘ titan ’, ‘ power3 ’, ‘ power4 ’, ‘ power5 ’,
‘ power5+ ’, ‘ power6 ’, ‘ power6x ’, ‘ power7 ’, ‘ power8 ’, ‘ power9 ’, ‘ power10 ’,
‘ powerpc ’, ‘ powerpc64 ’, ‘ powerpc64le ’, ‘ rs64 ’, and ‘ native ’.
‘ -mcpu=powerpc ’, ‘ -mcpu=powerpc64 ’, and ‘ -mcpu=powerpc64le ’ specify pure
32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.
Specifying ‘ native ’ as cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation.
‘ -mcpu=native ’ has no effect if GCC does not recognize the processor.
The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others.
The ‘ -mcpu ’ options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple
-mpopcntb -mpopcntd -mpowerpc64
-mpowerpc-gpopt -mpowerpc-gfxopt
-mmulhw -mdlmzb -mmfpgpr -mvsx
-mcrypto -mhtm -mpower8-fusion -mpower8-vector
-mquad-memory -mquad-memory-atomic -mfloat128
-mfloat128-hardware -mprefixed -mpcrel -mmma
-mrop-protect
The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that
CPU; it doesn’t necessarily reflect the actual hardware’s capabilities. If you wish to set an individual option to a particular value, you may specify it after the ‘ -mcpu ’ option, like ‘ -mcpu=970 -mno-altivec ’.
On AIX, the ‘ -maltivec ’ and ‘ -mpowerpc64 ’ options are not enabled or disabled by the ‘ -mcpu ’ option at present because AIX does not have full support for these options. You may still enable or disable them individually if you’re sure it’ll work in your environment.
-mtune= cpu_type
Set the instruction scheduling parameters for machine type cpu type , but do not set the architecture type or register usage, as ‘ -mcpu= cpu_type ’ does. The same values for cpu type are used for ‘ -mtune ’ as for ‘ -mcpu ’. If both are specified, the code generated uses the architecture and registers set by ‘ -mcpu ’, but the scheduling parameters set by ‘ -mtune ’.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space.
418 Using the GNU Compiler Collection (GCC)
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set ‘ -mabi=altivec ’ to adjust the current ABI with
AltiVec ABI enhancements.
When ‘ -maltivec ’ is used, the element order for AltiVec intrinsics such as vec_ splat , vec_extract , and vec_insert match array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so
to build executables and shared libraries with non-executable .plt
and .got
sections. This is a PowerPC 32-bit
SYSV ABI option.
-mbss-plt
Generate code that uses a BSS .plt
section that ld.so
fills in, and requires
.plt
and .got
sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-mvsx
-mno-vsx Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the
VSX instruction set.
-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC
ISA.
-mhtm
-mno-htm Enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors.
Chapter 3: GCC Command Options 419
-mpower8-vector
-mno-power8-vector
Generate code that uses (does not use) the vector and scalar instructions that were added in version 2.07 of the PowerPC ISA. Also enable the use of built-in functions that allow more direct access to the vector instructions.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad word memory instructions. The ‘ -mquad-memory ’ option requires use of 64-bit mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word memory instructions. The ‘ -mquad-memory-atomic ’ option requires use of 64-bit mode.
-mfloat128
-mno-float128
Enable/disable the float128 keyword for IEEE 128-bit floating point and use either software emulation for IEEE 128-bit floating point or hardware instructions.
The VSX instruction set (‘ -mvsx ’) must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point is only supported on
Linux.
The default for ‘ -mfloat128 ’ is enabled on PowerPC Linux systems using the
VSX instruction set, and disabled on other systems.
If you use the ISA 3.0 instruction set (‘ -mpower9-vector ’ or ‘ -mcpu=power9 ’) on a 64-bit system, the IEEE 128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not specify to generate ISA 3.0 instructions or you are targeting a 32bit big endian system, IEEE 128-bit floating point will be done with software emulation.
-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to support the float128 data type.
The default for ‘ -mfloat128-hardware ’ is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.
-m32
-m64 Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets
(including GNU/Linux). The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for ‘ -mpowerpc64 ’.
420 Using the GNU Compiler Collection (GCC)
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The ‘ -mfull-toc ’ option is selected by default. In that case,
GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the
TOC. However, only 16,384 entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the
‘ -mno-fp-in-toc ’ and ‘ -mno-sum-in-toc ’ options. ‘ -mno-fp-in-toc ’ prevents
GCC from putting floating-point constants in the TOC and ‘ -mno-sum-in-toc ’ forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify ‘ -mminimal-toc ’ instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code.
-maix64
-maix32 Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. Specifying ‘ -maix64 ’ implies ‘ -mpowerpc64 ’, while ‘ -maix32 ’ disables the 64-bit ABI and implies
‘ -mno-powerpc64 ’. GCC defaults to ‘ -maix32 ’.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument
FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to double. Use XL symbol names for long double support routines.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floatingpoint arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by
IBM XL compilers without optimization.
Chapter 3: GCC Command Options 421
-mpe Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to run. The system must have PE installed in the standard location (‘ /usr/lpp/ppe.poe/ ’), or the ‘ specs ’ file must be overridden with the
‘ -specs= ’ option to specify the appropriate directory location. The Parallel
Environment does not support threads, so the ‘ -mpe ’ option and the ‘ -pthread ’ option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option
‘ -malign-natural ’ overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option
‘ -malign-power ’ instructs GCC to follow the ABI-specified alignment rules.
GCC defaults to the standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and ‘ -malign-power ’ is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if you use the ‘ -msoft-float ’ option, and pass the option to GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use ‘ -mmultiple ’ on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are
PPC740 and PPC750 which permit these instructions in little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location.
These instructions are generated by default. If you use ‘ -mno-update ’, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting
Power6 and disabled otherwise.
422 Using the GNU Compiler Collection (GCC)
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent ‘ -mfused-madd ’ option is now mapped to the machine-independent ‘ -ffp-contract=fast ’ option, and
‘ -mno-fused-madd ’ is mapped to ‘ -ffp-contract=off ’.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and multiplyaccumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search ‘ dlmzb ’ instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 unsigned bitfields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using ‘ -mno-bit-align ’, the structure is aligned to a 1-byte boundary and is 1 byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are handled by the system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2
and 4-byte locations listed in the
.fixup
section, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with ‘ -mrelocatable ’ or ‘ -mrelocatable-lib ’. ‘ -mrelocatable ’ code aligns the stack to an 8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like ‘ -mrelocatable ’, ‘ -mrelocatable-lib ’ generates a .fixup
section to allow static executables to be relocated at run time, but ‘ -mrelocatable-lib ’ does not use the smaller stack alignment of ‘ -mrelocatable ’. Objects compiled with ‘ -mrelocatable-lib ’ may be linked with objects compiled with any combination of the ‘ -mrelocatable ’ options.
Chapter 3: GCC Command Options 423
-mno-toc
-mtoc On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The ‘ -mlittle-endian ’ option is the same as ‘ -mlittle ’.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode. The ‘ -mbig-endian ’ option is the same as ‘ -mbig ’.
-mdynamic-no-pic
On Darwin and Mac OS X systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins.
-mprioritize-restricted-insns= priority
This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value ‘ 0 ’, ‘ 1 ’, or ‘ 2 ’ to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions.
-msched-costly-dep= dependence_type
This option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence type takes one of the following values:
‘ no ’
‘ all ’
No dependence is costly.
All dependences are costly.
‘ true_store_to_load ’
A true dependence from store to load is costly.
‘ store_to_load ’
Any dependence from store to load is costly.
number Any dependence for which the latency is greater than or equal to number is costly.
-minsert-sched-nops= scheme
This option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:
‘ no ’ Don’t insert NOPs.
424 Using the GNU Compiler Collection (GCC)
‘ pad ’
‘ regroup_exact ’
Insert NOPs to force costly dependent insns into separate groups.
Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.
number
Pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler’s grouping.
Insert NOPs to force costly dependent insns into separate groups.
Insert number NOPs to force an insn to a new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that adhere to the March 1995 draft of the System V Application
Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using ‘ powerpc-*-eabiaix ’.
-mcall-sysv-eabi
-mcall-eabi
Specify both ‘ -mcall-sysv ’ and ‘ -meabi ’ options.
-mcall-sysv-noeabi
Specify both ‘ -mcall-sysv ’ and ‘ -mno-eabi ’ options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linuxbased GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system.
-mtraceback= traceback_type
Select the type of traceback table. Valid values for traceback type are ‘ full ’,
‘ part ’, and ‘ no ’.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4
ABI).
Chapter 3: GCC Command Options 425
-mabi= abi-type
Extend the current ABI with a particular extension, or remove such extension.
Valid values are: ‘ altivec ’, ‘ no-altivec ’, ‘ ibmlongdouble ’,
‘ ieeelongdouble ’, ‘ elfv1 ’, ‘ elfv2 ’, and for AIX: ‘ vec-extabi ’,
‘ vec-default ’.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is not likely to work if your system defaults to using IEEE extended-precision long double. If you change the long double type from IEEE extended-precision, the compiler will issue a warning unless you use the ‘ -Wno-psabi ’ option. Requires
‘ -mlong-double-128 ’ to be enabled.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is not likely to work if your system defaults to using IBM extended-precision long double. If you change the long double type from IBM extended-precision, the compiler will issue a warning unless you use the ‘ -Wno-psabi ’ option. Requires
‘ -mlong-double-128 ’ to be enabled.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu attribute assembly directives to set tag/value pairs in a
.gnu.attributes section that specify ABI variations in function parameters or return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly prototyped. Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ( CR ) to indicate whether floating-point values are passed in the floating-point registers in case the function takes variable arguments.
With ‘ -mprototype ’, only calls to prototyped variable argument functions set or clear the bit.
-msim
-mmvme
On embedded PowerPC systems, assume that the startup module is called
‘ sim-crt0.o
’ and that the standard C libraries are ‘ libsim.a
’ and ‘ libc.a
’.
This is the default for ‘ powerpc-*-eabisim ’ configurations.
On embedded PowerPC systems, assume that the startup module is called
‘ crt0.o
’ and the standard C libraries are ‘ libmvme.a
’ and ‘ libc.a
’.
426 Using the GNU Compiler Collection (GCC)
-mads On embedded PowerPC systems, assume that the startup module is called
‘ crt0.o
’ and the standard C libraries are ‘ libads.a
’ and ‘ libc.a
’.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called
‘ crt0.o
’ and the standard C libraries are ‘ libyk.a
’ and ‘ libc.a
’.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-memb On embedded PowerPC systems, set the PPC_EMB bit in the ELF flags header to indicate that ‘ eabi ’ extended relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the
Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. Selecting ‘ -meabi ’ means that the stack is aligned to an 8-byte boundary, a function __eabi is called from main to set up the EABI environment, and the ‘ -msdata ’ option can use both r2 and r13 to point to two separate small data areas. Selecting ‘ -mno-eabi ’ means that the stack is aligned to a 16-byte boundary, no EABI initialization function is called from main , and the ‘ -msdata ’ option only uses r13 to point to a single small data area. The ‘ -meabi ’ option is on by default if you configured GCC using one of the ‘ powerpc*-*-eabi* ’ options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized const global and static data in the .sdata2
section, which is pointed to by register r2 . Put small initialized nonconst global and static data in the .sdata
section, which is pointed to by register r13 . Put small uninitialized global and static data in the .sbss
section, which is adjacent to the .sdata
section. The
‘ -msdata=eabi ’ option is incompatible with the ‘ -mrelocatable ’ option. The
‘ -msdata=eabi ’ option also sets the ‘ -memb ’ option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata
section, which is pointed to by register r13 . Put small uninitialized global and static data in the .sbss
section, which is adjacent to the .sdata
section. The ‘ -msdata=sysv ’ option is incompatible with the
‘ -mrelocatable ’ option.
-msdata=default
-msdata On System V.4 and embedded PowerPC systems, if ‘ -meabi ’ is used, compile code the same as ‘ -msdata=eabi ’, otherwise compile code the same as
‘ -msdata=sysv ’.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data in the
.sdata
section. Put small uninitialized global data in the .sbss
section. Do
Chapter 3: GCC Command Options 427 not use register r13 to address small data however. This is the default behavior unless other ‘ -msdata ’ options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the .data
section, and all uninitialized data in the .bss
section.
-mreadonly-in-sdata
Put read-only objects in the .sdata
section as well. This is the default.
-mblock-move-inline-limit= num
Inline all block moves (such as calls to memcpy or structure copies) less than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific.
-mblock-compare-inline-limit= num
Generate non-looping inline code for all block compares (such as calls to memcmp or structure compares) less than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of block compare is disabled. The default value is target-specific.
-mblock-compare-inline-loop-limit= num
Generate an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length is not constant, at most num bytes will be compared before memcmp is called to compare the remainder of the block. The default value is target-specific.
-mstring-compare-inline-limit= num
Compare at most num string bytes with inline code. If the difference or end of string is not found at the end of the inline compare a call to strcmp or strncmp will take care of the rest of the comparison. The default is 64 bytes.
-G num On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or BSS sections instead of the normal data or
BSS section. By default, num is 8. The ‘ -G num ’ switch is also passed to the linker. All modules should be compiled with the same ‘ -G num ’ value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer and more expensive calling sequence is required. This is required for calls farther than 32 megabytes
(33,554,432 bytes) from the current location. A short call is generated if the compiler knows the call cannot be that far away. This setting can be overridden by the shortcall function attribute, or by #pragma longcall(0) .
428 Using the GNU Compiler Collection (GCC)
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for
32-bit PowerPC systems as well.
On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers,
GCC can generate long calls using an inline PLT call sequence (see ‘ -mpltseq ’).
PowerPC with ‘ -mbss-plt ’ and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls.
On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42 , plus a branch island (glue code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island. The branch island is appended to the body of the calling function; it computes the full
32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it.
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue.
-mpltseq
-mno-pltseq
Implement (do not implement) -fno-plt and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen’d shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and are enabled by default if the support is detected when configuring GCC, and, in the case of
32-bit PowerPC, if GCC is configured with ‘ --enable-secureplt ’. ‘ -mpltseq ’ code and ‘ -mbss-plt ’ 32-bit PowerPC relocatable objects may not be linked together.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to __tls_get_addr with a relocation specifying the function argument. The relocation allows the linker to reliably associate function call with argument setup instructions for TLS optimization, which in turn allows GCC to better schedule the sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. You should use the ‘ -ffast-math ’ option when using ‘ -mrecip ’
(or at least ‘ -funsafe-math-optimizations ’, ‘ -ffinite-math-only ’,
‘ -freciprocal-math ’ and ‘ -fno-trapping-math ’).
Note that while the
Chapter 3: GCC Command Options 429 throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots.
-mrecip= opt
This option controls which reciprocal estimate instructions may be used.
opt is a comma-separated list of options, which may be preceded by a !
to invert the option:
‘ all ’ Enable all estimate instructions.
‘ default ’ Enable the default instructions, equivalent to ‘ -mrecip ’.
‘ none ’ Disable all estimate instructions, equivalent to ‘ -mno-recip ’.
‘ div ’
‘
‘
‘ divf divd
’
’ rsqrt ’
Enable the reciprocal approximation instructions for both single and double precision.
Enable the single-precision reciprocal approximation instructions.
Enable the double-precision reciprocal approximation instructions.
Enable the reciprocal square root approximation instructions for both single and double precision.
‘ rsqrtf ’ Enable the single-precision reciprocal square root approximation instructions.
‘ rsqrtd ’ Enable the double-precision reciprocal square root approximation instructions.
So, for example, ‘ -mrecip=all,!rsqrtd
’ enables all of the reciprocal estimate instructions, except for the FRSQRTE , XSRSQRTEDP , and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. Selecting
‘ -mcpu=power6 ’, ‘ -mcpu=power7 ’ or ‘ -mcpu=power8 ’ automatically selects
‘ -mrecip-precision ’. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they do not provide an estimate that converges after three steps.
-mveclibabi= type
Specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is ‘ mass ’, which specifies to use
IBM’s Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries.
GCC currently emits calls to acosd2 , acosf4 , acoshd2 , acoshf4 , asind2 , asinf4 , asinhd2 , asinhf4 , atan2d2 , atan2f4 , atand2 , atanf4 , atanhd2 , atanhf4 , cbrtd2 , cbrtf4 , cosd2 , cosf4 , coshd2 , coshf4 , erfcd2 , erfcf4 , erfd2 , erff4 , exp2d2 , exp2f4 , expd2 , expf4 , expm1d2 , expm1f4 , hypotd2 , hypotf4 , lgammad2 , lgammaf4 , log10d2 ,
430 Using the GNU Compiler Collection (GCC) log10f4 , log1pd2 , log1pf4 , log2d2 , log2f4 , logd2 , logf4 , powd2 , powf4 , sind2 , sinf4 , sinhd2 , sinhf4 , sqrtd2 , sqrtf4 , tand2 , tanf4 , tanhd2 , and tanhf4 when generating code for power7.
