STMicroelectronics ST200 VLIW Micro Toolset software User Manual
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ST200 VLIW Series
ST200 Micro Toolset
User Manual
Last updated 15 June 2005
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Issued by the MCDT Documentation Group on behalf of STMicroelectronics
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics.
Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without the express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics.
All other names are the property of their respective owners.
The ST200 series cores are based on technology jointly developed by Hewlett-Packard Laboratories and
STMicroelectronics.
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®
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TM and Insight
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Contents
ST200 document identification and control
Conventions used in this guide
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Call Trace instrumentation options
Symbol visibility specification
Sending options to a specific phase
Specifying the run-time environment
C++ specific command-line options
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Extensions to the C++ language
Variable, functions and type attributes
Standard Template Libraries (STL) support
Supported backward compatibility headers:
Checking operator new return value
Status of the known limitations
Pragmas short description and syntax
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Memory dependences in C programs
Aliasing rules in C/C++ programs
Advanced data prefetching options
Instrumenting functions: -finstrument-functions 81
Instrumenting call to functions: -minstrument-calls 81
Profiling feedback optimization (PFO) 83
Interprocedural analysis optimization (IPA)
Symbol visibility specification
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Usage of the -fvisibility option
Usage of the -mvisibility-decl option
The visibility specification file
6 GNU C extensions supported by st200cc
Extensions to the C language family
Statements and declarations in expressions
Referring to a type with typeof
Conditionals with omitted operands
Specifying a register for a local variable
Macro with variable number of arguments
Strings literals with embedded newlines
Non-Lvalue arrays may have subscripts
Arithmetic on void and function pointers
Dollar signs in identifier names
Prototypes and old-style function definitions
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Inquiring on alignment of types or variables
Understanding intrinsic models
Understanding fundamentals operators
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Division and modulus built-ins
10 ICache/dead code optimization
Shared libraries, call shared executable
Passing other options to the optimization phase
Compiling with static ICache optimizations
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Compiling a relocatable object with static ICache optimizations
To keep a generated layout file
Using profile driven ICache optimizations 164
Using profile driven ICache optimizations using -O2 or higher 165
Compiling with st200gprof file-driven ICache optimizations
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Preface
This document is part of the ST200 documentation suite detailed below. Comments on this or other manuals in the ST200 documentation suite should be made by contacting your local STMicroelectronics Limited sales office or distributor.
ST200 document identification and control
Each book in the ST200 documentation suite carries a unique ADCS identifier in the form:
ADCS nnnnnnnx
Where, nnnnnnn is the document number and x is the revision.
Whenever making comments on a ST200 document the complete identification
ADCS nnnnnnnx should be quoted.
ST200 documentation suite
The ST200 documentation suite comprises the following volumes:
ST200 Micro Toolset User Manual
(ADCS 7508723) This manual describes the software provided as part of the ST200 tools. It supports the development of ST200 applications for embedded systems.
Applications may be developed in either a stand-alone environment, or under the
OS21 real-time operating system.
This manual also contains reference material relating to the ST200 Micro Toolset.
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ST200 Cross Development Manual
(ADCS 7521642) This manual describes the cross development tools and platforms.
ST200 Run-time Architecture Manual
(ADCS 7521848) This manual describes the common software conventions for the
ST200 processor runtime architecture.
OS21 for ST200 User Manual
(ADCS 7410372) This manual describes the use of OS21 on ST200 platforms. It describes how specific ST200 facilities are exploited by the OS21 API. It also describes the OS21 board support packages for ST200 platforms.
ST220 Core and Instruction Set Architecture
(ADCS 7395369) This manual describes the architecture and the instruction set of the ST220 core as used by STMicroelectronics.
ST231 Core and Instruction Set Architecture
(ADCS 7645929) This manual describes the architecture and the instruction set of the ST231 core as used by STMicroelectronics.
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Conventions used in this guide
General notation
The notation in this document uses the following conventions:
• sample code, keyboard input and file names,
• variables and code variables,
• code comments,
• screens, windows and dialog boxes,
• instructions.
Hardware notation
The following conventions are used for hardware notation:
• REGISTER NAMES and FIELD NAMES,
• PIN NAMES and SIGNAL NAMES.
Software notation
Syntax definitions are presented in a modified Backus-Naur Form (BNF). Briefly:
1 Terminal strings of the language, that is, those not built up by rules of the language, are printed in teletype font. For example, void.
2 Nonterminal strings of the language, that is, those built up by rules of the language, are printed in italic teletype font. For example, name.
3 If a nonterminal string of the language starts with a non-italicized part, it is equivalent to the same non-terminal string without that non-italicized part. For example, vspace-name.
4 Each phrase definition is built up using a double colon and an equals sign to separate the two sides (‘::=’).
5 Alternatives are separated by vertical bars (‘|’).
6 Optional sequences are enclosed in square brackets (‘[’ and ‘]’).
7 Items which may be repeated appear in braces (‘{’ and ‘}’).
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ST200 development system
1
1.1 ST200 compiler overview
The purpose of the st200cc compilation driver is to translate a program written in the C language, into the ST220 assembly language so that is suitable for assembly, linking, and execution. The assembler file is compiled using st200as and linked using st200ld to provide an ST200 binary image. All these phases are hidden using the driver tool st200cc.
The st200cc compiler uses the GNU C language parser, and implements state-of-the art compiler optimizations. Thanks to this GNU C language parser, the
st200cc compiler is closely compatible with the GNU C compiler, both at the driver level, and on C language extensions (GNU Compiler Collection project http://www.gnu.org/software/gcc/gcc.html
). The processor-independent compiler optimizations available in the st200cc compiler are mostly inherited from the
Open64 project hosted on SourceForge http://open64.sourceforge.net
. Other compiler optimizations that are specific to the ST200 family of processors were developed by STMicroelectronics at the CMG Compilation and Simulation Expertise center.
These include:
• the exploitation of the ST200 SELECT instruction,
• aggressive instruction selection including mapping of the user boolean variables to the branch registers,
• instruction scheduling,
• software pipelining of the inner loops,
• compiler intrinsics and builtins support,
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2 ST200 C++ compiler
• reordering of object code to minimize instruction cache conflicts,
• dead code and dead data elimination,
• automatic generation of prefetch instructions,
• profiling feedback optimization,
• interprocedural analysis optimization.
The binary image can be executed on an ST220, or ST231 hardware target or by using st200gdb. The binary format used for the image is ELF and the debug format is DWARF2. The ST200 Micro Toolset supports both little endian and big endian code generation for ST200 targets.
Where applicable, the available options are accessible through a command-line interface similar to the UNIX style. This will be familiar to most gcc and cc users.
The toolset is installed in a directory structure which also follows the UNIX structure, that is bin and lib. This is described in more detail in
Micro Toolset interface on page 5 .
The compiler supports the ANSI C89 standard and partially supports the ANSI C99 standard, see
Section 2.4: C99 support on page 27 .
1.2 ST200 C++ compiler
A GNU C++ compiler is provided which is documented in
Chapter 3: st200c++ on page 31 . Command line options which are common to both st200cc and st200c++
are documented in
, so C++ users should read both chapters.
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Toolset overview
1.3 Toolset overview
The ST200 Micro Toolset is a set of tools that allow C and C++ programs compiled for an ST200 target to be simulated on a host workstation or executed on an ST200 target. Supported platforms are:
• Solaris 2.5.8,
• RedHat Linux 7.2,
• RedHat Enterprise Linux 3.0,
• Windows XP with any Cygwin environment from version 1.5.11 or higher,
• Windows 2000 with any Cygwin environment from version 1.5.11 or higher.
The ST200 Micro Toolset is mainly intended for tool developers, for operating system development and for applications that require modeling interrupts and real-time behavior. It includes the whole set of tools that manipulate ST200 object files, including the ST200 assembler, compiler, linker, load/run tool, debugger and archiver. Here, ST200 assembler files are translated to ST200 object files that the linker merges to produce an ST200 executable image. This image file does not run natively on the host workstation and requires an interpreter to be executed. See
Section 1.4.1: Example command-lines on page 6
for details.
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Figure 1 shows the main components of the ST200 Micro Toolset.
Toolset overview
.c
source files .cxx
source files
ST200 C/C++ Compiler
ST200 assembler files (.s)
ST200 assembler
(st200as)
ST200 object file ST200 object file ST200 libraries target board boot and sysconf files
ST200 linker (st200ld)
ST200 binary (.elf)
ST200 load/run tool
(st200run)
ST200 debugger
(st200gdb)
Figure 1: Components of the ST200 Micro Toolset interfaces
Note:
Figure 1 does not include the binary optimizer tool (instruction cache placement,
dead code and dead data elimination) nor the PFO (Profiling Feedback
Optimization) nor the IPA (Interprocedural Analysis Optimization).