Both ‘ -ftree-vectorize ’ and
‘ -funsafe-math-optimizations ’ must also be enabled. The MASS libraries must be specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the friz instruction when the
‘ -funsafe-math-optimizations ’ option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. The friz instruction does not return the same value if the floating-point number is too large to fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register ( r11 ) when calling through a pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address, TOC value to be loaded in register r2 , and static chain value to be loaded in register r11 . The
‘ -mpointers-to-nested-functions ’ is on by default. You cannot call through pointers to nested functions or pointers to functions compiled in other languages that use the static chain if you use ‘ -mno-pointers-to-nested-functions ’.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value is not saved in the prologue, it is saved just before the call through the pointer. The ‘ -mno-save-toc-indirect ’ option is the default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC.
This option may be used to generate code that is compatible with functions compiled with older versions of GCC.
The ‘ -mno-compat-align-parm ’ option is the default.
-mstack-protector-guard= guard
-mstack-protector-guard-reg= reg
-mstack-protector-guard-offset= offset
-mstack-protector-guard-symbol= symbol
Generate stack protection code using canary at guard . Supported locations are
‘ global ’ for global canary or ‘ tls ’ for per-thread canary in the TLS block (the default with GNU libc version 2.4 or later).
Chapter 3: GCC Command Options 431
With the latter choice the options ‘ -mstack-protector-guard-reg= reg ’ and
‘ -mstack-protector-guard-offset= offset ’ furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI.
‘ -mstack-protector-guard-symbol= symbol ’ overrides the offset with a symbol reference to a canary in the TLS block.
-mpcrel
-mno-pcrel
Generate (do not generate) pc-relative addressing. The ‘ -mpcrel ’ option requires that the medium code model (‘ -mcmodel=medium ’) and prefixed addressing (‘ -mprefixed ’) options are enabled.
-mprefixed
-mno-prefixed
Generate (do not generate) addressing modes using prefixed load and store instructions. The ‘ -mprefixed ’ option requires that the option ‘ -mcpu=power10 ’
(or later) is enabled.
-mmma
-mno-mma Generate (do not generate) the MMA instructions. The ‘ -mma ’ option requires that the option ‘ -mcpu=power10 ’ (or later) is enabled.
-mrop-protect
-mno-rop-protect
Generate (do not generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization
(‘ -fshrink-wrap ’).
-mprivileged
-mno-privileged
Generate (do not generate) code that will run in privileged state.
-mblock-ops-unaligned-vsx
-mno-block-ops-unaligned-vsx
Generate (do not generate) unaligned vsx loads and stores for inline expansion of memcpy and memmove .
3.19.45 RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the double data type be 64 bits (‘ -m64bit-doubles ’) or 32 bits
(‘ -m32bit-doubles ’) in size.
The default is ‘ -m32bit-doubles ’.
Note RX floating-point hardware only works on 32-bit values, which is why the default is ‘ -m32bit-doubles ’.
-fpu
-nofpu Enables (‘ -fpu ’) or disables (‘ -nofpu ’) the use of RX floating-point hardware.
The default is enabled for the RX600 series and disabled for the RX200 series.
432 Using the GNU Compiler Collection (GCC)
Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the ‘ -m64bit-doubles ’ option is used.
Note If the ‘ -fpu ’ option is enabled then ‘ -funsafe-math-optimizations ’ is also enabled automatically. This is because the RX FPU instructions are themselves unsafe.
-mcpu= name
Selects the type of RX CPU to be targeted. Currently three types are supported, the generic ‘ RX600 ’ and ‘ RX200 ’ series hardware and the specific ‘ RX610 ’
CPU. The default is ‘ RX600 ’.
The only difference between ‘ RX600 ’ and ‘ RX610 ’ is that the ‘ RX610 ’ does not support the MVTIPL instruction.
The ‘ RX200 ’ series does not have a hardware floating-point unit and so ‘ -nofpu ’ is enabled by default when this type is selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format.
The default is
‘ -mlittle-endian-data ’, i.e. to store data in the little-endian format.
-msmall-data-limit= N
Specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer to ensure that the area does not overflow. Also when the small data area is used one of the RX’s registers (usually r13 ) is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register.
Note, common variables (variables that have not been initialized) and constants are not placed into the small data area as they are assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher optimization levels (‘ -O2 ’ etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program.
See the description of the ‘ -mpid ’ option for a description of how the actual register to hold the small data area pointer is chosen.
-msim
-mno-sim Use the simulator runtime. The default is to use the libgloss board-specific runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with Renesas’s AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default.
Chapter 3: GCC Command Options 433
-mmax-constant-size= N
Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction.
Although the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction. Thus in some circumstances it can be beneficial to restrict the size of constants that are used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed.
-mrelax Enable linker relaxation.
Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default.
-mint-register= N
Specify the number of registers to reserve for fast interrupt handler functions.
The value N can be between 0 and 4. A value of 1 means that register r13 is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves r13 and r12 . A value of 3 reserves r13 , r12 and r11 , and a value of 4 reserves r13 through r10 . A value of 0, the default, does not reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator as this makes the interrupt handlers faster.
-mpid
-mno-pid Enables the generation of position independent data. When enabled any access to constant data is done via an offset from a base address held in a register.
This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option.
Note, using this feature reserves a register, usually r13 , for the constant data base address. This can result in slower and/or larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address depends upon whether the ‘ -msmall-data-limit ’ and/or the ‘ -mint-register ’ commandline options are enabled. Starting with register r13 and proceeding downwards, registers are allocated first to satisfy the requirements of ‘ -mint-register ’, then ‘ -mpid ’ and finally ‘ -msmall-data-limit ’. Thus it is possible for the small data area register to be r8 if both ‘ -mint-register=4 ’ and ‘ -mpid ’ are specified on the command line.
By default this feature is not enabled. The default can be restored via the
‘ -mno-pid ’ command-line option.
434 Using the GNU Compiler Collection (GCC)
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt.
-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation instructions SMOVF ,
SCMPU , SMOVB , SMOVU , SUNTIL SWHILE and also the RMPA instruction. These instructions may prefetch data, which is not safe to do if accessing an I/O register. (See section 12.2.7 of the RX62N Group User’s Manual for more information).
The default is to allow these instructions, but it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. Instead it is reliant upon the programmer to use the ‘ -mno-allow-string-insns ’ option if their program accesses I/O space.
When the instructions are enabled GCC defines the C preprocessor symbol _
_RX_ALLOW_STRING_INSNS__ , otherwise it defines the symbol __RX_DISALLOW_
STRING_INSNS__ .
-mjsr
-mno-jsr Use only (or not only) JSR instructions to access functions. This option can be used when code size exceeds the range of BSR instructions. Note that ‘ -mno-jsr ’ does not mean to not use JSR but instead means that any type of branch may be used.
Note: The generic GCC command-line option ‘ -ffixedreg ’ has special significance to the RX port when used with the interrupt function attribute. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers r10 , r11 , r12 and/or r13 and only provided that the normal use of the corresponding registers have been restricted via the ‘ -ffixedreg ’ or ‘ -mint-register ’ command-line options.
3.19.46 S/390 and zSeries Options
These are the ‘ -m ’ options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-point operations.
When ‘ -msoft-float ’ is specified, functions in ‘ libgcc.a
’ are used to perform floating-point operations.
When
‘ -mhard-float ’ is specified, the compiler generates IEEE floating-point instructions. This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations.
When ‘ -mno-hard-dfp ’ is specified,
Chapter 3: GCC Command Options 435 functions in ‘ libgcc.a
’ are used to perform decimal-floating-point operations.
When ‘ -mhard-dfp ’ is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for ‘ -march=z9-ec ’ or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller’s frame as backchain pointer into the callee’s stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF call frame information. When
‘ -mno-packed-stack ’ is in effect, the backchain pointer is stored at the bottom of the stack frame; when ‘ -mpacked-stack ’ is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.
In general, code compiled with ‘ -mbackchain ’ is call-compatible with code compiled with ‘ -mno-backchain ’; however, use of the backchain for debugging purposes usually requires that the whole binary is built with ‘ -mbackchain ’. Note that the combination of ‘ -mbackchain ’, ‘ -mpacked-stack ’ and ‘ -mhard-float ’ is not supported. In order to build a linux kernel use ‘ -msoft-float ’.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When ‘ -mno-packed-stack ’ is specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When
‘ -mpacked-stack ’ is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when ‘ -mbackchain ’ is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated with ‘ -mpacked-stack ’ is call-compatible with code generated with
‘ -mno-packed-stack ’.
Note that some non-FSF releases of GCC 2.95 for
S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-compatible with code compiled with ‘ -mpacked-stack ’.
Also, note that the combination of
‘ -mbackchain ’, ‘ -mpacked-stack ’ and ‘ -mhard-float ’ is not supported. In order to build a linux kernel use ‘ -msoft-float ’.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the bras instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k.
436 Using the GNU Compiler Collection (GCC)
The default is to use the basr instruction instead, which does not have this limitation.
-m64
-m31 When ‘ -m31 ’ is specified, generate code compliant to the GNU/Linux for S/390
ABI. When ‘ -m64 ’ is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the ‘ s390 ’ targets, the default is ‘ -m31 ’, while the ‘ s390x ’ targets default to
‘ -m64 ’.
-mzarch
-mesa When ‘ -mzarch ’ is specified, generate code using the instructions available on z/Architecture. When ‘ -mesa ’ is specified, generate code using the instructions available on ESA/390. Note that ‘ -mesa ’ is not possible with ‘ -m64 ’. When generating code compliant to the GNU/Linux for S/390 ABI, the default is
‘ -mesa ’. When generating code compliant to the GNU/Linux for zSeries ABI, the default is ‘ -mzarch ’.
-mhtm
-mno-htm The ‘ -mhtm ’ option enables a set of builtins making use of instructions available with the transactional execution facility introduced with the IBM zEnterprise
EC12 machine generation
Section 6.60.30 [S/390 System z Built-in Functions], page 832
. ‘ -mhtm ’ is enabled by default when using ‘ -march=zEC12 ’.
-mvx
-mno-vx When ‘ -mvx ’ is specified, generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option changes the ABI for some vector type values with regard to alignment and calling conventions. In case vector type values are being used in an ABIrelevant context a GAS ‘ .gnu_attribute
’ command will be added to mark the resulting binary with the ABI used. ‘ -mvx ’ is enabled by default when using
‘ -march=z13 ’.
-mzvector
-mno-zvector
The ‘ -mzvector ’ option enables vector language extensions and builtins using instructions available with the vector extension facility introduced with the
IBM z13 machine generation. This option adds support for ‘ vector ’ to be used as a keyword to define vector type variables and arguments. ‘ vector ’ is only available when GNU extensions are enabled. It will not be expanded when requesting strict standard compliance e.g. with ‘ -std=c99 ’. In addition to the GCC low-level builtins ‘ -mzvector ’ enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. In order to make use of these builtins the header file ‘ vecintrin.h
’ needs to be included.
‘ -mzvector ’ is disabled by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the mvcle instruction to perform block moves. When ‘ -mno-mvcle ’ is specified, use a mvc loop instead. This is the default unless optimizing for size.
Chapter 3: GCC Command Options 437
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print debug information.
-march= cpu-type
Generate code that runs on cpu-type , which is the name of a system representing a certain processor type.
Possible values for cpu-type are
‘ z900 ’/‘ arch5 ’, ‘ z990 ’/‘ arch6 ’, ‘ z9-109 ’, ‘ z9-ec ’/‘ arch7 ’, ‘ z10 ’/‘ arch8 ’,
‘ z196 ’/‘ arch9 ’, ‘ zEC12 ’, ‘ z13 ’/‘ arch11 ’, ‘ z14 ’/‘ arch12 ’, ‘ z15 ’/‘ arch13 ’, and
‘ native ’.
The default is ‘ -march=z900 ’.
Specifying ‘ native ’ as cpu type can be used to select the best architecture option for the host processor. ‘ -march=native ’ has no effect if GCC does not recognize the processor.
-mtune= cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for ‘ -march ’. The default is the value used for ‘ -march ’.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS.
-mtpf-trace-skip
-mno-tpf-trace-skip
Generate code that changes (does not change) the default branch targets enabled by ‘ -mtpf-trace ’ to point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying
‘ -mtpf-trace ’.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used.
-mwarn-framesize= framesize
Emit a warning if the current function exceeds the given frame size. Because this is a compile-time check it doesn’t need to be a real problem when the program runs. It is intended to identify functions that most probably cause a stack overflow. It is useful to be used in an environment with limited stack size e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls alloca or uses dynamically-sized arrays.
This is generally a bad idea with a limited stack size.
438 Using the GNU Compiler Collection (GCC)
-mstack-guard= stack-guard
-mstack-size= stack-size
If these options are provided the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390 grows downward).
If the stack-guard option is omitted the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are intended to be used to help debugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation. The given values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size . The stack-guard option can only be used in conjunction with stack-size .
-mhotpatch= pre-halfwords , post-halfwords
If the hotpatch option is enabled, a “hot-patching” function prologue is generated for all functions in the compilation unit. The funtion label is prepended with the given number of two-byte NOP instructions ( pre-halfwords , maximum
1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP like instructions the architecture allows (maximum 1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the hotpatch attribute.
3.19.47 Score Options
These options are defined for Score implementations:
-meb
-mel
Compile code for big-endian mode. This is the default.
Compile code for little-endian mode.
-mnhwloop
Disable generation of bcnz instructions.
-muls
-mmac
Enable generation of unaligned load and store instructions.
Enable the use of multiply-accumulate instructions. Disabled by default.
-mscore5 Specify the SCORE5 as the target architecture.
-mscore5u
Specify the SCORE5U of the target architecture.
-mscore7 Specify the SCORE7 as the target architecture. This is the default.
-mscore7d
Specify the SCORE7D as the target architecture.
3.19.48 SH Options
These ‘ -m ’ options are defined for the SH implementations:
Chapter 3: GCC Command Options 439
-m1
-m2
-m2e
Generate code for the SH1.
Generate code for the SH2.
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in singleprecision mode by default.
-m2a Generate code for the SH2a-FPU assuming the floating-point unit is in doubleprecision mode by default.
-m3
-m3e
-m4-nofpu
Generate code for the SH3.
Generate code for the SH3e.
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports singleprecision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-m4-100 Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-point unit is not used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
-m4-200 Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the floating-point unit is not used.
440 Using the GNU Compiler Collection (GCC)
-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
-m4-300 Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the floating-point unit is not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
-m4-340 Generate code for SH4-340 (no MMU, no FPU).
-m4-500 Generate code for SH4-500 (no FPU). Passes ‘ -isa=sh4-nofpu ’ to the assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floatingpoint unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floatingpoint operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
-m4a
-m4al
Generate code for the SH4a.
Same as ‘ -m4a-nofpu ’, except that it implicitly passes ‘ -dsp ’ to the assembler.
GCC doesn’t generate any DSP instructions at the moment.
-mb
-ml
Compile code for the processor in big-endian mode.
Compile code for the processor in little-endian mode.
-mdalign Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with ‘ -mdalign ’.
-mrelax Shorten some address references at link time, when possible; uses the linker option ‘ -relax ’.
-mbigtable
Use 32-bit offsets in switch tables. The default is to use 16-bit offsets.
Chapter 3: GCC Command Options 441
-mbitops Enable the use of bit manipulation instructions on SH2A.
-mfmovd Enable the use of the instruction fmovd . Check ‘ -mdalign ’ for alignment constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain.
-mnomacsave
Mark the MAC register as call-clobbered, even if ‘ -mrenesas ’ is given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default
‘ -mieee ’ is implicitly enabled. If ‘ -ffinite-math-only ’ is enabled ‘ -mno-ieee ’ is implicitly set, which results in faster floating-point greater-equal and lessequal comparisons. The implicit settings can be overridden by specifying either
‘ -mieee ’ or ‘ -mno-ieee ’.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if ‘ -musermode ’ is in effect and the selected code generation option (e.g. ‘ -m4 ’) does not allow the use of the icbi instruction. If the selected code generation option does not allow the use of the icbi instruction, and ‘ -musermode ’ is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write.
This not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped.
-misize Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
-matomic-model= model
Sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see
[ atomic Builtins], page 713 . The following models and parameters are sup-
ported:
‘ none ’ Disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not sh*-
*-linux* .