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1.4 ST200 Micro Toolset interface
The interface to the toolset is a command-line interface and makes the assumption that a command-line shell (like sh, ksh, csh, tcsh, or similar) is available to the user. On Windows XP/2000 systems, this is provided by the
CYGWIN32 system (see http://sources.redhat.com/cygwin for details). Most of the
ST200 tools work as command-line interfaces, with the exception of the debugger
(st200gdb) that is optionally available with a Tcl/Tk interface.
The steps involved in getting from a C program to an ST200 executable image are hidden from the user and implemented by the st200cc driver. The ST200 Micro
Toolset supports the ST220 and ST231 cores, both little endian and big endian code generation and both bare machine and OS21 run-time environments. It is installed in a directory tree with the structure shown below. (The example shows part of a
Windows directory tree.)
<tools-dir>/bin st200ar.exe
st200as.exe
st200cc.exe
st200c++.exe
st200gdb.exe
st200ld.exe
st200run.exe
(archiver)
(assembler)
(C compiler driver)
(C++ compiler driver)
(debugger)
(linker) driver for simulator or hardware
<tools-dir>/lib/cmplrs
<tools-dir>/lib/<core_target>/
<endianess>/<run-time>
<other binary utilities>
<all the compiler components> crtxxx.o
libgcc.a
libgloss.a
libgprof.a
libc.a
libm.a
C runtime start-up files libraries
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<tools-dir>/lib/engine/
<core_target>/<endianess>
<tools-dir>/include
<tools-dir>/target
<tools-dir>/target/board/
<board_target>/<core_target>/
<endianess>/<run-time>
(See
<tools-dir>/man adaptor.rcu
basicsim.dll
st200emu.dll
gdiserv.exe
gdiserv.dll
stmrpc.dll
st200sim.dll
<all the include files> execution engines targets.cfg
bootboard.o
libboard.a
<man pages> the target tree containing all of the
BSP files relative to the core, board and soc, see the ST200 Cross
Development Manual for more information.
file containing all the known targets of the toolset (used by
st200gdb and
st200run) the boot code and board specific code for target board_target
1.4.1 Example command-lines
The ST200 Micro Toolset produces an ST200 object file in ST200 object file formats
(elf).
Assuming that we want to compile two files file1.c and file2.c into an ST220, little endian executable a.elf, the set of commands to issue is:
$[1] st200cc –c file1.c
$[2] st200cc –c file2.c
$[3] st200cc –o a.elf file1.o file2.o
This assumes that the $PATH environment variable has been updated with
<tools-dir>/bin .
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Command [1] causes the following steps to be executed:
<tools-dir>/bin/st200cc # st200cc driver
<tools-dir>/lib/cpp <cpp_flags> file1.c file1.i
# C preprocessor
<tools-dir>/lib/cmplrs/<C compiler> <C Compiler flags> file1.i file1.s
# C compiler
<tools-dir>/bin/st200as <st200as_flags> file1.s file1.o
# ST200
# Assembler
Command [2] causes the following steps to be executed:
<tools-dir>/bin/st200cc # st200cc driver
<tools-dir>/lib/cpp <cpp_flags> file2.c file2.i
# C preprocessor
<tools-dir>/lib/cmplrs/<C compiler> <C Compiler flags> file2.i file2.s
# C compiler
<tools-dir/bin/st200as <st200as_flags> file2.s file2.o
# ST200
# Assembler
Command [3] causes the following steps to be executed:
<tools-dir>/bin/st200cc
<tools-dir>/bin/st200ld <st200ld_flags>
# st200cc driver
# ST200 linker
<tools-dir>/lib/st220/le/bare/crt1.o
# ST200 startup files
<tools-dir>/lib/st220/le/bare/crti.o
<tools-dir>/lib/st220/le/bare/crtbegin.o
file1.o file2.o
# ST200 startup files
# ST200 startup files
# Application objects
<tools-dir>/lib/st220/le/bare/libc.a
<tools-dir>/lib/st220/le/bare/libgcc.a
<tools-dir>/lib/st220/le/bare/crtn.o
<tools-dir>/lib/st220/le/bare/crtend.o
–o a.elf
# ST200 C libraries
# run-time
# arithmetic library
# ST200 startup files
# ST200 startup files
# Output binary
Once steps [1] to [3] are completed, an ST200 executable binary a.elf is generated.
This can be executed in two different ways:
• Use the stand-alone driver for the load/run tool (available as st200run) in the following way:
$[4] st200run --target=sim220 a.elf
This causes the a.elf ST200 binary to be “interpreted” by the st200run command. The simulator also provides some minimal cycle counting and statistics facilities.
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• Use the ST200 gdb debugger:
$[4] st200gdb -nw a.elf
This causes the ST200 version of gdb running on the host platform to load the a.elf
image.