‘ soft-gusa ’
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences
442 Using the GNU Compiler Collection (GCC)
-mtas require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* singlecore systems. This option is enabled by default when the target is sh*-*-linux* and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions movli.l
and movco.l
to create more efficient code, unless ‘ strict ’ is specified.
‘ soft-tcb ’
Generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets.
The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems.
When using this model, the
‘ gbr-offset= ’ parameter has to be specified as well.
‘ soft-imask ’
Generate software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111 . This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is sh*-*-linux* and SH1* or SH2*.
‘ hard-llcs ’
Generate hardware atomic sequences using the movli.l
and movco.l
instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables access to 8 or 16 bit variables is emulated with 32 bit accesses.
Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model.
‘ gbr-offset= ’
This parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the ‘ soft-tcb ’ model has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range 0-1020.
‘ strict ’ This parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only.
Generate the tas.b
opcode for __atomic_test_and_set . Notice that depending on the particular hardware and software configuration this can degrade overall performance due to the operand cache line flushes that are implied by
Chapter 3: GCC Command Options 443 the tas.b
instruction. On multi-core SH4A processors the tas.b
instruction must be used with caution since it can result in data corruption for certain cache configurations.
-mprefergot
When generating position-independent code, emit function calls using the
Global Offset Table instead of the Procedure Linkage Table.
-musermode
-mno-usermode
Don’t allow (allow) the compiler generating privileged mode code. Specifying
‘ -musermode ’ also implies ‘ -mno-inline-ic_invalidate ’ if the inlined code would not work in user mode. ‘ -musermode ’ is the default when the target is sh*-*-linux* . If the target is SH1* or SH2* ‘ -musermode ’ has no effect, since there is no user mode.
-multcost= number
Set the cost to assume for a multiply insn.
-mdiv= strategy
Set the division strategy to be used for integer division operations.
strategy can be one of:
‘ call-div1 ’
Calls a library function that uses the single-step division instruction div1 to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for
SH4, SH2A and SHcompact.
‘ call-fp ’ Calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that do not have a double precision FPU defaults to call-div1 .
‘ call-table ’
Calls a library function that uses a lookup table for small divisors and the div1 instruction with case distinction for larger divisors.
Division by zero calculates an unspecified result and does not trap.
This is the default for SH4. Specifying this for targets that do not have dynamic shift instructions defaults to call-div1 .
When a division strategy has not been specified the default strategy is selected based on the current target. For SH2A the default strategy is to use the divs and divu instructions instead of library function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code.
444 Using the GNU Compiler Collection (GCC)
-mdivsi3_libfunc= name
Set the name of the library function used for 32-bit signed division to name .
This only affects the name used in the ‘ call ’ division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present.
-mfixed-range= register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash.
Multiple register ranges can be specified separated by a comma.
-mbranch-cost= num
Assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified the value is selected depending on the processor type that is being compiled for.
-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement conditional branch instructions bt and bf are fast. If ‘ -mzdcbranch ’ is specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying
‘ -mno-zdcbranch ’.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches, which stuffs the delay slot with a nop if a suitable instruction cannot be found. By default this option is disabled. It can be enabled to work around hardware bugs as found in the original SH7055.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent ‘ -mfused-madd ’ option is now mapped to the machine-independent ‘ -ffp-contract=fast ’ option, and
‘ -mno-fused-madd ’ is mapped to ‘ -ffp-contract=off ’.
-mfsca
-mno-fsca
Allow or disallow the compiler to emit the fsca instruction for sine and cosine approximations. The option ‘ -mfsca ’ must be used in combination with
‘ -funsafe-math-optimizations ’. It is enabled by default when generating code for SH4A. Using ‘ -mno-fsca ’ disables sine and cosine approximations even if ‘ -funsafe-math-optimizations ’ is in effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the fsrra instruction for reciprocal square root approximations. The option ‘ -mfsrra ’ must be used in combina-
Chapter 3: GCC Command Options 445 tion with ‘ -funsafe-math-optimizations ’ and ‘ -ffinite-math-only ’. It is enabled by default when generating code for SH4A. Using ‘ -mno-fsrra ’ disables reciprocal square root approximations even if ‘ -funsafe-math-optimizations ’ and ‘ -ffinite-math-only ’ are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor.
-mfdpic Generate code using the FDPIC ABI.
3.19.49 Solaris 2 Options
These ‘ -m ’ options are supported on Solaris 2:
-mclear-hwcap
‘ -mclear-hwcap ’ tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them.
-mimpure-text
‘ -mimpure-text ’, used in addition to ‘ -shared ’, tells the compiler to not pass
‘ -z text ’ to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.
‘ -mimpure-text ’ suppresses the “relocations remain against allocatable but non-writable sections” linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using ‘ -mimpure-text ’, you should compile all source code with ‘ -fpic ’ or ‘ -fPIC ’.
These switches are supported in addition to the above on Solaris 2:
-pthreads
This is a synonym for ‘ -pthread ’.
3.19.50 SPARC Options
These ‘ -m ’ options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify ‘ -mapp-regs ’ to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Like the global register
1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss, specify
‘ -mno-app-regs ’. You should compile libraries and system software with this option.
-mflat
-mno-flat
With ‘ -mflat ’, the compiler does not generate save/restore instructions and uses a “flat” or single register window model. This model is compatible with
446 Using the GNU Compiler Collection (GCC) the regular register window model. The local registers and the input registers
(0–5) are still treated as “call-saved” registers and are saved on the stack as needed.
With ‘ -mno-flat ’ (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine’s usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets ‘ sparc-*-aout ’ and ‘ sparclite-*-* ’ do provide software floating-point support.
‘ -msoft-float ’ changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile ‘ libgcc.a
’, the library that comes with GCC, with
‘ -msoft-float ’ in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floatingpoint instructions. The functions called are those specified in the SPARC ABI.
This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the ‘ -msoft-quad-float ’ option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With ‘ -munaligned-doubles ’, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code.
Chapter 3: GCC Command Options 447
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode. This is relevant only for the casa instruction emitted for the LEON3 processor. This is the default.
-mfaster-structs
-mno-faster-structs
With ‘ -mfaster-structs ’, the compiler assumes that structures should have
8-byte alignment. This enables the use of pairs of ldd and std instructions for copies in structure assignment, in place of twice as many ld and st pairs.
However, the use of this changed alignment directly violates the SPARC ABI.
Thus, it’s intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI.
-mstd-struct-return
-mno-std-struct-return
With ‘ -mstd-struct-return ’, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.
The default is ‘ -mno-std-struct-return ’. This option has no effect in 64-bit mode.
-mlra
-mno-lra Enable Local Register Allocation. This is the default for SPARC since GCC 7 so ‘ -mno-lra ’ needs to be passed to get old Reload.
-mcpu= cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu type . Supported values for cpu type are ‘ v7 ’, ‘ cypress ’,
‘ v8 ’, ‘ supersparc ’, ‘ hypersparc ’, ‘ leon ’, ‘ leon3 ’, ‘ leon3v7 ’, ‘ leon5 ’,
‘ sparclite ’, ‘ f930 ’, ‘ f934 ’, ‘ sparclite86x ’, ‘ sparclet ’, ‘ tsc701 ’, ‘ v9 ’,
‘ ultrasparc ’, ‘ ultrasparc3 ’, ‘ niagara ’, ‘ niagara2 ’, ‘ niagara3 ’, ‘ niagara4 ’,
‘ niagara7 ’ and ‘ m8 ’.
Native Solaris and GNU/Linux toolchains also support the value
‘ native ’, which selects the best architecture option for the host processor.
‘ -mcpu=native ’ has no effect if GCC does not recognize the processor.
Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are ‘ v7 ’, ‘ v8 ’, ‘ sparclite ’,
‘ sparclet ’, ‘ v9 ’.
Here is a list of each supported architecture and their supported implementations.
v7 cypress, leon3v7 v8 supersparc, hypersparc, leon, leon3, leon5 sparclite f930, f934, sparclite86x sparclet tsc701
448 Using the GNU Compiler Collection (GCC) v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With ‘ -mcpu=cypress ’, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARC-
Station 1, 2, IPX etc.
With ‘ -mcpu=v8 ’, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in
SPARC-V7. With ‘ -mcpu=supersparc ’, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
With ‘ -mcpu=sparclite ’, GCC generates code for the SPARClite variant of the
SPARC architecture. This adds the integer multiply, integer divide step and scan ( ffs ) instructions which exist in SPARClite but not in SPARC-V7. With
‘ -mcpu=f930 ’, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With ‘ -mcpu=f934 ’, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.
With ‘ -mcpu=sparclet ’, GCC generates code for the SPARClet variant of the
SPARC architecture.
This adds the integer multiply, multiply/accumulate, integer divide step and scan ( ffs ) instructions which exist in SPARClet but not in SPARC-V7. With ‘ -mcpu=tsc701 ’, the compiler additionally optimizes it for the TEMIC SPARClet chip.
With ‘ -mcpu=v9 ’, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With ‘ -mcpu=ultrasparc ’, the compiler additionally optimizes it for the
Sun UltraSPARC I/II/IIi chips. With ‘ -mcpu=ultrasparc3 ’, the compiler additionally optimizes it for the Sun UltraSPARC III/III + /IIIi/IIIi + /IV/IV + chips.
With ‘ -mcpu=niagara ’, the compiler additionally optimizes it for Sun Ultra-
SPARC T1 chips. With ‘ -mcpu=niagara2 ’, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With ‘ -mcpu=niagara3 ’, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With ‘ -mcpu=niagara4 ’, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. With
‘ -mcpu=niagara7 ’, the compiler additionally optimizes it for Oracle SPARC
M7 chips. With ‘ -mcpu=m8 ’, the compiler additionally optimizes it for Oracle
M8 chips.
-mtune= cpu_type
Set the instruction scheduling parameters for machine type cpu type , but do not set the instruction set or register set that the option ‘ -mcpu= cpu_type ’ does.
The same values for ‘ -mcpu= cpu_type ’ can be used for ‘ -mtune= cpu_type ’, but the only useful values are those that select a particular CPU implemen-
Chapter 3: GCC Command Options 449 tation.
Those are ‘ cypress ’, ‘ supersparc ’, ‘ hypersparc ’, ‘ leon ’, ‘ leon3 ’,
‘ leon3v7 ’, ‘ leon5 ’, ‘ f930 ’, ‘ f934 ’, ‘ sparclite86x ’, ‘ tsc701 ’, ‘ ultrasparc ’,
‘ ultrasparc3 ’, ‘ niagara ’, ‘ niagara2 ’, ‘ niagara3 ’, ‘ niagara4 ’, ‘ niagara7 ’ and
‘ m8 ’. With native Solaris and GNU/Linux toolchains, ‘ native ’ can also be used.
-mv8plus
-mno-v8plus
With ‘ -mv8plus ’, GCC generates code for the SPARC-V8 + ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.
-mvis
-mno-vis With ‘ -mvis ’, GCC generates code that takes advantage of the UltraSPARC
Visual Instruction Set extensions. The default is ‘ -mno-vis ’.
-mvis2
-mno-vis2
With ‘ -mvis2 ’, GCC generates code that takes advantage of version 2.0 of the
UltraSPARC Visual Instruction Set extensions. The default is ‘ -mvis2 ’ when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting ‘ -mvis2 ’ also sets ‘ -mvis ’.
-mvis3
-mno-vis3
With ‘ -mvis3 ’, GCC generates code that takes advantage of version 3.0 of the
UltraSPARC Visual Instruction Set extensions. The default is ‘ -mvis3 ’ when targeting a cpu that supports such instructions, such as niagara-3 and later.
Setting ‘ -mvis3 ’ also sets ‘ -mvis2 ’ and ‘ -mvis ’.
-mvis4
-mno-vis4
With ‘ -mvis4 ’, GCC generates code that takes advantage of version 4.0 of the
UltraSPARC Visual Instruction Set extensions. The default is ‘ -mvis4 ’ when targeting a cpu that supports such instructions, such as niagara-7 and later.
Setting ‘ -mvis4 ’ also sets ‘ -mvis3 ’, ‘ -mvis2 ’ and ‘ -mvis ’.
-mvis4b
-mno-vis4b
With ‘ -mvis4b ’, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017. The default is ‘ -mvis4b ’ when targeting a cpu that supports such instructions, such as m8 and later. Setting ‘ -mvis4b ’ also sets ‘ -mvis4 ’, ‘ -mvis3 ’, ‘ -mvis2 ’ and ‘ -mvis ’.
-mcbcond
-mno-cbcond
With ‘ -mcbcond ’, GCC generates code that takes advantage of the
UltraSPARC Compare-and-Branch-on-Condition instructions. The default is
‘ -mcbcond ’ when targeting a CPU that supports such instructions, such as
Niagara-4 and later.
450 Using the GNU Compiler Collection (GCC)
-mfmaf
-mno-fmaf
With ‘ -mfmaf ’, GCC generates code that takes advantage of the UltraSPARC
Fused Multiply-Add Floating-point instructions. The default is ‘ -mfmaf ’ when targeting a CPU that supports such instructions, such as Niagara-3 and later.
-mfsmuld
-mno-fsmuld
With ‘ -mfsmuld ’, GCC generates code that takes advantage of the
Floating-point Multiply Single to Double (FsMULd) instruction. The default is ‘ -mfsmuld ’ when targeting a CPU supporting the architecture versions V8 or V9 with FPU except ‘ -mcpu=leon ’.
-mpopc
-mno-popc
With ‘ -mpopc ’, GCC generates code that takes advantage of the UltraSPARC
Population Count instruction. The default is ‘ -mpopc ’ when targeting a CPU that supports such an instruction, such as Niagara-2 and later.
-msubxc
-mno-subxc
With ‘ -msubxc ’, GCC generates code that takes advantage of the UltraSPARC
Subtract-Extended-with-Carry instruction. The default is ‘ -msubxc ’ when targeting a CPU that supports such an instruction, such as Niagara-7 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).
-mfix-ut699
Enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
-mfix-ut700
Enable the documented workaround for the back-to-back store errata of the
UT699E/UT700 processor.
-mfix-gr712rc
Enable the documented workaround for the back-to-back store errata of the
GR712RC processor.
These ‘ -m ’ options are supported in addition to the above on SPARC-V9 processors in
64-bit environments:
-m32
-m64 Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel= which
Set the code model to one of
Chapter 3: GCC Command Options 451
‘ medlow ’ The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
‘ medmid ’ The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
‘ medany ’ The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
‘ embmedany ’
The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time).
The global register %g4 points to the base of the data segment.
Programs are statically linked and PIC is not supported.
-mmemory-model= mem-model
Set the memory model in force on the processor to one of
‘ default ’ The default memory model for the processor and operating system.
‘ rmo ’ Relaxed Memory Order
‘ pso ’
‘ tso ’
Partial Store Order
Total Store Order
‘ sc ’ Sequential Consistency
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor’s PSTATE.MM
field.
-mstack-bias
-mno-stack-bias
With ‘ -mstack-bias ’, GCC assumes that the stack pointer, and frame pointer if present, are offset by − 2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present.
3.19.51 Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
-G
-Qy
-Qn
Create a shared object. It is recommended that ‘ -symbolic ’ or ‘ -shared ’ be used instead.
Identify the versions of each tool used by the compiler, in a .ident
assembler directive in the output.
Refrain from adding .ident
directives to the output file (this is the default).
452 Using the GNU Compiler Collection (GCC)
-YP, dirs Search the directories dirs , and no others, for libraries specified with ‘ -l ’.
-Ym, dir Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
3.19.52 TILE-Gx Options
These ‘ -m ’ options are supported on the TILE-Gx:
-mcmodel=small
Generate code for the small model. The distance for direct calls is limited to
500M in either direction. PC-relative addresses are 32 bits. Absolute addresses support the full address range.
-mcmodel=large
Generate code for the large model. There is no limitation on call distance, pc-relative addresses, or absolute addresses.
-mcpu= name
Selects the type of CPU to be targeted. Currently the only supported type is
‘ tilegx ’.
-m32
-m64 Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long, and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mbig-endian
-mlittle-endian
Generate code in big/little endian mode, respectively.
3.19.53 TILEPro Options
These ‘ -m ’ options are supported on the TILEPro:
-mcpu= name
Selects the type of CPU to be targeted. Currently the only supported type is
‘ tilepro ’.
-m32 Generate code for a 32-bit environment, which sets int, long, and pointer to 32 bits. This is the only supported behavior so the flag is essentially ignored.
3.19.54 V850 Options
These ‘ -m ’ options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function’s address into a register, and calls indirect through the pointer.
-mno-ep
-mep Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the ep register, and use the shorter sld and sst instructions. The ‘ -mep ’ option is on by default if you optimize.
Chapter 3: GCC Command Options 453
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers.