Once started, the minimal sequence of commands to issue is:
(gdb)> target gdi -t sim220
(gdb)> load
#inform gdb to use the
#built-in simulator
#load the binary to the
#simulator environment
#start the execution (gdb)> run
The version of st200gdb shipped with the ST200 emulation toolchain is a fully functional debugger. If the binary is compiled with the –g flag, source-level debugging is possible. Otherwise, assembly level debugging is always supported for ST200 binaries. The st200gdb debugger has both a graphical user interface as well as a command-line interface (supported from a terminal shell). The facilities provided by the st200gdb debugger are introduced in the Getting
Started chapter of the ST200 Cross Development Manual.
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st200cc
2
2.1 Getting started
The st200cc compiler is similar to any command-line compiler. It is either invoked from a command line interpreter or from a Makefile and implicitly recognizes files by their extension.
2.1.1 Invoking the compiler
The C compiler is invoked using the st200cc command: st200cc {<argument>} where:
<argument> = <option> | <input_file>
Examples: st200cc -S file.c # produces file.s
st200cc -c file.c # produces file.o
Conflicting options are resolved by using the last option on the command line. Any string that is not recognized as an option or a recognized input file is passed down to the link phase.
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2.1.2 Input and output
File extension naming conventions are summarized in
.c
.h
.i
.s
.S
Extension Convention
C language source file to be pre-processed and compiled
C language header file
C language source file already pre-processed
Assembly language source file to be assembled
Assembly language source file to be pre-processed and assembled
Table 1: Input Names conventions
.s
.o
Extension Convention
Assembly language output file
Object file
Table 2: Output Names conventions
Produced by option(s)
-S
-c
The final executable file does not need to have a specific file extension. If no output file name is specified through the -o option, the executable generated is named a.out
.
Examples: st200cc file.c # generates the executable a.out
st200cc file.c -o file.u # generates the executable file.u
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Command-line options 11
2.2 Command-line options
2.2.1 Getting help
If the compiler driver is given the -help option, it displays the list of available options, and then terminates.
Additionally, the -help option can be followed by an additional keyword separated from the help option by a colon. All entries matching the keyword are displayed on the standard output, for example: st200cc -help:-W
This command displays all options containing the -W string. In this example, all options related to the emission of compiler warnings will be listed.
2.2.2 Overall options
The options in
Table 3 control the type of processing performed by st200cc and the
output it generates, for example: an executable, an object file, an assembler file, a pre-processed file, an archive or a dependency list.
Output files produced by these option default to:
<original_file_name>.<output_extension> and can be renamed using the
-o option.
-c
-S
-E
-v
Option
--version
-dumpversion
-keep
Description
Compile or assemble the source file, but do not link.
Stop after the compilation phase.
Stop after the preprocessing phase. Output is send to stdout.
Print on stderr the commands executed to run the compilation phases.
The message generated indicates the release identity.
Display the version numbers of the invoked compiler and stop.
Print the compiler front-end version (for example, 2.95.2) and stop.
Keep intermediary files produced by the compilation phases.
Table 3: Overall options
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2.2.3 C preprocessor options
The preprocessor is run on each C source file before actual compilation. The options in
control how the sources are pre-processed.
Option Description
-E
-C
-P
-Ddef
-Ddef=defn
-M
-MM
-MG
-H
Only the preprocessor is run.
The preprocessor does not discard comments. Use with the -E option.
The preprocessor does discard #line information. Use with the -E option.
Define the macro definition with the string 1 as the definition.
Define the macro definition as defn.
Generates a list of object file dependencies suitable for a Makefile.
Similar to -M, but ignores system header files, that is, header files included by <header.h>.
Along with -M or -MM, treat missing files as generated in the local directory.
Display the name and path of the header in use.
-dM
-dD
Print a list of macros definitions in use after preprocessing. Use with the -E option.
Print a list of macro definitions in use while preprocessing. Use with the
-E option.
-dN Same as -dD, except that the macro arguments are not shown. Use with the -E option.
-fpreprocessed Indicate to the preprocessor that the input file has already been preprocessed.
Table 4: Preprocessor options
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Command-line options 13
2.2.4 C dialect options
The option -std=value instructs the compiler front-end to select the appropriate C language dialect to use. For instance, the C99 restrict keyword is only recognized with the -std=c99 option. However, this keyword also exists as a GNU extension keyword, either __restrict or __restrict__ that are recognized by default.