The ‘ -mprolog-function ’ option is on by default if you optimize.
-mspace Try to make the code as small as possible. At present, this just turns on the
‘ -mep ’ and ‘ -mprolog-function ’ options.
-mtda= n Put static or global variables whose size is n bytes or less into the tiny data area that register ep points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
-msda= n Put static or global variables whose size is n bytes or less into the small data area that register gp points to. The small data area can hold up to 64 kilobytes.
-mzda= n Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
-mv850 Specify that the target processor is the V850.
-mv850e3v5
Specify that the target processor is the V850E3V5. The preprocessor constant
__v850e3v5__ is defined if this option is used.
-mv850e2v4
Specify that the target processor is the V850E3V5. This is an alias for the
‘ -mv850e3v5 ’ option.
-mv850e2v3
Specify that the target processor is the V850E2V3. The preprocessor constant
__v850e2v3__ is defined if this option is used.
-mv850e2 Specify that the target processor is the V850E2. The preprocessor constant
__v850e2__ is defined if this option is used.
-mv850e1 Specify that the target processor is the V850E1. The preprocessor constants
__v850e1__ and __v850e__ are defined if this option is used.
-mv850es Specify that the target processor is the V850ES. This is an alias for the
‘ -mv850e1 ’ option.
-mv850e Specify that the target processor is the V850E. The preprocessor constant
__v850e__ is defined if this option is used.
If neither ‘ -mv850 ’ nor ‘ -mv850e ’ nor ‘ -mv850e1 ’ nor ‘ -mv850e2 ’ nor
‘ -mv850e2v3 ’ nor ‘ -mv850e3v5 ’ are defined then a default target processor is chosen and the relevant ‘ __v850*__ ’ preprocessor constant is defined.
The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the CALLT instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.
454 Using the GNU Compiler Collection (GCC)
This option is enabled by default when the RH850 ABI is in use (see
‘ -mrh850-abi ’), and disabled by default when the GCC ABI is in use.
If
CALLT instructions are being generated then the C preprocessor symbol
__V850_CALLT__ is defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the ‘ -mrelax ’ command-line option to the assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating point instructions.
This option is only significant when the target architecture is ‘ V850E2V3 ’ or higher. If hardware floating point instructions are being generated then the C preprocessor symbol __FPU_OK__ is defined, otherwise the symbol __NO_FPU__ is defined.
-mloop Enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental.
-mrh850-abi
-mghs Enables support for the RH850 version of the V850 ABI. This is the default.
With this version of the ABI the following rules apply:
• Integer sized structures and unions are returned via a memory pointer rather than a register.
• Large structures and unions (more than 8 bytes in size) are passed by value.
• Functions are aligned to 16-bit boundaries.
• The ‘ -m8byte-align ’ command-line option is supported.
• The ‘ -mdisable-callt ’ command-line option is enabled by default. The
‘ -mno-disable-callt ’ command-line option is not supported.
When this version of the ABI is enabled the C preprocessor symbol __V850_
RH850_ABI__ is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:
• Integer sized structures and unions are returned in register r10 .
• Large structures and unions (more than 8 bytes in size) are passed by reference.
• Functions are aligned to 32-bit boundaries, unless optimizing for size.
• The ‘ -m8byte-align ’ command-line option is not supported.
• The ‘ -mdisable-callt ’ command-line option is supported but not enabled by default.
Chapter 3: GCC Command Options 455
When this version of the ABI is enabled the C preprocessor symbol __V850_
GCC_ABI__ is defined.
-m8byte-align
-mno-8byte-align
Enables support for double and long long types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes.
When ‘ -m8byte-align ’ is in effect the C preprocessor symbol __V850_8BYTE_
ALIGN__ is defined.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code generated by the compiler.
This setting is the default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed registers.
3.19.55 VAX Options
These ‘ -m ’ options are defined for the VAX:
-munix Do not output certain jump instructions ( aobleq and so on) that the Unix assembler for the VAX cannot handle across long ranges.
-mgnu
-mg
Do output those jump instructions, on the assumption that the GNU assembler is being used.
Output code for G-format floating-point numbers instead of D-format.
-mlra
-mno-lra Enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload.
3.19.56 Visium Options
-mdebug A program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the libraries libc.a and libdebug.a
to be linked. The program should be run on the target under the control of the
GDB remote debugging stub.
-msim A program which performs file I/O and is destined to run on the simulator should be linked with option. This causes libraries libc.a and libsim.a to be linked.
-mfpu
-mhard-float
Generate code containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate code containing library calls for floating-point.
456 Using the GNU Compiler Collection (GCC)
‘ -msoft-float ’ changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile ‘ libgcc.a
’, the library that comes with GCC, with
‘ -msoft-float ’ in order for this to work.
-mcpu= cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu type . Supported values for cpu type are ‘ mcm ’, ‘ gr5 ’ and
‘ gr6 ’.
‘ mcm ’ is a synonym of ‘ gr5 ’ present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.
With ‘ -mcpu=gr6 ’, GCC generates code for the GR6 variant of the Visium architecture.
The only difference from GR5 code is that the compiler will generate block move instructions.
-mtune= cpu_type
Set the instruction scheduling parameters for machine type cpu type , but do not set the instruction set or register set that the option ‘ -mcpu= cpu_type ’ would.
-msv-mode
Generate code for the supervisor mode, where there are no restrictions on the access to general registers. This is the default.
-muser-mode
Generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected.
3.19.57 VMS Options
These ‘ -m ’ options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from main . The default is to return POSIX-style condition (e.g. error) codes.
-mdebug-main= prefix
Flag the first routine whose name starts with prefix as the main routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
-mpointer-size= size
Set the default size of pointers. Possible options for size are ‘ 32 ’ or ‘ short ’ for
32 bit pointers, ‘ 64 ’ or ‘ long ’ for 64 bit pointers, and ‘ no ’ for supporting only
32 bit pointers. The later option disables pragma pointer_size .
Chapter 3: GCC Command Options 457
3.19.58 VxWorks Options
The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target.
-mrtp GCC can generate code for both VxWorks kernels and real time processes
(RTPs). This option switches from the former to the latter. It also defines the preprocessor macro __RTP__ .
-non-static
Link an RTP executable against shared libraries rather than static libraries.
The options ‘ -static ’ and ‘ -shared ’ can also be used for RTPs (see
-static ’ is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to ‘ -Wl,-z,now ’ and is defined for compatibility with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.
3.19.59 x86 Options
These ‘ -m ’ options are defined for the x86 family of computers.
-march= cpu-type
Generate instructions for the machine type cpu-type .
In contrast to
‘ -mtune= cpu-type ’, which merely tunes the generated code for the specified cpu-type , ‘ -march= cpu-type ’ allows GCC to generate code that may not run at all on processors other than the one indicated.
Specifying
‘ -march= cpu-type ’ implies ‘ -mtune= cpu-type ’, except where noted otherwise.
The choices for cpu-type are:
‘ native ’ This selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using
‘ -march=native ’ enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using ‘ -mtune=native ’ produces code optimized for the local machine under the constraints of the selected instruction set.
‘ x86-64 ’ A generic CPU with 64-bit extensions.
‘ x86-64-v2 ’
‘ x86-64-v3 ’
‘ x86-64-v4 ’
These choices for cpu-type select the corresponding microarchitecture level from the x86-64 psABI. On ABIs other than the
458 Using the GNU Compiler Collection (GCC) x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.
Since these cpu-type values do not have a corresponding ‘ -mtune ’ setting, using ‘ -march ’ with these values enables generic tuning.
Specific tuning can be enabled using the ‘ -mtune= other-cpu-type ’ option with an appropriate other-cpu-type value.
Original Intel i386 CPU.
Intel i486 CPU. (No scheduling is implemented for this chip.)
‘ i386 ’
‘ i486 ’
‘ i586 ’
‘ pentium ’ Intel Pentium CPU with no MMX support.
‘ lakemont ’
Intel Lakemont MCU, based on Intel Pentium CPU.
‘ pentium-mmx ’
Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
‘ pentiumpro ’
Intel Pentium Pro CPU.
‘ i686 ’ When used with ‘ -march ’, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with ‘ -mtune ’, it has the same meaning as ‘ generic ’.
‘ pentium2 ’
Intel Pentium II CPU, based on Pentium Pro core with MMX instruction set support.
‘ pentium3 ’
‘ pentium3m ’
Intel Pentium III CPU, based on Pentium Pro core with MMX and
SSE instruction set support.
‘ pentium-m ’
Intel Pentium M; low-power version of Intel Pentium III CPU with
MMX, SSE and SSE2 instruction set support. Used by Centrino notebooks.
‘ pentium4 ’
‘ pentium4m ’
Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support.
‘ prescott ’
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support.
‘ nocona ’ Improved version of Intel Pentium 4 CPU with 64-bit extensions,
MMX, SSE, SSE2 and SSE3 instruction set support.
Chapter 3: GCC Command Options 459
‘ core2 ’ Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
‘ nehalem ’ Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2 and POPCNT instruction set support.
‘ westmere ’
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES and PCLMUL instruction set support.
‘ sandybridge ’
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES and
PCLMUL instruction set support.
‘ ivybridge ’
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2,
SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AES, PCLMUL,
FSGSBASE, RDRND and F16C instruction set support.
‘ haswell ’ Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2,
AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2 and
F16C instruction set support.
‘ broadwell ’
Intel Broadwell CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2,
AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED ADCX and PREFETCHW instruction set support.
‘ skylake ’ Intel Skylake CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2,
AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, CLFLUSHOPT, XSAVEC and
XSAVES instruction set support.
‘ bonnell ’ Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE,
SSE2, SSE3 and SSSE3 instruction set support.
‘ silvermont ’
Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL and RDRND instruction set support.
‘ goldmont ’
Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC,
XSAVES, XSAVEOPT and FSGSBASE instruction set support.
460 Using the GNU Compiler Collection (GCC)
‘ goldmont-plus ’
Intel Goldmont Plus CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC,
XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX and UMIP instruction set support.
‘
‘ tremont ’ Intel Tremont CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC,
XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID,
SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B,
CLDEMOTE and WAITPKG instruction set support.
‘ knl ’ knm ’
Intel Knight’s Landing CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX,
AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2,
F16C, RDSEED, ADCX, PREFETCHW, PREFETCHWT1,
AVX512F, AVX512PF, AVX512ER and AVX512CD instruction set support.
Intel Knights Mill CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AVX, AVX2,
AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI, BMI2, F16C,
RDSEED, ADCX, PREFETCHW, PREFETCHWT1, AVX512F,
AVX512PF, AVX512ER, AVX512CD, AVX5124VNNIW,
AVX5124FMAPS and AVX512VPOPCNTDQ instruction set support.
‘ skylake-avx512 ’
Intel Skylake Server CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW,
AVX512DQ and AVX512CD instruction set support.
‘ cannonlake ’
Intel Cannonlake Server CPU with 64-bit extensions, MOVBE,
MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
PKU, AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND,
FMA, BMI, BMI2, F16C, RDSEED, ADCX, PREFETCHW,
CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL,
AVX512BW, AVX512DQ, AVX512CD, AVX512VBMI,
AVX512IFMA, SHA and UMIP instruction set support.
‘ icelake-client ’
Intel Icelake Client CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
Chapter 3: GCC Command Options 461
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW,
AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA,
SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2,
AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI,
VPCLMULQDQ, VAES instruction set support.
‘ icelake-server ’
Intel Icelake Server CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW,
AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA,
SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2,
AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI,
VPCLMULQDQ, VAES, PCONFIG and WBNOINVD instruction set support.
‘ cascadelake ’
Intel Cascadelake CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW,
AVX512DQ, AVX512CD and AVX512VNNI instruction set support.
‘ cooperlake ’
Intel cooperlake CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW,
AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.
‘ tigerlake ’
Intel Tigerlake CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW,
AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA,
SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2,
AVX512VPOPCNTDQ, AVX512BITALG, AVX512VNNI,
VPCLMULQDQ, VAES, PCONFIG, WBNOINVD, MOVDIRI,
MOVDIR64B, AVX512VP2INTERSECT and KEYLOCKER instruction set support.
462 Using the GNU Compiler Collection (GCC)
‘ sapphirerapids ’
Intel sapphirerapids CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, CLWB, AVX512VL, AVX512BW,
AVX512DQ, AVX512CD, AVX512VNNI, AVX512BF16,
MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, EN-
QCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE,
TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8 and
AVX-VNNI instruction set support.
‘ alderlake ’
Intel Alderlake CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC,
XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID,
SGX, UMIP, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B,
CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2,
F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ,
SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support.
‘ rocketlake ’
Intel Rocketlake CPU with 64-bit extensions, MOVBE, MMX,
SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, PKU,
AVX, AVX2, AES, PCLMUL, FSGSBASE, RDRND, FMA, BMI,
BMI2, F16C, RDSEED, ADCX, PREFETCHW, CLFLUSHOPT,
XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW,
AVX512DQ, AVX512CD, AVX512VBMI, AVX512IFMA,
SHA, CLWB, UMIP, RDPID, GFNI, AVX512VBMI2,
AVX512VPOPCNTDQ, AVX512BITALG,
VPCLMULQDQ, VAES instruction set support.
AVX512VNNI,
‘ k6 ’
‘ k6-2 ’
‘ k6-3 ’
AMD K6 CPU with MMX instruction set support.
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
‘ athlon ’
‘ athlon-tbird ’
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and
SSE prefetch instructions support.
‘ athlon-4 ’
‘ athlon-xp ’
‘ athlon-mp ’
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced
3DNow! and full SSE instruction set support.
Chapter 3: GCC Command Options 463
‘ k8 ’
‘ opteron ’
‘ athlon64 ’
‘ athlon-fx ’
Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced
3DNow! and 64-bit instruction set extensions.)
‘ k8-sse3 ’
‘ opteron-sse3 ’
‘ athlon64-sse3 ’
Improved versions of AMD K8 cores with SSE3 instruction set support.
‘ amdfam10 ’
‘ barcelona ’
CPUs based on AMD Family 10h cores with x86-64 instruction set support.
(This supersets MMX, SSE, SSE2, SSE3, SSE4A,
3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
‘ bdver1 ’ CPUs based on AMD Family 15h cores with x86-64 instruction set support.
(This supersets FMA4, AVX, XOP, LWP, AES,
PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3,
SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
‘ bdver2 ’ AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP,
LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A,
SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
‘ bdver3 ’ AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FS-
GSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE,
SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
‘ bdver4 ’ AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4,
FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16,
MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1,
SSE4.2, ABM and 64-bit instruction set extensions.)
‘ znver1 ’ AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE,
AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO,
AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES,
CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)
464 Using the GNU Compiler Collection (GCC)
‘ znver2 ’ AMD Family 17h core based CPUs with x86-64 instruction set support.
(This supersets BMI, BMI2, CLWB, F16C, FMA,
FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA,
CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2,
SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES,
CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.)
‘ znver3 ’ AMD Family 19h core based CPUs with x86-64 instruction set support.
(This supersets BMI, BMI2, CLWB, F16C, FMA,
FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA,
CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2,
SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES,
CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU,
VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)
‘ btver1 ’ CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A,
CX16, ABM and 64-bit instruction set extensions.)
‘ btver2 ’ CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL,
AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2,
SSE, MMX and 64-bit instruction set extensions.
‘ winchip-c6 ’
IDT WinChip C6 CPU, dealt in same way as i486 with additional
MMX instruction set support.
‘ winchip2 ’
IDT WinChip 2 CPU, dealt in same way as i486 with additional
MMX and 3DNow! instruction set support.
‘ c3 ’ VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
‘
‘ c3-2 c7 ’
’ VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
‘ samuel-2 ’
VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
‘ nehemiah ’
VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
‘ esther ’ VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
Chapter 3: GCC Command Options 465
‘ eden-x2 ’ VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
‘ eden-x4 ’ VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3,
SSE4.1, SSE4.2, AVX and AVX2 instruction set support.
(No scheduling is implemented for this chip.)
‘ nano ’ Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and
SSSE3 instruction set support. (No scheduling is implemented for this chip.)
‘ nano-1000 ’
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and
SSSE3 instruction set support. (No scheduling is implemented for this chip.)
‘ nano-2000 ’
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and
SSSE3 instruction set support. (No scheduling is implemented for this chip.)
‘ nano-3000 ’
VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
‘ nano-x2 ’ VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
‘ nano-x4 ’ VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3,
SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
‘ geode ’ AMD Geode embedded processor with MMX and 3DNow! instruction set support.
-mtune= cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. While picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a ‘ -march= cpu-type ’ option. For example, if GCC is configured for i686-pclinux-gnu then ‘ -mtune=pentium4 ’ generates code that is tuned for Pentium 4 but still runs on i686 machines.