Possible values for -std are listed in
Option Description
-std=iso9899:1990 Same as -ansi
-std=iso9899:199409 ISO C as modified in amendment 1
-std=iso9899:1999
-std=c89
ISO C 99
Same as -std=iso9899:1990
-std=c99
-std=gnu89
-std=gnu99
Same as -std=iso9899:1999
This is the default, iso9899:1990 + gnu extensions iso9899:1999 + gnu extensions
Table 5: C Dialect options
2.2.5 Warning options
Diagnostic messages can be requested from the compiler to notify potentially erroneous or dangerous C program constructions.
Table 6 lists a subset of the GCC
options.
Option
-Wall
-w
-Werror
-pedantic
-pedantic-error
Description
Enables all warnings.
Disables all warnings.
Turns warnings into errors.
Issues all warnings needed by strict ANSI C compliance.
Turn all pedantic warnings into errors.
Table 6: General warning options
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The options in
Table 7 take a positive form; a negative form can be systematically
constructed by changing the -W prefix to a -Wno prefix, for example -Wnoformat.
Option
-Wchar-subscripts
-Wformat
-Wimplicit-int
Description
Warn if an array subscript has type char.
Check calls to the printf and scanf family of library functions.
Check that all declarations specify a type, which is int by default in c89.
Warn when a function is used but not declared.
-Wimplicit-functiondeclaration
-Werror-implicit-functiondeclaration
-Wimplicit
-Wmissing-braces
-Wparentheses
-Wreturn-type
-Wswitch
-Wtrigraph
-Wunused
-Wunknown-pragmas
-Wmultichar
-Wshadow
-Wpointer-arith
-Wbad-function-cast
Output error when a function is used but not declared.
-Wimplicit-int and
-Wimplicit-function-declaration .
Warn if an aggregate or union initializer is not fully bracketed.
Warn if parentheses are omitted in certain contexts.
Warn when a function is defined with a return-type that defaults to int.
Warn whenever a switch statement may be incomplete.
Warn if any trigraphs are encountered that might change the meaning of the program.
Warn whenever a static function, a label, a parameter, a value is not used.
Warn when a #pragma is encountered which is not understood by st200cc.
Warn if a multi-character constant is used.
Warn whenever a local variable shadows another variable.
Warn about anything that depends on the “size of” a function type or of void.
Warn whenever a function call is cast to a non-matching type.
Table 7: Detailed warning options
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Command-line options 15
-Wcast-qual
Option
-Wcast-align
-Wwrite-strings
-Wconversion
-Wsign-compare
-Waggregate-return
-Wstrict-prototypes
-Wmissing-prototypes
-Wmissing-declarations
-Wmissing-noreturn
-Wredundant-decls
-Wnested-externs
-Wlong-long
-Wpacked
-Wpadded
Description
Warn whenever a pointer is cast so as to remove a type qualifier from the target type.
Warn whenever a pointer is cast such that the required alignment of the target is increased.
Warn when trying to write to a string constant.
Warn if a prototype causes a type conversion that is different from what would happen in the absence of a prototype.
Warn when a comparison between signed and unsigned values could produce an incorrect result.
Warn if any functions that return structures or unions are defined or called.
Warn if a function is declared or defined without specifying the argument types.
Warn if a global function is defined without a previous prototype declaration
Warn if a global function is defined without a previous declaration.
Warn about functions which might be candidates for attribute noreturn .
Warn if anything is declared more than once in the same scope.
Warn if an extern declaration is encountered within a function.
Warn if long long type is used. Only active along with
-pedantic .
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure.
Warn if padding is included in a structure, either to align an element of the structure or to align the whole structure.
Table 7: Detailed warning options
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2.2.6 Debugging options
The -g option instructs st200cc to generate symbolic information for debugging.
DWARF2 format is used.
Note: The -g option may be used with optimization up to level -O2 and with -Os (see
Section 2.2.9: Optimization options on page 16 ).
2.2.7 Profiling options
The -pg option instructs st200cc to generate profiling information, see
Note: Using the -pg option together with -g enhances the precision of the profile symbolic information.
2.2.8 Call Trace instrumentation options
The -finstrument-functions and -minstrument-calls instructs st200cc to generate instrumentation calls, see
Section 5.7: Call trace instrumentation on page 81 .
2.2.9 Optimization options
The options in
Table 8 control optimization levels.
-O0
-O1
-Os
-O2
-O3
Option Description
No optimization
Minimal optimization
Optimization without code size expansion
Global optimization
Aggressive optimization
Table 8: Optimize options
Note: -O optimization is equivalent to -O2. -Os optimization applies the optimizations of
-O2 , except for those that increase the code size (such as unrolling).
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Command-line options
The options in
Table 9
enable finer control of the optimization level.
Option Description
-f[no-]strict-aliasing -fstrict-aliasing allows 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 types are almost the same (the aliasing rules are stated in the ANSI C standard, in clause
6.5 (7) Expressions. For example an unsigned int can alias an int, but not a void * or a double. A character type may alias any other type.
The default is -fno-strict-aliasing, for legacy reasons. In future releases, the -fstrict-aliasing option will be used by default.
-f[no-]unroll-loops -funroll-loops forces loop unrolling. This is the default at -O2 and -O3. Loops are unrolled up to 4 times at -O2, and up to 8 times at -O3, with a limit on code size expansion.
Loops with a #pragma unroll directive are not affected by this option.
-fno-unroll-loops disable loop unrolling. This is the default at -Os. Loops with a #pragma unroll directive are not affected by this option.
Table 9: Advanced Optimization options
2.2.10 Code generation options
The options in
Table 10 control various aspects of the code generation.
-fb <name>
Option
-fb_create <name>
Description
The feedback file to use for feedback instrumentation. See
Section 5.8: Profiling feedback optimization (PFO) on page 83 .
Generate an executable with feedback instrumentation.
See
Section 5.8: Profiling feedback optimization (PFO) on page 83 .
Table 10: Code generation options
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18 Command-line options
Option Description
-fsigned-char
-funsigned-char
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
-ffixed-reg=<register-list>
-fsigned-char implements type ‘char’ as signed.
-funsigned-char implements char as unsigned.
Note that when the -funsigned-char option is used, the
__CHAR_UNSIGNED__ preprocessor symbol is defined.
The compiler default is signed.
These options control whether a bitfield is signed or unsigned, when the declaration does not use either
‘signed’ or ‘unsigned’.
The compiler default is signed.
<register-list> is a list of one or several comma-separated register names or dash-separated register ranges, either general purpose registers or boolean registers. The syntax used for a general register is ri where i can be 0 to 63 and bj where j can be from 0 to 7.
This option makes the given registers fixed registers; that is the code generated by the compiler never uses them.
There are however, some registers that are used by the compiler for ABI register conventions. See the table of general registers in the ST200 Run-time Architecture
Manual. Those registers with a specified use must not be reserved with this option.
Note that specific care must be taken when using this option since low-level library and run-time support code are not specifically built to support non-ABI register usage. For instance, reserving the r62 register does not prevent the already compiled library code from using it. Using this option generally requires rebuilding a set of libraries either with the same option (for C/C++ code) or to take into account that this option has been used.
Examples: st200cc -ffixed-reg=r62,b0 st200cc -ffixed-reg=r24-r62
Table 10: Code generation options
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Command-line options
Option Description
-f[no-]zero-initialized-in-bss
-fdismissible-load
-fno-dismissible-load
-fverbose-asm
-fno-verbose-asm
st200cc by default puts variables that are initialized to zero into the .bss section that is cleared upon startup.
Note that the -fno-zero-initialized-in-bss option turns off this behavior because some programs explicitly rely on variables going to the data section. If
-fno-zero-initialized-in-bss is specified then zero-initialized variables are placed in the data section.
The default is -fzero-initialized-in-bss.
The -fdismissible-load option globally enables and the -fno-dismissible-load globally disables the dismissible load generation in all compiler phases.
Dismissible loads are only generated at -Os, -O2 and -O3 optimization levels.
Libraries, for example libc, libgcc are generally built with -O2 or -Os options and thus may include legitimate dismissible loads. This is the case even if
-fno-dismissible-load is used to compile the application.
The -fno-dismissible-load option reduces performance and does not reduce code size. This option is intended to be used by OS or library writers and should not be used by the general user.
Dismissible load generation is enabled by default.
The -fno-verbose-asm removes extra commentary information in the generated assembly code.
The default is to have verbose asm output.
Table 10: Code generation options
2.2.11 -OPT options
The options -OPT:unroll_size, -OPT:cray_ivdep and -OPT:liberal_ivdep modify the behavior of pragmas and are documented in
Section 4.2.1: #pragma unroll on page 49 and
Section 4.2.2: #pragma ivdep on page 50 .
The -OPT:alias option is documented in
Section 5.3: Memory dependences in C programs on page 69 .
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2.2.12 -Data prefetching options
The option -mauto-prefetch triggers automatic data prefetching which is documented in
Section 5.5: Data prefetching on page 75 .
2.2.13 Inlining options
The -inline, -noinline and -INLINE options are provided to control inlining of functions. They are listed in
described in
Section 5.2.1: Single file inlining on page 59 .