The choices for cpu-type are the same as for ‘ -march ’. In addition, ‘ -mtune ’ supports 2 extra choices for cpu-type :
‘ generic ’ Produce code optimized for the most common IA32/AMD64/
EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding ‘ -mtune ’ or ‘ -march ’ option instead of ‘ -mtune=generic ’.
But, if you do not know
466 Using the GNU Compiler Collection (GCC)
‘ intel ’ exactly what CPU users of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released.
There is no ‘ -march=generic ’ option because ‘ -march ’ indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, ‘ -mtune ’ indicates the processor (or, in this case, collection of processors) for which the code is optimized.
Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding ‘ -mtune ’ or ‘ -march ’ option instead of ‘ -mtune=intel ’.
But, if you want your application performs better on both Haswell and Silvermont, then you should use this option.
As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.
There is no ‘ -march=intel ’ option because ‘ -march ’ indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors.
In contrast, ‘ -mtune ’ indicates the processor (or, in this case, collection of processors) for which the code is optimized.
-mcpu= cpu-type
A deprecated synonym for ‘ -mtune ’.
-mfpmath= unit
Generate floating-point arithmetic for selected unit unit . The choices for unit are:
‘ 387 ’ Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See ‘ -ffloat-store ’ for more detailed description.
This is the default choice for non-Darwin x86-32 targets.
‘ sse ’ Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only
Chapter 3: GCC Command Options 467 single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.
For the x86-32 compiler, you must use ‘ -march= cpu-type ’, ‘ -msse ’ or ‘ -msse2 ’ switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default.
The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when ‘ -ffast-math ’ is enabled.
‘ sse,387 ’
‘ sse+387 ’
‘ both ’ Attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too.
Use this option with care, as it is still experimental, because the
GCC register allocator does not model separate functional units well, resulting in unstable performance.
-masm= dialect
Output assembly instructions using selected dialect . Also affects which dialect is used for basic asm (see
Section 6.47.1 [Basic Asm], page 648
) and extended asm
(see
Section 6.47.2 [Extended Asm], page 649 ). Supported choices (in dialect
order) are ‘ att ’ or ‘ intel ’. The default is ‘ att ’. Darwin does not support
‘ intel ’.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point comparisons.
These correctly handle the case where the result of a comparison is unordered.
-m80387
-mhard-float
Generate output containing 80387 instructions for floating point.
-mno-80387
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine’s usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.
468 Using the GNU Compiler Collection (GCC)
On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if ‘ -msoft-float ’ is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.
The option ‘ -mno-fp-ret-in-387 ’ causes such values to be returned in ordinary
CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the sin , cos and sqrt instructions for the
387. Specify this option to avoid generating those instructions. This option is overridden when ‘ -march ’ indicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the ‘ -funsafe-math-optimizations ’ switch.
-malign-double
-mno-align-double
Control whether GCC aligns double , long double , and long long variables on a two-word boundary or a one-word boundary. Aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory.
On x86-64, ‘ -malign-double ’ is enabled by default.
Warning: if you use the ‘ -malign-double ’ switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of long double type. The x86-32 application binary interface specifies the size to be 96 bits, so ‘ -m96bit-long-double ’ is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying ‘ -m128bit-long-double ’ aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero.
In the x86-64 compiler, ‘ -m128bit-long-double ’ is the default choice as its
ABI specifies that long double is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double .
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as
Chapter 3: GCC Command Options 469 modifying the function calling convention for functions taking long double .
Hence they are not binary-compatible with code compiled without that switch.
-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default for 32-bit
Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 64-bit Bionic C library.
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double .
Hence they are not binary-compatible with code compiled without that switch.
-malign-data= type
Control how GCC aligns variables. Supported values for type are ‘ compat ’ uses increased alignment value compatible uses GCC 4.8 and earlier, ‘ abi ’ uses alignment value as specified by the psABI, and ‘ cacheline ’ uses increased alignment value to match the cache line size. ‘ compat ’ is the default.
-mlarge-data-threshold= threshold
When ‘ -mcmodel=medium ’ is specified, data objects larger than threshold are placed in the large data section. This value must be the same across all objects linked into the binary, and defaults to 65535.
-mrtd Use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function attribute stdcall . You can also override the ‘ -mrtd ’ option by using the function attribute cdecl . See
Section 6.33 [Function Attributes], page 548
.
Warning: this calling convention is incompatible with the one normally used on
Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf ); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
-mregparm= num
Control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm . See
Section 6.33 [Function Attributes], page 548
.
470 Using the GNU Compiler Collection (GCC)
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm . See
Section 6.33 [Function Attributes], page 548 .
Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version
12.
Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When ‘ -mpc32 ’ is specified, the significands of results of floating-point operations are rounded to 24 bits
(single precision); ‘ -mpc64 ’ rounds the significands of results of floating-point operations to 53 bits (double precision) and ‘ -mpc80 ’ rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called “catastrophic cancellation”, when this option is used to set the precision to less than extended precision.
-mstackrealign
Realign the stack at entry. On the x86, the ‘ -mstackrealign ’ option generates an alternate prologue and epilogue that realigns the run-time stack if necessary.
This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer , applicable to individual functions.
-mpreferred-stack-boundary= num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary.
If ‘ -mpreferred-stack-boundary ’ is not specified, the default is 4 (16 bytes or
128 bits).
Warning: When generating code for the x86-64 architecture with SSE extensions disabled, ‘ -mpreferred-stack-boundary=3 ’ can be used to keep the stack
Chapter 3: GCC Command Options 471 boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and int128), leading to wrong results. You must build all modules with ‘ -mpreferred-stack-boundary=3 ’, including any libraries. This includes the system libraries and startup modules.
-mincoming-stack-boundary= num
Assume the incoming stack is aligned to a 2 raised to num byte boundary.
If ‘ -mincoming-stack-boundary ’ is not specified, the one specified by
‘ -mpreferred-stack-boundary ’ is used.
On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see ‘ -malign-double ’) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension
(SSE) data type __m128 may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting.
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to
‘ -mpreferred-stack-boundary=2 ’.
472
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mavx512bf16
-mavx512fp16
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
Using the GNU Compiler Collection (GCC)
Chapter 3: GCC Command Options 473
-mmovdir64b
-menqcmd
-muintr
-mtsxldtrk
-mavx512vpopcntdq
-mavx512vp2intersect
-mavx5124fmaps
-mavx512vnni
-mavxvnni
-mavx5124vnniw
-mcldemote
-mserialize
-mamx-tile
-mamx-int8
-mamx-bf16
-mhreset
-mkl
-mwidekl These switches enable the use of instructions in the MMX, SSE, SSE2,
SSE3, SSSE3, SSE4, SSE4A, SSE4.1, SSE4.2, AVX, AVX2, AVX512F,
AVX512PF, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ,
AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT,
CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG,
WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, RDSEED,
SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI,
BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM,
HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES,
WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B,
AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX5124FMAPS,
AVX512VNNI, AVX5124VNNIW, SERIALIZE, UINTR, HRESET,
AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512FP16 or CLDEMOTE extended instruction sets. Each has a corresponding ‘ -mno’ option to disable use of these instructions.
These extensions are also available as built-in functions: see
Built-in Functions], page 839 , for details of the functions enabled and disabled
by these switches.
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see ‘ -mfpmath=sse ’.
GCC depresses SSEx instructions when ‘ -mavx ’ is used.
Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed.
These options enable GCC to use these extended instructions in generated code, even without ‘ -mfpmath=sse ’. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options.
474 Using the GNU Compiler Collection (GCC)
-mdump-tune-features
This option instructs GCC to dump the names of the x86 performance tuning features and default settings.
‘ -mtune-ctrl= feature-list ’.
The names can be used in
-mtune-ctrl= feature-list
This option is used to do fine grain control of x86 code generation features.
feature-list is a comma separated list of feature names. See also
‘ -mdump-tune-features ’. When specified, the feature is turned on if it is not preceded with ‘ ^ ’, otherwise, it is turned off. ‘ -mtune-ctrl= feature-list ’ is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors.
-mno-default
This option instructs GCC to turn off all tunable features.
See also
‘ -mtune-ctrl= feature-list ’ and ‘ -mdump-tune-features ’.
-mcld This option instructs GCC to emit a cld instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the
DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the ‘ --enable-cld ’ configure option. Generation of cld instructions can be suppressed with the
‘ -mno-cld ’ compiler option in this case.
-mvzeroupper
This option instructs GCC to emit a vzeroupper instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary zeroupper intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead of 256-bit
AVX instructions in the auto-vectorizer.
-mprefer-vector-width= opt
This option instructs GCC to use opt -bit vector width in instructions instead of default on the selected platform.
-mmove-max= bits
This option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits . The valid bits are 128, 256 and
512.
-mstore-max= bits
This option instructs GCC to set the maximum number of bits can be stored to memory efficiently to bits . The valid bits are 128, 256 and 512.
‘ none ’ No extra limitations applied to GCC other than defined by the selected platform.
Chapter 3: GCC Command Options 475
‘ 128 ’
‘ 256 ’
Prefer 128-bit vector width for instructions.
Prefer 256-bit vector width for instructions.
-mcx16
‘ 512 ’ Prefer 512-bit vector width for instructions.
This option enables GCC to generate CMPXCHG16B instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement
However, for
Section 6.55 [ atomic Builtins], page 713
operating on 128-bit integers, a library call is always used.
-msahf This option enables generation of SAHF instructions in 64-bit code. Early Intel
Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium
4 G1 step in December 2005, lacked the LAHF and SAHF instructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize fmod , drem , and remainder built-in functions; see
Section 6.59 [Other Builtins], page 722
for details.
-mmovbe This option enables use of the movbe instruction to implement __builtin_ bswap32 and __builtin_bswap64 .
-mshstk The ‘ -mshstk ’ option enables shadow stack built-in functions from x86 Controlflow Enforcement Technology (CET).
-mcrc32 This option enables built-in functions __builtin_ia32_crc32qi , __builtin_ ia32_crc32hi , __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction.
-mmwait This option enables built-in functions __builtin_ia32_monitor , and
__builtin_ia32_mwait to generate the monitor and mwait machine instructions.
-mrecip This option enables use of RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS ) with an additional Newton-Raphson step to increase precision instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when ‘ -funsafe-math-optimizations ’ is enabled together with ‘ -ffinite-math-only ’ and ‘ -fno-trapping-math ’. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e.
the inverse of 1.0 equals 0.99999994).
Note that GCC implements 1.0f/sqrtf( x ) in terms of RSQRTSS (or RSQRTPS ) already with ‘ -ffast-math ’ (or the above option combination), and doesn’t need ‘ -mrecip ’.
Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf( x ) already with
‘ -ffast-math ’ (or the above option combination), and doesn’t need ‘ -mrecip ’.
476 Using the GNU Compiler Collection (GCC)
-mrecip= opt
This option controls which reciprocal estimate instructions may be used.
opt is a comma-separated list of options, which may be preceded by a ‘ !
’ to invert the option:
‘ all ’ Enable all estimate instructions.
‘ default ’ Enable the default instructions, equivalent to ‘ -mrecip ’.
‘ none ’
‘ div ’
Disable all estimate instructions, equivalent to ‘ -mno-recip ’.
Enable the approximation for scalar division.
‘ vec-div ’ Enable the approximation for vectorized division.
‘ sqrt ’ Enable the approximation for scalar square root.
‘ vec-sqrt ’
Enable the approximation for vectorized square root.
So, for example, ‘ -mrecip=all,!sqrt
’ enables all of the reciprocal approximations, except for square root.
-mveclibabi= type
Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are ‘ svml ’ for the Intel short vector math library and ‘ acml ’ for the AMD math core library. To use this option, both
‘ -ftree-vectorize ’ and ‘ -funsafe-math-optimizations ’ have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time.
GCC currently emits calls to vmldExp2 , vmldLn2 , vmldLog102 , vmldPow2 , vmldTanh2 , vmldTan2 , vmldAtan2 , vmldAtanh2 , vmldCbrt2 , vmldSinh2 , vmldSin2 , vmldAsinh2 , vmldAsin2 , vmldCosh2 , vmldCos2 , vmldAcosh2 , vmldAcos2 , vmlsExp4 , vmlsLn4 , vmlsLog104 , vmlsPow4 , vmlsTanh4 , vmlsTan4 , vmlsAtan4 , vmlsAtanh4 , vmlsCbrt4 , vmlsSinh4 , vmlsSin4 , vmlsAsinh4 , vmlsAsin4 , vmlsCosh4 , vmlsCos4 , vmlsAcosh4 and vmlsAcos4 for corresponding function type when ‘ -mveclibabi=svml ’ is used, and __vrd2_sin , __vrd2_cos , __vrd2_exp , __vrd2_log , __vrd2_log2 ,
__vrd2_log10 , __vrs4_sinf , __vrs4_cosf , __vrs4_expf , __vrs4_logf ,
__vrs4_log2f , __vrs4_log10f and __vrs4_powf for the corresponding function type when ‘ -mveclibabi=acml ’ is used.
-mabi= name
Generate code for the specified calling convention. Permissible values are ‘ sysv ’ for the ABI used on GNU/Linux and other systems, and ‘ ms ’ for the Microsoft
ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi . See
Section 6.33 [Function Attributes], page 548
.
-mforce-indirect-call
Force all calls to functions to be indirect.
This is useful when using Intel
Processor Trace where it generates more precise timing information for function calls.
Chapter 3: GCC Command Options 477
-mmanual-endbr
Insert ENDBR instruction at function entry only via the cf_check function attribute. This is useful when used with the option ‘ -fcf-protection=branch ’ to control ENDBR insertion at the function entry.
-mcall-ms2sysv-xlogues
Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, resulting in fairly lengthy prologues and epilogues. Using ‘ -mcall-ms2sysv-xlogues ’ emits prologues and epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing function size at the cost of a few extra instructions.
-mtls-dialect= type
Generate code to access thread-local storage using the ‘ gnu ’ or ‘ gnu2 ’ conventions. ‘ gnu ’ is the conservative default; ‘ gnu2 ’ is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies ‘ -mno-push-args ’.
-mthreads
Support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the
‘ -mthreads ’ option.
When compiling, ‘ -mthreads ’ defines ‘ -D_MT ’; when linking, it links in a special thread helper library ‘ -lmingwthrd ’ which cleans up per-thread exception-handling data.
-mms-bitfields
-mno-ms-bitfields
Enable/disable bit-field layout compatible with the native Microsoft Windows compiler.
If packed is used on a structure, or if bit-fields are used, it may be that the
Microsoft ABI lays out the structure differently than the way GCC normally does. Particularly when moving packed data between functions compiled with
GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.
478 Using the GNU Compiler Collection (GCC)
This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see
Section 6.34.15 [x86 Variable Attributes], page 628
and
Section 6.35.7 [x86 Type Attributes], page 637
.
The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing. The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determine by these rules:
1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the aligned attribute or the pack pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that: offset % alignment_requirement == 0
3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.
MSVC interprets zero-length bit-fields in the following ways:
1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.
For example: struct
{ unsigned long bf_1 : 12; unsigned long : 0; unsigned long bf_2 : 12;
} t1;
The size of t1 is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, t1 ’s size would be 4 bytes.
2. If a zero-length bit-field is inserted after a bit-field, foo , and the alignment of the zero-length bit-field is greater than the member that follows it, bar , bar is aligned as the type of the zero-length bit-field.
For example: struct
{ char foo : 4; short : 0; char bar;
} t2; struct
{
Chapter 3: GCC Command Options 479 char foo : 4; short : 0; double bar;
} t3;
For t2 , bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3 , the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.
Taking this into account, it is important to note the following:
1. If a zero-length bit-field follows a normal bit-field, the type of the zerolength bit-field may affect the alignment of the structure as whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short.
2. Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure: struct
{ char foo : 6; long : 0;
} t4;
Here, t4 takes up 4 bytes.
3. Zero-length bit-fields following non-bit-field members are ignored: struct
{ char foo; long : 0; char bar;
} t5;
Here, t5 takes up 2 bytes.
-mno-align-stringops
Do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn’t know about it.
-minline-all-stringops
By default GCC inlines string operations only when the destination is known to be aligned to least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks.
-mstringop-strategy= alg
Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
480 Using the GNU Compiler Collection (GCC)
‘ rep_byte ’
‘ rep_4byte ’
‘ rep_8byte ’
Expand using i386 rep prefix of the specified size.
‘ byte_loop ’
‘ loop ’
‘ unrolled_loop ’
Expand into an inline loop.
‘ libcall ’ Always use a library call.
-mmemcpy-strategy= strategy
Override the internal decision heuristic to decide if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known.
strategy is a comma-separated list of alg : max size : dest align triplets.
alg is specified in ‘ -mstringop-strategy ’, max size specifies the max byte size with which inline algorithm alg is allowed.
For the last triplet, the max size must be -1 . The max size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is
0 for the first triplet and max_size + 1 of the preceding range.
-mmemset-strategy= strategy
The option is similar to ‘ -mmemcpy-strategy= ’ except that it is to control __ builtin_memset expansion.
-momit-leaf-frame-pointer
Don’t keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option ‘ -fomit-leaf-frame-pointer ’ removes the frame pointer for leaf functions, which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register ( %gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX prefix.
The option ‘ -mavx ’ turns this on by default.
-mfentry
-mno-fentry
If profiling is active (‘ -pg ’), put the profiling counter call before the prologue.
Note: On x86 architectures the attribute ms_hook_prologue isn’t possible at the moment for ‘ -mfentry ’ and ‘ -pg ’.
Chapter 3: GCC Command Options 481
-mrecord-mcount
-mno-record-mcount
If profiling is active (‘ -pg ’), generate a mcount loc section that contains pointers to each profiling call. This is useful for automatically patching and out calls.
-mnop-mcount
-mno-nop-mcount
If profiling is active (‘ -pg ’), generate the calls to the profiling functions as NOPs.
This is useful when they should be patched in later dynamically. This is likely only useful together with ‘ -mrecord-mcount ’.
-minstrument-return= type
Instrument function exit in -pg -mfentry instrumented functions with call to specified function. This only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types are none to not instrument, call to generate a call to return , or nop5 to generate a 5 byte nop.
-mrecord-return
-mno-record-return
Generate a return loc section pointing to all return instrumentation code.
-mfentry-name= name
Set name of fentry symbol called at function entry for -pg -mfentry functions.
-mfentry-section= name
Set name of section to record -mrecord-mcount calls (default mcount loc).
-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with SSE extensions disabled,
‘ -mskip-rax-setup ’ can be used to skip setting up RAX register when there are no variable arguments passed in vector registers.
Warning: Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location.
GCC 4.4 or newer don’t have those issues, regardless the RAX register value.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
482 Using the GNU Compiler Collection (GCC)
-mstack-protector-guard= guard
-mstack-protector-guard-reg= reg
-mstack-protector-guard-offset= offset
Generate stack protection code using canary at guard . Supported locations are ‘ global ’ for global canary or ‘ tls ’ for per-thread canary in the TLS block
(the default).
This option has effect only when ‘ -fstack-protector ’ or
‘ -fstack-protector-all ’ is specified.
With the latter choice the options ‘ -mstack-protector-guard-reg= reg ’ and
‘ -mstack-protector-guard-offset= offset ’ furthermore specify which segment register ( %fs or %gs ) to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI.
-mgeneral-regs-only
Generate code that uses only the general-purpose registers. This prevents the compiler from using floating-point, vector, mask and bound registers.
-mrelax-cmpxchg-loop
Relax cmpxchg loop by emitting an early load and compare before cmpxchg, execute pause if load value is not expected. This reduces excessive cachline bouncing when and works for all atomic logic fetch builtins that generates compare and swap loop.
-mindirect-branch= choice
Convert indirect call and jump with choice . The default is ‘ keep ’, which keeps indirect call and jump unmodified. ‘ thunk ’ converts indirect call and jump to call and return thunk. ‘ thunk-inline ’ converts indirect call and jump to inlined call and return thunk. ‘ thunk-extern ’ converts indirect call and jump to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute indirect_branch . See
Section 6.33 [Function Attributes], page 548 .
Note that ‘ -mcmodel=large ’ is incompatible with ‘ -mindirect-branch=thunk ’ and ‘ -mindirect-branch=thunk-extern ’ since the thunk function may not be reachable in the large code model.
Note that ‘ -mindirect-branch=thunk-extern ’ is compatible with
‘ -fcf-protection=branch ’ since the external thunk can be made to enable control-flow check.
-mfunction-return= choice
Convert function return with choice . The default is ‘ keep ’, which keeps function return unmodified. ‘ thunk ’ converts function return to call and return thunk.
‘ thunk-inline ’ converts function return to inlined call and return thunk. ‘ thunk-extern ’ converts function return to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute function_return .
See
Section 6.33 [Function Attributes], page 548
.
Note that ‘ -mindirect-return=thunk-extern ’ is compatible with
‘ -fcf-protection=branch ’ since the external thunk can be made to enable control-flow check.
Chapter 3: GCC Command Options 483
Note that ‘ -mcmodel=large ’ is incompatible with ‘ -mfunction-return=thunk ’ and ‘ -mfunction-return=thunk-extern ’ since the thunk function may not be reachable in the large code model.
-mindirect-branch-register
Force indirect call and jump via register.
-mharden-sls= choice
Generate code to mitigate against straight line speculation (SLS) with choice .
The default is ‘ none ’ which disables all SLS hardening. ‘ return ’ enables SLS hardening for function returns. ‘ indirect-jmp ’ enables SLS hardening for indirect jumps. ‘ all ’ enables all SLS hardening.
-mindirect-branch-cs-prefix
Add CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers so that the call and jmp instruction length is 6 bytes to allow them to be replaced with ‘ lfence; call *%r8-r15 ’ or ‘ lfence; jmp *%r8-r15 ’ at run-time.
These ‘ -m ’ switches are supported in addition to the above on x86-64 processors in 64-bit environments.
-m32
-m64
-mx32
-m16
-miamcu Generate code for a 16-bit, 32-bit or 64-bit environment. The ‘ -m32 ’ option sets int , long , and pointer types to 32 bits, and generates code that runs on any i386 system.
The ‘ -m64 ’ option sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only the ‘ -m64 ’ option also turns off the ‘ -fno-pic ’ and ‘ -mdynamic-no-pic ’ options.
The ‘ -mx32 ’ option sets int , long , and pointer types to 32 bits, and generates code for the x86-64 architecture.
The ‘ -m16 ’ option is the same as ‘ -m32 ’, except for that it outputs the
.code16gcc
assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.
The ‘ -miamcu ’ option generates code which conforms to Intel MCU psABI. It requires the ‘ -m32 ’ option to be turned on.
-mno-red-zone
Do not use a so-called “red zone” for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag ‘ -mno-red-zone ’ disables this red zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model.
484 Using the GNU Compiler Collection (GCC)
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than ‘ -mlarge-data-threshold ’ are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no assumptions about addresses and sizes of sections.
-maddress-mode=long
Generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments.
-mneeded
-mno-needed
Emit GNU PROPERTY X86 ISA 1 NEEDED GNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary.
3.19.60 x86 Windows Options
These additional options are available for Microsoft Windows targets:
-mconsole
-mdll
This option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications.
This option is available for Cygwin and MinGW targets and is enabled by default on those targets.
This option is available for Cygwin and MinGW targets. It specifies that a
DLL—a dynamic link library—is to be generated, enabling the selection of the required runtime startup object and entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies that the dllimport attribute should be ignored.
-mthreads
This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used.
-municode
This option is available for MinGW-w64 targets. It causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code.
Chapter 3: GCC Command Options 485
-mwin32 This option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn’t set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn’t available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets.
It specifies that relocated-data in read-only section is put into the .data
section. This is a necessary for older runtimes not supporting modification of .rdata
sections for pseudo-relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies that the
GNU extension to the PE file format that permits the correct alignment of
COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature.
See also under
Section 3.19.59 [x86 Options], page 457
for standard options.
3.19.61 Xstormy16 Options
These options are defined for Xstormy16:
-msim Choose startup files and linker script suitable for the simulator.
3.19.62 Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of CONST16 instructions for loading constant values. The
CONST16 instruction is currently not a standard option from Tensilica. When enabled, CONST16 instructions are always used in place of the standard L32R instructions. The use of CONST16 is enabled by default only if the L32R instruction is not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option.
This has no effect if the floating-point option
486 Using the GNU Compiler Collection (GCC) is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict
IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler’s ability to combine multiply and add/subtract operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency.
The default is
‘ -mserialize-volatile ’. Use ‘ -mno-serialize-volatile ’ to omit the MEMW instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code.
-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools.
The default is
‘ -mno-text-section-literals ’, which places literals in a separate section in the output file.
This allows the literal pool to be placed in a data
RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With
‘ -mtext-section-literals ’, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function.
-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools.
The default is
‘ -mno-auto-litpools ’, which places literals in a separate section in the output file unless ‘ -mtext-section-literals ’ is used. With ‘ -mauto-litpools ’ the literals are interspersed in the text section by the assembler. Compiler does not produce explicit .literal
directives and loads literals into registers with
MOVI instructions instead of L32R to let the assembler do relaxation and place literals as necessary.
This option allows assembler to create several literal pools per function and assemble very big functions, which may not be possible with ‘ -mtext-section-literals ’.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The
Chapter 3: GCC Command Options 487 assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is
‘ -mtarget-align ’. These options do not affect the treatment of auto-aligned instructions like LOOP , which the assembler always aligns, either by widening density instructions or by inserting NOP instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct CALL instruction into an L32R followed by a CALLX instruction. The default is ‘ -mno-longcalls ’. This option should be used in programs where the call target can potentially be out of range. This option is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions—look at the disassembled object code to see the actual instructions. Note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range.
-mabi= name
Generate code for the specified ABI.
Permissible values are: ‘ call0 ’,
‘ windowed ’. Default ABI is chosen by the Xtensa core configuration.
-mabi=call0
When this option is enabled function parameters are passed in registers a2 through a7 , registers a12 through a15 are caller-saved, and register a15 may be used as a frame pointer. When this version of the ABI is enabled the C preprocessor symbol __XTENSA_CALL0_ABI__ is defined.
-mabi=windowed
When this option is enabled function parameters are passed in registers a10 through a15 , and called function rotates register window by 8 registers on entry so that its arguments are found in registers a2 through a7 . Register a7 may be used as a frame pointer. Register window is rotated 8 registers back upon return. When this version of the ABI is enabled the C preprocessor symbol
__XTENSA_WINDOWED_ABI__ is defined.
3.19.63 zSeries Options
These are listed under See
Section 3.19.46 [S/390 and zSeries Options], page 434 .
3.20 Specifying Subprocesses and the Switches to Pass to
Them
gcc is a driver program. It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking. GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines. This behavior is controlled by spec strings . In
488 Using the GNU Compiler Collection (GCC) most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior. The spec strings built into
GCC can be overridden by using the ‘ -specs= ’ command-line switch to specify a spec file.
Spec files are plain-text files that are used to construct spec strings. They consist of a sequence of directives separated by blank lines. The type of directive is determined by the first non-whitespace character on the line, which can be one of the following:
% command Issues a command to the spec file processor. The commands that can appear here are:
%include < file >
Search for file and insert its text at the current point in the specs file.
%include_noerr < file >
Just like ‘ %include ’, but do not generate an error message if the include file cannot be found.
%rename old_name new_name
Rename the spec string old name to new name .
*[ spec_name ]:
This tells the compiler to create, override or delete the named spec string. All lines after this directive up to the next directive or blank line are considered to be the text for the spec string. If this results in an empty string then the spec is deleted. (Or, if the spec did not exist, then nothing happens.) Otherwise, if the spec does not currently exist a new spec is created. If the spec does exist then its contents are overridden by the text of this directive, unless the first character of that text is the ‘ + ’ character, in which case the text is appended to the spec.
[ suffix ]:
Creates a new ‘ [ suffix ] spec ’ pair. All lines after this directive and up to the next directive or blank line are considered to make up the spec string for the indicated suffix. When the compiler encounters an input file with the named suffix, it processes the spec string in order to work out how to compile that file.
For example:
.ZZ: z-compile -input %i
This says that any input file whose name ends in ‘ .ZZ
’ should be passed to the program ‘ z-compile ’, which should be invoked with the command-line switch
‘ -input ’ and with the result of performing the ‘ %i ’ substitution. (See below.)
As an alternative to providing a spec string, the text following a suffix directive can be one of the following:
@ language
This says that the suffix is an alias for a known language . This is similar to using the ‘ -x ’ command-line switch to GCC to specify a language explicitly. For example:
.ZZ:
Chapter 3: GCC Command Options 489
# name
@c++
Says that .ZZ files are, in fact, C ++ source files.
This causes an error messages saying: name compiler not installed on this system.
GCC already has an extensive list of suffixes built into it. This directive adds an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.
GCC has the following spec strings built into it. Spec files can override these strings or create their own. Note that individual targets can also add their own spec strings to this list.
asm asm_final cpp cc1 cc1plus endfile link
Options to pass to the assembler
Options to pass to the assembler post-processor
Options to pass to the C preprocessor
Options to pass to the C compiler
Options to pass to the C++ compiler
Object files to include at the end of the link
Options to pass to the linker lib libgcc
Libraries to include on the command line to the linker
Decides which GCC support library to pass to the linker linker Sets the name of the linker predefines Defines to be passed to the C preprocessor signed_char Defines to pass to CPP to say whether char is signed by default startfile Object files to include at the start of the link
Here is a small example of a spec file:
%rename lib old_lib
*lib:
--start-group -lgcc -lc -leval1 --end-group %(old_lib)
This example renames the spec called ‘ lib ’ to ‘ old_lib ’ and then overrides the previous definition of ‘ lib ’ with a new one. The new definition adds in some extra command-line options before including the text of the old definition.
Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain ‘ % ’-prefixed sequences to substitute variable text or to conditionally insert text into the command line. Using these constructs it is possible to generate quite complex command lines.
Here is a table of all defined ‘ % ’-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together or combine them with constant text in a single argument.
%%
%"
%i
Substitute one ‘ % ’ into the program name or argument.
Substitute an empty argument.
%b
Substitute the name of the input file being processed.
Substitute the basename for outputs related with the input file being processed.
This is often the substring up to (and not including) the last period and not including the directory but, unless %w is active, it expands to the basename
490 Using the GNU Compiler Collection (GCC)
%B for auxiliary outputs, which may be influenced by an explicit output name, and by various other options that control how auxiliary outputs are named.
This is the same as ‘ %b ’, but include the file suffix (text after the last period).
Without %w, it expands to the basename for dump outputs.
%d Marks the argument containing or following the ‘ %d ’ as a temporary file name, so that that file is deleted if GCC exits successfully. Unlike ‘ %g ’, this contributes no text to the argument.
%g suffix Substitute a file name that has suffix suffix and is chosen once per compilation, and mark the argument in the same way as ‘ %d ’. To reduce exposure to denialof-service attacks, the file name is now chosen in a way that is hard to predict even when previously chosen file names are known. For example, ‘ %g.s ...
%g.o ... %g.s
’ might turn into ‘ ccUVUUAU.s ccXYAXZ12.o ccUVUUAU.s
’.
suffix matches the regexp ‘ [.A-Za-z]* ’ or the special string ‘ %O ’, which is treated exactly as if ‘ %O ’ had been preprocessed. Previously, ‘ %g ’ was simply substituted with a file name chosen once per compilation, without regard to any appended suffix (which was therefore treated just like ordinary text), making such attacks more likely to succeed.
%u suffix Like ‘ %g ’, but generates a new temporary file name each time it appears instead of once per compilation.
%U suffix Substitutes the last file name generated with ‘ %u suffix ’, generating a new one if there is no such last file name. In the absence of any ‘ %u suffix ’, this is just like ‘ %g suffix ’, except they don’t share the same suffix space , so ‘ %g.s
... %U.s ... %g.s ... %U.s
’ involves the generation of two distinct file names, one for each ‘ %g.s
’ and another for each ‘ %U.s
’. Previously, ‘ %U ’ was simply substituted with a file name chosen for the previous ‘ %u ’, without regard to any appended suffix.
%j suffix Substitutes the name of the HOST_BIT_BUCKET , if any, and if it is writable, and if ‘ -save-temps ’ is not used; otherwise, substitute the name of a temporary file, just like ‘ %u ’. This temporary file is not meant for communication between processes, but rather as a junk disposal mechanism.
%| suffix
%m suffix Like ‘ %g ’, except if ‘ -pipe ’ is in effect. In that case ‘ %| ’ substitutes a single dash and ‘ %m ’ substitutes nothing at all. These are the two most common ways to instruct a program that it should read from standard input or write to standard output. If you need something more elaborate you can use an
‘ %{pipe:X} ’ construct: see for example ‘ gcc/fortran/lang-specs.h
’.
%.
SUFFIX Substitutes .SUFFIX
for the suffixes of a matched switch’s args when it is subsequently output with ‘ %* ’.
SUFFIX is terminated by the next space or %.
%w
%V
Marks the argument containing or following the ‘ %w ’ as the designated output file of this compilation. This puts the argument into the sequence of arguments that ‘ %o ’ substitutes.