Only functions marked with the inline keyword are subject to inlining unless specified otherwise.
2.2.14 Interprocedural analysis
The -ipa option enables interprocedural analysis, and is described in
Section 5.9 on page 86 . This section documents a range of advanced -IPA options that provide
control over the optimizations performed.
2.2.15 Symbol visibility specification
The options -fvisibility and -mvisibility-decl are provided to specify symbol visibility, they are documented in
Section 5.10: Symbol visibility specification on page 89 .
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Command-line options 21
2.2.16 Sending options to a specific phase
The -W<phase>,<arg> option passes the specified argument <arg> to a specific processing phase <phase> of st200cc.
Table 11 lists the different values of <phase>.
Value of phase
Description p f a l o
Preprocessor cpp
Compiler front-end
Assembler st200as
Linker st200ld
Binary optimizer tool binopt
Table 11: Possible value for phase
There must be a comma between the option -W<phase> and the argument and no spaces. Anything occurring after a space is treated as the next option to st200cc.
Also the argument is only passed to <phase> if <phase> is normally run from the specified command. For example: st200cc -O3 -Wo,--help a.out
This command executes the ICache optimizer, which is enabled by the -O3 option and the argument --help is passed in.
Multiple arguments must be separated by commas, with no spaces: st200cc -O3, -Wo,--dump,--static a.out
Full details of binopt options are given in
Chapter 10: ICache/dead code optimization .
2.2.17 Specifying the run-time environment
The -mruntime=name option selects the run-time environment for the compilation and link. name may be either bare to select a bare machine or os21 to select the
OS21 run-time libraries. The default is a bare machine.
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To select a run-time other than the default, the -mruntime option must be used for both the compilation and link.
The OS21 for ST200 User Manual describes the use of OS21 for ST200 cores.
2.2.18 Selecting the endianness
The -EL option selects code generation for a little endian core, and propagates this option to all the tools that are invoked by the compiler. The startup and libraries are also selected based on this information to be little endian. This is the default.
The -EB option selects the big endian mode for compiler code generation, startup and libraries selection.
2.2.19 Configuration options
Options are provided which enable system configuration parameters (called sysconf parameters) or a boot code file to be selected at link time. These options and the modules they select to be linked are described in more detail in the Getting started chapter of the ST200 Cross Development Manual; they are listed in
Option Description
-mboard=<board_target> Select a board target. bootboard.o, libboard.a and board.x
located in target/board/<board_target>/
<core_target>/<endianess>/<runtime> are linked.
-mcore=<core_target>
-msoc=<soc_target>
Selects the core_target code generation mode in the compiler. Available targets are st220 and st231.
bootcore.o
, libcore.a and core.x located in target/core/<core_target>/<endianess>/
<runtime> are linked.
Select a SOC target. bootsoc.o, libsoc.a and soc.x located in target/soc/<soc_target>/
<core_target>/<endianess>/<runtime> are linked.
Table 12: Configuration options
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Command-line options 23
2.2.20 Directory and library options
lists the options that select header files, libraries and compiler executables.
Option
-Idirectory
-nostdinc
-l<library>
-Ldirectory
-nostdlib
Description
Add directory to the beginning of the search list for include files.
No predefined include search path.
Search the library named lib<library>.a when linking. The linker looks for the library in the directories specified by the -L options and then in a standard list of directories.
The position of this option on the command line makes a difference.
The linker processes object files and libraries in the order that they are specified on the command line. For example, if the following is specified: st200cc file1.o file2.o -lmylib then the files will be processed in the order file1.o, file2.o, libmylib.a
.
However, if the following is specified: st200cc file1.o -lmylib file2.o
then the files will be processed in the order file1.o, libmylib.a, file2.o
.
In this case, file2.o should not refer to any symbols defined in libmylib.a
.
Add directory to the beginning of the search list for library files.
No predefined libraries search path.
Table 13: Directory options
The search path for the various phases of the compiler can be overridden by using the option: -Y<phase>,<path> where <phase> can take the values listed in
and <path> is the path of the required tool. There must be a comma and no spaces separating -Y<phase> and
<path> .
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2.2.21 Environment variables
Currently there are no special environment variables that affect st200cc.
2.3 Predefined macros
Predefined macros are described in
.
Note: 1 The list of macros currently defined can be obtained by typing: st200cc -E -dM filename.c
where filename.c can be any .c file including an empty file.
2 Do not rely on a macro that is not documented, even if it is currently defined.
3 Some macros values are subject to change along with compiler evolutions, for instance front-end identifications values.