Indicates that this compilation produces no output file.
Chapter 3: GCC Command Options 491
%o
%O
%I
%s
%T
Substitutes the names of all the output files, with spaces automatically placed around them. You should write spaces around the ‘ %o ’ as well or the results are undefined. ‘ %o ’ is for use in the specs for running the linker. Input files whose names have no recognized suffix are not compiled at all, but they are included among the output files, so they are linked.
Substitutes the suffix for object files. Note that this is handled specially when it immediately follows ‘ %g, %u, or %U ’, because of the need for those to form complete file names. The handling is such that ‘ %O ’ is treated exactly as if it had already been substituted, except that ‘ %g, %u, and %U ’ do not currently support additional suffix characters following ‘ %O ’ as they do following, for example, ‘ .o
’.
Substitute any of ‘ -iprefix ’ (made from GCC_EXEC_PREFIX ), ‘ -isysroot ’
(made from TARGET_SYSTEM_ROOT ), ‘ -isystem ’ (made from COMPILER_PATH and ‘ -B ’ options) and ‘ -imultilib ’ as necessary.
Current argument is the name of a library or startup file of some sort. Search for that file in a standard list of directories and substitute the full name found.
The current working directory is included in the list of directories scanned.
Current argument is the name of a linker script. Search for that file in the current list of directories to scan for libraries. If the file is located insert a
‘ --script ’ option into the command line followed by the full path name found.
If the file is not found then generate an error message.
Note: the current working directory is not searched.
%e str Print str as an error message.
str is terminated by a newline. Use this when inconsistent options are detected.
%n str Print str as a notice.
str is terminated by a newline.
%( name ) Substitute the contents of spec string name at this point.
%x{ option }
Accumulate an option for ‘ %X ’.
%X
%Y
Output the accumulated linker options specified by a ‘ %x ’ spec string.
Output the accumulated assembler options specified by ‘ -Wa ’.
%Z
%M
%R
%a
%A
%l
Output the accumulated preprocessor options specified by ‘ -Wp ’.
Output multilib_os_dir .
Output the concatenation of target_system_root and target_sysroot_ suffix .
Process the asm spec. This is used to compute the switches to be passed to the assembler.
Process the asm_final spec. This is a spec string for passing switches to an assembler post-processor, if such a program is needed.
Process the link spec. This is the spec for computing the command line passed to the linker. Typically it makes use of the ‘ %L %G %S %D and %E ’ sequences.
492 Using the GNU Compiler Collection (GCC)
%D
%L
%G
%S
%E
%C
%1
%2
%*
%<S
Dump out a ‘ -L ’ option for each directory that GCC believes might contain startup files. If the target supports multilibs then the current multilib directory is prepended to each of these paths.
Process the lib spec. This is a spec string for deciding which libraries are included on the command line to the linker.
Process the libgcc spec. This is a spec string for deciding which GCC support library is included on the command line to the linker.
Process the startfile spec. This is a spec for deciding which object files are the first ones passed to the linker. Typically this might be a file named ‘ crt0.o
’.
Process the endfile spec. This is a spec string that specifies the last object files that are passed to the linker.
Process the cpp spec. This is used to construct the arguments to be passed to the C preprocessor.
Process the cc1 spec. This is used to construct the options to be passed to the actual C compiler ( cc1 ).
Process the cc1plus spec. This is used to construct the options to be passed to the actual C ++ compiler ( cc1plus ).
Substitute the variable part of a matched option. See below. Note that each comma in the substituted string is replaced by a single space.
Remove all occurrences of -S from the command line. Note—this command is position dependent. ‘ % ’ commands in the spec string before this one see -S , ‘ % ’ commands in the spec string after this one do not.
Similar to ‘ %<S ’, but match all switches beginning with -S .
Similar to ‘ %<S ’, but keep -S in the GCC command line.
%<S*
%>S
%: function ( args )
Call the named function function , passing it args .
args is first processed as a nested spec string, then split into an argument vector in the usual fashion. The function returns a string which is processed as if it had appeared literally as part of the current spec.
The following built-in spec functions are provided: getenv The getenv spec function takes two arguments: an environment variable name and a string.
If the environment variable is not defined, a fatal error is issued. Otherwise, the return value is the value of the environment variable concatenated with the string. For example, if TOPDIR is defined as ‘ /path/to/top ’, then:
%:getenv(TOPDIR /include) expands to ‘ /path/to/top/include ’.
if-exists
The if-exists spec function takes one argument, an absolute pathname to a file. If the file exists, if-exists returns the pathname. Here is a small example of its usage:
Chapter 3: GCC Command Options 493
*startfile: crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s if-exists-else
The if-exists-else spec function is similar to the if-exists spec function, except that it takes two arguments. The first argument is an absolute pathname to a file. If the file exists, if-exists-else returns the pathname. If it does not exist, it returns the second argument. This way, if-exists-else can be used to select one file or another, based on the existence of the first. Here is a small example of its usage:
*startfile: crt0%O%s %:if-exists(crti%O%s) \
%:if-exists-else(crtbeginT%O%s crtbegin%O%s) if-exists-then-else
The if-exists-then-else spec function takes at least two arguments and an optional third one. The first argument is an absolute pathname to a file. If the file exists, the function returns the second argument. If the file does not exist, the function returns the third argument if there is one, or NULL otherwise. This can be used to expand one text, or optionally another, based on the existence of a file. Here is a small example of its usage:
-l%:if-exists-then-else(%:getenv(VSB_DIR rtnet.h) rtnet net) sanitize The sanitize spec function takes no arguments. It returns non-
NULL if any address, thread or undefined behavior sanitizers are active.
%{%:sanitize(address):-funwind-tables} replace-outfile
The replace-outfile spec function takes two arguments. It looks for the first argument in the outfiles array and replaces it with the second argument. Here is a small example of its usage:
%{fgnu-runtime:%:replace-outfile(-lobjc -lobjc-gnu)} remove-outfile
The remove-outfile spec function takes one argument. It looks for the first argument in the outfiles array and removes it. Here is a small example its usage:
%:remove-outfile(-lm) version-compare
The version-compare spec function takes four or five arguments of the following form:
<comparison-op> <arg1> [<arg2>] <switch> <result>
It returns result if the comparison evaluates to true, and NULL if it doesn’t. The supported comparison-op values are:
>=
!>
True if switch is a later (or same) version than arg1
Opposite of >=
494 Using the GNU Compiler Collection (GCC)
<
!<
True if switch is an earlier version than arg1
Opposite of <
><
<>
True if switch is arg1 or later, and earlier than arg2
True if switch is earlier than arg1 , or is arg2 or later
If the switch is not present at all, the condition is false unless the first character of the comparison-op is !
.
%:version-compare(>= 10.3 mmacosx-version-min= -lmx)
The above example would add ‘ -lmx ’ if ‘ -mmacosx-version-min=10.3.9
’ was passed.
include The include spec function behaves much like %include , with the advantage that it can be nested inside a spec and thus be conditionalized. It takes one argument, the filename, and looks for it in the startfile path. It always returns NULL.
%{static-libasan|static:%:include(libsanitizer.spec)%(link_libasan)} pass-through-libs
The pass-through-libs spec function takes any number of arguments. It finds any ‘ -l ’ options and any non-options ending in
‘ .a
’ (which it assumes are the names of linker input library archive files) and returns a result containing all the found arguments each prepended by ‘ -plugin-opt=-pass-through= ’ and joined by spaces. This list is intended to be passed to the LTO linker plugin.
%:pass-through-libs(%G %L %G) print-asm-header
The print-asm-header function takes no arguments and simply prints a banner like:
Assembler options
================= gt
Use "-Wa,OPTION" to pass "OPTION" to the assembler.
It is used to separate compiler options from assembler options in the ‘ --target-help ’ output.
The gt spec function takes two or more arguments. It returns ""
(the empty string) if the second-to-last argument is greater than the last argument, and NULL otherwise. The following example inserts the link_gomp spec if the last ‘ -ftree-parallelize-loops= ’ option given on the command line is greater than 1:
%{%:gt(%{ftree-parallelize-loops=*:%*} 1):%:include(libgomp.spec)%(link_gomp)} debug-level-gt
The debug-level-gt spec function takes one argument and returns
"" (the empty string) if debug_info_level is greater than the specified number, and NULL otherwise.
%{%:debug-level-gt(0):%{gdwarf*:--gdwarf2}}
Chapter 3: GCC Command Options 495
%{S}
%W{S}
Substitutes the -S switch, if that switch is given to GCC. If that switch is not specified, this substitutes nothing. Note that the leading dash is omitted when specifying this option, and it is automatically inserted if the substitution is performed. Thus the spec string ‘ %{foo} ’ matches the command-line option
‘ -foo ’ and outputs the command-line option ‘ -foo ’.
Like % { S } but mark last argument supplied within as a file to be deleted on failure.
%@{S}
%{S*}
Like % { S } but puts the result into a FILE and substitutes @FILE if an @file argument has been supplied.
Substitutes all the switches specified to GCC whose names start with -S , but which also take an argument. This is used for switches like ‘ -o ’, ‘ -D ’, ‘ -I ’, etc.
GCC considers ‘ -o foo ’ as being one switch whose name starts with ‘ o ’. % { o* } substitutes this text, including the space. Thus two arguments are generated.
%{S*&T*} Like % { S * } , but preserve order of S and T options (the order of S and T in the spec is not significant). There can be any number of ampersand-separated variables; for each the wild card is optional. Useful for CPP as ‘ %{D*&U*&A*} ’.
%{S:X} Substitutes X , if the ‘ -S ’ switch is given to GCC.
%{!S:X} Substitutes X , if the ‘ -S ’ switch is not given to GCC.
%{S*:X} Substitutes X if one or more switches whose names start with -S are specified to
GCC. Normally X is substituted only once, no matter how many such switches appeared. However, if %* appears somewhere in X , then X is substituted once for each matching switch, with the %* replaced by the part of that switch matching the * .
If %* appears as the last part of a spec sequence then a space is added after the end of the last substitution. If there is more text in the sequence, however, then a space is not generated. This allows the %* substitution to be used as part of a larger string. For example, a spec string like this:
%{mcu=*:--script=%*/memory.ld} when matching an option like ‘ -mcu=newchip ’ produces:
--script=newchip/memory.ld
%{.S:X} Substitutes X , if processing a file with suffix S .
%{!.S:X} Substitutes X , if not processing a file with suffix S .
%{,S:X} Substitutes X , if processing a file for language S .
%{!,S:X} Substitutes X , if not processing a file for language S .
%{S|P:X} Substitutes X if either -S or -P is given to GCC. This may be combined with
‘ !
’, ‘ .
’, ‘ , ’, and * sequences as well, although they have a stronger binding than the ‘ | ’. If %* appears in X , all of the alternatives must be starred, and only the first matching alternative is substituted.
For example, a spec string like this:
%{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle} outputs the following command-line options from the following input commandline options:
496 Using the GNU Compiler Collection (GCC) fred.c
jim.d
-d fred.c
-d jim.d
-foo -baz
-bar -boggle
-foo -baz -boggle
-bar -baz -boggle
%{%: function ( args ):X}
Call function named function with args args . If the function returns non-NULL, then X is substituted, if it returns NULL, it isn’t substituted.
%{S:X; T:Y; :D}
If S is given to GCC, substitutes X ; else if T is given to GCC, substitutes Y ; else substitutes D . There can be as many clauses as you need. This may be combined with .
, , , !
, | , and * as needed.
The switch matching text S in a ‘ %{S} ’, ‘ %{S:X} ’ or similar construct can use a backslash to ignore the special meaning of the character following it, thus allowing literal matching of a character that is otherwise specially treated. For example, ‘ %{std=iso9899\:1999:X} ’ substitutes X if the ‘ -std=iso9899:1999 ’ option is given.
The conditional text X in a ‘ %{S:X} ’ or similar construct may contain other nested ‘ % ’ constructs or spaces, or even newlines. They are processed as usual, as described above.
Trailing white space in X is ignored. White space may also appear anywhere on the left side of the colon in these constructs, except between .
or * and the corresponding word.
The ‘ -O ’, ‘ -f ’, ‘ -m ’, and ‘ -W ’ switches are handled specifically in these constructs. If another value of ‘ -O ’ or the negated form of a ‘ -f ’, ‘ -m ’, or ‘ -W ’ switch is found later in the command line, the earlier switch value is ignored, except with { S * } where S is just one letter, which passes all matching options.
The character ‘ | ’ at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if ‘ -pipe ’ is specified.
It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler’s spec to say which switches take arguments. But this cannot be done in a consistent fashion. GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).
GCC also knows implicitly that arguments starting in ‘ -l ’ are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.
3.21 Environment Variables Affecting GCC
This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as ‘ -B ’, ‘ -I ’ and
‘ -L ’ (see
Section 3.16 [Directory Options], page 262
). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See Section “Controlling the Compilation Driver ‘ gcc ’” in GNU
Compiler Collection (GCC) Internals .
Chapter 3: GCC Command Options 497
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL These environment variables control the way that GCC uses localization information which allows GCC to work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is ‘ en_GB.UTF-8 ’ for English in the United
Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and
LC_MESSAGES ; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the
LANG environment variable. If none of these variables are set, GCC defaults to traditional C English behavior.
TMPDIR If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing ‘ -fcompare-debug ’ to the compiler driver. See the documentation of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is ‘ prefix /lib/gcc/ ’ where prefix is the prefix to the installed compiler. In many cases prefix is the value of prefix when you ran the ‘ configure ’ script.
Other prefixes specified with ‘ -B ’ take precedence over this prefix.
This prefix is also used for finding files such as ‘ crt0.o
’ that are used for linking.
In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with ‘ /usr/local/lib/gcc ’ (more precisely, with the value of
GCC_INCLUDE_DIR ), GCC tries replacing that beginning with the specified prefix
498 Using the GNU Compiler Collection (GCC) to produce an alternate directory name. Thus, with ‘ -Bfoo/ ’, GCC searches
‘ foo/bar ’ just before it searches the standard directory ‘ /usr/local/lib/bar ’.
If a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like
PATH . GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX .
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like
PATH . When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using GCC_
EXEC_PREFIX . Linking using GCC also uses these directories when searching for ordinary libraries for the ‘ -l ’ option (but directories specified with ‘ -L ’ come first).
LANG This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C ++ . When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:
‘ C-JIS ’ Recognize JIS characters.
‘ C-SJIS ’ Recognize SJIS characters.
‘ C-EUCJP ’ Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.
GCC_EXTRA_DIAGNOSTIC_OUTPUT
If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fixit hints are emitted.
‘ -fdiagnostics-parseable-fixits ’ and
‘ -fno-diagnostics-parseable-fixits ’ take precedence over this environment variable.
‘ fixits-v1 ’
Emit parseable fix-it hints, equivalent to ‘ -fdiagnostics-parseable-fixits ’.
In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.
‘ fixits-v2 ’
As fixits-v1 , but columns are expressed as display columns, as per ‘ -fdiagnostics-column-unit=display ’.
Some additional environment variables affect the behavior of the preprocessor.
Chapter 3: GCC Command Options 499
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable’s value is a list of directories separated by a special character, much like PATH , in which to look for header files. The special character, PATH_
SEPARATOR , is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if specified with ‘ -I ’, but after any paths given with ‘ -I ’ options on the command line. This environment variable is used regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with ‘ -isystem ’, but after any paths given with ‘ -isystem ’ options on the command line.
In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is :/special/include , that has the same effect as ‘ -I. -I/special/include ’.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the
Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form ‘ file target ’, in which case the rules are written to file file using target as the target name.
In other words, this environment variable is equivalent to combining the options
‘ -MM ’ and ‘ -MF ’ (see
Section 3.13 [Preprocessor Options], page 248
), with an optional ‘ -MT ’ switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies ‘ -M ’ rather than ‘ -MM ’. However, the dependence on the main input file is omitted. See
.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of date +%s on GNU/Linux and other systems that support the %s extension in the date command.
500 Using the GNU Compiler Collection (GCC)
The value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process.
3.22 Using Precompiled Headers
Often large projects have many header files that are included in every source file. The time the compiler takes to process these header files over and over again can account for nearly all of the time required to build the project. To make builds faster, GCC allows you to precompile a header file.
To create a precompiled header file, simply compile it as you would any other file, if necessary using the ‘ -x ’ option to make the driver treat it as a C or C ++ header file. You may want to use a tool like make to keep the precompiled header up-to-date when the headers it contains change.
A precompiled header file is searched for when #include is seen in the compilation. As it searches for the included file (see Section “Search Path” in The C Preprocessor ) the compiler looks for a precompiled header in each directory just before it looks for the include file in that directory. The name searched for is the name specified in the #include with
‘ .gch
’ appended. If the precompiled header file cannot be used, it is ignored.
For instance, if you have #include "all.h" , and you have ‘ all.h.gch
’ in the same directory as ‘ all.h
’, then the precompiled header file is used if possible, and the original header is used otherwise.
Alternatively, you might decide to put the precompiled header file in a directory and use
‘ -I ’ to ensure that directory is searched before (or instead of) the directory containing the original header. Then, if you want to check that the precompiled header file is always used, you can put a file of the same name as the original header in this directory containing an
#error command.
This also works with ‘ -include ’. So yet another way to use precompiled headers, good for projects not designed with precompiled header files in mind, is to simply take most of the header files used by a project, include them from another header file, precompile that header file, and ‘ -include ’ the precompiled header. If the header files have guards against multiple inclusion, they are skipped because they’ve already been included (in the precompiled header).
If you need to precompile the same header file for different languages, targets, or compiler options, you can instead make a directory named like ‘ all.h.gch
’, and put each precompiled header in the directory, perhaps using ‘ -o ’. It doesn’t matter what you call the files in the directory; every precompiled header in the directory is considered. The first precompiled header encountered in the directory that is valid for this compilation is used; they’re searched in no particular order.
There are many other possibilities, limited only by your imagination, good sense, and the constraints of your build system.
A precompiled header file can be used only when these conditions apply:
• Only one precompiled header can be used in a particular compilation.
• A precompiled header cannot be used once the first C token is seen. You can have preprocessor directives before a precompiled header; you cannot include a precompiled header from inside another header.
Chapter 3: GCC Command Options 501
• The precompiled header file must be produced for the same language as the current compilation. You cannot use a C precompiled header for a C ++ compilation.
• The precompiled header file must have been produced by the same compiler binary as the current compilation is using.
• Any macros defined before the precompiled header is included must either be defined in the same way as when the precompiled header was generated, or must not affect the precompiled header, which usually means that they don’t appear in the precompiled header at all.
The ‘ -D ’ option is one way to define a macro before a precompiled header is included; using a #define can also do it. There are also some options that define macros implicitly, like ‘ -O ’ and ‘ -Wdeprecated ’; the same rule applies to macros defined this way.
• If debugging information is output when using the precompiled header, using ‘ -g ’ or similar, the same kind of debugging information must have been output when building the precompiled header. However, a precompiled header built using ‘ -g ’ can be used in a compilation when no debugging information is being output.
• The same ‘ -m ’ options must generally be used when building and using the precompiled header. See
Section 3.19 [Submodel Options], page 292
, for any cases where this rule is relaxed.
• Each of the following options must be the same when building and using the precompiled header:
-fexceptions
• Some other command-line options starting with ‘ -f ’, ‘ -p ’, or ‘ -O ’ must be defined in the same way as when the precompiled header was generated. At present, it’s not clear which options are safe to change and which are not; the safest choice is to use exactly the same options when generating and using the precompiled header. The following are known to be safe:
-fmessage-length= -fpreprocessed -fsched-interblock
-fsched-spec -fsched-spec-load -fsched-spec-load-dangerous
-fsched-verbose= number -fschedule-insns -fvisibility=
-pedantic-errors
• Address space layout randomization (ASLR) can lead to not binary identical PCH files.
If you rely on stable PCH file contents disable ASLR when generating PCH files.
For all of these except the last, the compiler automatically ignores the precompiled header if the conditions aren’t met. If you find an option combination that doesn’t work and doesn’t cause the precompiled header to be ignored, please consider filing a bug report, see
If you do use differing options when generating and using the precompiled header, the actual behavior is a mixture of the behavior for the options. For instance, if you use ‘ -g ’ to generate the precompiled header but not when using it, you may or may not get debugging information for routines in the precompiled header.
3.23 C
++
Modules
Modules are a C ++ 20 language feature. As the name suggests, they provides a modular compilation system, intending to provide both faster builds and better library isolation.
502 Using the GNU Compiler Collection (GCC)
The “Merging Modules” paper https://wg21.link/p1103 , provides the easiest to read set of changes to the standard, although it does not capture later changes. That specification is now part of C ++ 20, [email protected]:cplusplus/draft.git
, it is considered complete
(there may be defect reports to come).
G ++ ’s modules support is not complete.
Other than bugs, the known missing pieces are:
Private Module Fragment
The Private Module Fragment is recognized, but an error is emitted.
Partition definition visibility rules
Entities may be defined in implementation partitions, and those definitions are not available outside of the module. This is not implemented, and the definitions are available to extra-module use.
Textual merging of reachable GM entities
Entities may be multiply defined across different header-units. These must be de-duplicated, and this is implemented across imports, or when an import redefines a textually-defined entity. However the reverse is not implemented— textually redefining an entity that has been defined in an imported header-unit.
A redefinition error is emitted.
Translation-Unit local referencing rules
Papers p1815 ( https://wg21.link/p1815 ) and p2003 ( https://wg21.link/ p2003 ) add limitations on which entities an exported region may reference (for instance, the entities an exported template definition may reference). These are not fully implemented.
Language-linkage module attachment
Declarations with explicit language linkage ( extern "C" or extern "C++" ) are attached to the global module, even when in the purview of a named module.
This is not implemented. Such declarations will be attached to the module, if any, in which they are declared.
Standard Library Header Units
The Standard Library is not provided as importable header units. If you want to import such units, you must explicitly build them first. If you do not do this with care, you may have multiple declarations, which the module machinery must merge—compiler resource usage can be affected by how you partition header files into header units.
Modular compilation is not enabled with just the ‘ -std=c++20 ’ option. You must explicitly enable it with the ‘ -fmodules-ts ’ option. It is independent of the language version selected, although in pre-C ++ 20 versions, it is of course an extension.
No new source file suffixes are required or supported. If you wish to use a non-standard suffix (see
Section 3.2 [Overall Options], page 33 ), you also need to provide a ‘
-x c++ ’ option too.
2
2
Some users like to distinguish module interface files with a new suffix, such as naming the source module.cppm
, which involves teaching all tools about the new suffix. A different scheme, such as naming module-m.cpp
would be less invasive.
Chapter 3: GCC Command Options 503
Compiling a module interface unit produces an additional output (to the assembly or object file), called a Compiled Module Interface (CMI). This encodes the exported declarations of the module. Importing a module reads in the CMI. The import graph is a Directed
Acyclic Graph (DAG). You must build imports before the importer.
Header files may themselves be compiled to header units, which are a transitional ability aiming at faster compilation. The ‘ -fmodule-header ’ option is used to enable this, and implies the ‘ -fmodules-ts ’ option. These CMIs are named by the fully resolved underlying header file, and thus may be a complete pathname containing subdirectories. If the header file is found at an absolute pathname, the CMI location is still relative to a CMI root directory.
As header files often have no suffix, you commonly have to specify a ‘ -x ’ option to tell the compiler the source is a header file. You may use ‘ -x c++-header ’, ‘ -x c++-user-header ’ or ‘ -x c++-system-header ’. When used in conjunction with ‘ -fmodules-ts ’, these all imply an appropriate ‘ -fmodule-header ’ option. The latter two variants use the user or system include path to search for the file specified. This allows you to, for instance, compile standard library header files as header units, without needing to know exactly where they are installed. Specifying the language as one of these variants also inhibits output of the object file, as header files have no associated object file.
The ‘ -fmodule-only ’ option disables generation of the associated object file for compiling a module interface. Only the CMI is generated. This option is implied when using the
‘ -fmodule-header ’ option.
The ‘ -flang-info-include-translate ’ and ‘ -flang-info-include-translate-not ’ options notes whether include translation occurs or not. With no argument, the first will note all include translation. The second will note all non-translations of include files not known to intentionally be textual. With an argument, queries about include translation of a header files with that particular trailing pathname are noted. You may repeat this form to cover several different header files. This option may be helpful in determining whether include translation is happening—if it is working correctly, it behaves as if it isn’t there at all.
The ‘ -flang-info-module-cmi ’ option can be used to determine where the compiler is reading a CMI from. Without the option, the compiler is silent when such a read is successful. This option has an optional argument, which will restrict the notification to just the set of named modules or header units specified.
The ‘ -Winvalid-imported-macros ’ option causes all imported macros to be resolved at the end of compilation. Without this, imported macros are only resolved when expanded or (re)defined. This option detects conflicting import definitions for all macros.
For details of the ‘ -fmodule-mapper ’ family of options, see
3.23.1 Module Mapper
A module mapper provides a server or file that the compiler queries to determine the mapping between module names and CMI files. It is also used to build CMIs on demand.
Mapper functionality is in its infancy and is intended for experimentation with build system interactions.
504 Using the GNU Compiler Collection (GCC)
You can specify a mapper with the ‘ -fmodule-mapper= val ’ option or CXX_MODULE_
MAPPER environment variable. The value may have one of the following forms:
[ hostname ] : port [ ?
ident ]
An optional hostname and a numeric port number to connect to. If the hostname is omitted, the loopback address is used. If the hostname corresponds to multiple IPV6 addresses, these are tried in turn, until one is successful. If your host lacks IPv6, this form is non-functional. If you must use IPv4 use
‘ -fmodule-mapper=’|ncat ipv4host port ’ ’.
= socket [ ?
ident ]
A local domain socket. If your host lacks local domain sockets, this form is non-functional.
| program [ ?
ident ] [ args...
]
A program to spawn, and communicate with on its stdin/stdout streams. Your
PATH environment variable is searched for the program. Arguments are separated by space characters, (it is not possible for one of the arguments delivered to the program to contain a space). An exception is if program begins with @.
In that case program (sans @) is looked for in the compiler’s internal binary directory. Thus the sample mapper-server can be specified with @g++-mapperserver .
<> [ ?
ident ]
<> inout [ ?
ident ]
< in > out [ ?
ident ]
Named pipes or file descriptors to communicate over. The first form, ‘ <> ’, communicates over stdin and stdout. The other forms allow you to specify a file descriptor or name a pipe. A numeric value is interpreted as a file descriptor, otherwise named pipe is opened. The second form specifies a bidirectional pipe and the last form allows specifying two independent pipes. Using file descriptors directly in this manner is fragile in general, as it can require the cooperation of intermediate processes. In particular using stdin & stdout is fraught with danger as other compiler options might also cause the compiler to read stdin or write stdout, and it can have unfortunate interactions with signal delivery from the terminal.
file [ ?
ident ]
A mapping file consisting of space-separated module-name, filename pairs, one per line. Only the mappings for the direct imports and any module export name need be provided. If other mappings are provided, they override those stored in any imported CMI files. A repository root may be specified in the mapping file by using ‘ $root ’ as the module name in the first active line. Use of this option will disable any default module> CMI name mapping.
As shown, an optional ident may suffix the first word of the option, indicated by a ‘ ?
’ prefix. The value is used in the initial handshake with the module server, or to specify a prefix on mapping file lines. In the server case, the main source file name is used if no ident is specified. In the file case, all non-blank lines are significant, unless a value is specified, in which case only lines beginning with ident are significant. The ident must be separated by
Chapter 3: GCC Command Options 505 whitespace from the module name. Be aware that ‘ < ’, ‘ > ’, ‘ ?
’, and ‘ | ’ characters are often significant to the shell, and therefore may need quoting.
The mapper is connected to or loaded lazily, when the first module mapping is required.
The networking protocols are only supported on hosts that provide networking.
If no mapper is specified a default is provided.
A project-specific mapper is expected to be provided by the build system that invokes the compiler. It is not expected that a general-purpose server is provided for all compilations.
As such, the server will know the build configuration, the compiler it invoked, and the environment (such as working directory) in which that is operating. As it may parallelize builds, several compilations may connect to the same socket.
The default mapper generates CMI files in a ‘ gcm.cache
’ directory. CMI files have a
‘ .gcm
’ suffix. The module unit name is used directly to provide the basename. Header units construct a relative path using the underlying header file name. If the path is already relative, a ‘ , ’ directory is prepended. Internal ‘ ..
’ components are translated to ‘ ,, ’. No attempt is made to canonicalize these filenames beyond that done by the preprocessor’s include search algorithm, as in general it is ambiguous when symbolic links are present.
The mapper protocol was published as “A Module Mapper” https://wg21.link/p1184 .
The implementation is provided by libcody , https://github.com/urnathan/libcody , which specifies the canonical protocol definition. A proof of concept server implementation embedded in make was described in ”Make Me A Module”, https://wg21.link/p1602 .
3.23.2 Module Preprocessing
Modules affect preprocessing because of header units and include translation. Some uses of the preprocessor as a separate step either do not produce a correct output, or require CMIs to be available.
Header units import macros. These macros can affect later conditional inclusion, which therefore can cascade to differing import sets. When preprocessing, it is necessary to load the CMI. If a header unit is unavailable, the preprocessor issues a warning and continue
(when not just preprocessing, an error is emitted). Detecting such imports requires preprocessor tokenization of the input stream to phase 4 (macro expansion).
Include translation converts #include , #include_next and #import directives to internal import declarations. Whether a particular directive is translated is controlled by the module mapper. Header unit names are canonicalized during preprocessing.
Dependency information can be emitted for macro import, extending the functionality of ‘ -MD ’ and ‘ -MMD ’ options. Detection of import declarations also requires phase 4 preprocessing, and thus requires full preprocessing (or compilation).
The ‘ -M ’, ‘ -MM ’ and ‘ -E -fdirectives-only ’ options halt preprocessing before phase 4.
The ‘ -save-temps ’ option uses ‘ -fdirectives-only ’ for preprocessing, and preserve the macro definitions in the preprocessed output. Usually you also want to use this option when explicitly preprocessing a header-unit, or consuming such preprocessed output: g++ -fmodules-ts -E -fdirectives-only my-header.hh -o my-header.ii
g++ -x c++-header -fmodules-ts -fpreprocessed -fdirectives-only my-header.ii
506 Using the GNU Compiler Collection (GCC)
3.23.3 Compiled Module Interface
CMIs are an additional artifact when compiling named module interfaces, partitions or header units. These are read when importing. CMI contents are implementation-specific, and in GCC’s case tied to the compiler version. Consider them a rebuildable cache artifact, not a distributable object.
When creating an output CMI, any missing directory components are created in a manner that is safe for concurrent builds creating multiple, different, CMIs within a common subdirectory tree.
CMI contents are written to a temporary file, which is then atomically renamed. Observers either see old contents (if there is an existing file), or complete new contents. They do not observe the CMI during its creation. This is unlike object file writing, which may be observed by an external process.
CMIs are read in lazily, if the host OS provides mmap functionality. Generally blocks are read when name lookup or template instantiation occurs.
To inhibit this, the
‘ -fno-module-lazy ’ option may be used.
The ‘ --param lazy-modules= n ’ parameter controls the limit on the number of concurrently open module files during lazy loading. Should more modules be imported, an LRU algorithm is used to determine which files to close—until that file is needed again. This limit may be exceeded with deep module dependency hierarchies. With large code bases there may be more imports than the process limit of file descriptors. By default, the limit is a few less than the per-process file descriptor hard limit, if that is determinable.
3
GCC CMIs use ELF32 as an architecture-neutral encapsulation mechanism. You may use readelf to inspect them, although section contents are largely undecipherable. There is a section named .gnu.c++.README
, which contains human-readable text. Other than the first line, each line consists of tag : value tuples.
> readelf -p.gnu.c++.README gcm.cache/foo.gcm
String dump of section ’.gnu.c++.README’:
[ 0] GNU C++ primary module interface
[
[
21] compiler: 11.0.0 20201116 (experimental) [c++-modules revision 20201116-0454]
6f] version: 2020/11/16-04:54
[
[
[
[
[
89]
95] module: foo source: c_b.ii
a4] dialect: C++20/coroutines be] cwd: /data/users/nathans/modules/obj/x86_64/gcc ee] repository: gcm.cache
[ 104] buildtime: 2020/11/16 15:03:21 UTC
[ 127] localtime: 2020/11/16 07:03:21 PST
[ 14a] export: foo:part1 foo-part1.gcm
Amongst other things, this lists the source that was built, C ++ dialect used and imports of the module.
4 The timestamp is the same value as that provided by the __DATE__ &
__TIME__ macros, and may be explicitly specified with the environment variable SOURCE_
DATE_EPOCH . For further details see
Section 3.21 [Environment Variables], page 496 .
A set of related CMIs may be copied, provided the relative pathnames are preserved.
3
Where applicable the soft limit is incremented as needed towards the hard limit.
4
The precise contents of this output may change.
Chapter 3: GCC Command Options 507
The .gnu.c++.README
contents do not affect CMI integrity, and it may be removed or altered. The section numbering of the sections whose names do not begin with .gnu.c++.
, or are not the string section is significant and must not be altered.
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