Name
__open64__
__GNUC__
__GNUC_MINOR__
__ST200CC__
__ST200CC_MINOR__
__ST200CC_PATCHLEVEL__
Default definition Purpose
Defined
2
95
Defined, value depends on major compiler version
Defined, value depends on minor compiler version
Defined, value depends on compiler patch level
Compiler technology identification
Front end major release identification
Front end minor release identification
Compiler identification
Compiler identification
Compiler identification
Table 14: Predefined macros
See also
-no-gcc
-no-gcc
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Predefined macros
Name
__ST200CC_DATE__
__ST200CC_VERSION__
__ST200
__ST200__
__st200__
__ST220__
__st220__
__ST231__
__st231__
__LITTLE_ENDIAN__
__BIG_ENDIAN__
_LANGUAGE_C
_LANGUAGE_ASSEMBLY
__STRICT_ANSI__
Default definition
Defined, value depends on compiler release date
Defined, value is an identification string
Defined
Defined
Purpose
Compiler identification
Compiler identification
Architecture identification
Architecture identification
Defined by default and when
-mcore =st220
Defined when
-mcore =st231
Architecture identification
Architecture identification
Defined by default and when using the
-EL option
Defined by the -EB option
Defined for C source
Defined for ASM source
Defined when
-std=c89 or
-std=c99 or
-ansi
Endianness identification
Endianness identification
Language currently processed is C language.
Language currently processed is assembly language.
Compiler is in strict ansi mode.
Table 14: Predefined macros
-mcore
-mcore
-EL
-EB
-std
See also
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26 Predefined macros
Name
__STDC_VERSION__
__OPTIMIZE__
__INLINE_INTRINSICS
__STDC_HOSTED__
__BARE_BOARD__
__OS21_BOARD__
Default definition
Defined when
-std=c99 with value 199901L
Defined as soon as optimization is on.
Defined
Defined by default.
Purpose
Compiler is in
C99 ansi mode
Optimization mode detection.
Intrinsics inlining mode detection.
Hosting mode.
Defined by default
Defined when
-mruntime=os21
Run-time support identification
Run-time support identification
Table 14: Predefined macros
-std
-O
See also
-OPT:inline_ intrinsics
-f[no-]hosted
-f[no-]freestanding
-mruntime
-mruntime=os21
Note: That the C standard guarantees that the __cplusplus symbol is never defined when compiling C source code.
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C99 support 27
2.4 C99 support
The st200cc compiler supports a subset of the C99 standard. Most features are implicitly available through default compiler command line options, with the notable exception of the restrict keyword that requires the -std=c99 command line option to be specified.
It is recommended that any code fragment that depends upon C99 specific behavior be guarded by the following preprocessing definitions, which are correctly triggered when the -std=c99 command line option is used:
#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
// Your C99 dependent code here
#else
#error "This source file depends upon C99 features not available with this compiler."
#endif
Table 15 summarizes the status of the st200cc compiler C99 support.
Feature as described in the C99 standard
Restricted character set support via digraphs
(<iso646.h>)
Wide character library support
Status
NO: include file not provided.
More precise aliasing rules via effective type
Restricted pointer
Variable length arrays
Flexible array members
Static and type qualifiers in parameter array declarators
Complex support (<complex.h>)
Type generic math macros (<tgmath.h>)
Table 15: C99 support in st200cc
NO: type not supported and library not provided.
YES: provided that the
-fstrict-aliasing option is used
YES
PARTIAL: only local allocation, but no other features
NO
NO
NO
NO: include file not provided
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C99 support
Feature as described in the C99 standard Status
The long long int type and library functions
Increased minimum translation unit
Additional floating-point characteristics (<float.h>)
Remove implicit int
Reliable integer division
Universal character names
Extended identifiers
Hexadecimal floating-point constants
Compound literals
Designated initializers
// comments
Extended integer type and library functions in
<inttypes.h> and <stdint.h>
Remove implicit function declaration
Preprocessor arithmetic done in intmax_t/uintmax_t
Mixed declaration and code
New block scope for selection and iteration statements
Integer constant type rules
Integer promotion rules vararg macro
The vscanf family of function in <stdio.h> and
<wchar.h>
Additional math library functions in <math.h>
Floating point environment access in <fenv.h>
NO
NO
Table 15: C99 support in st200cc
YES but C libraries may not provide full support
YES
NO
NO, but can get warning
YES
NO
NO
YES, but no %a %A printf scanf /scanf conversion specifiers
YES
YES
YES
NO: include file not provided
NO, can get warning
YES
NO
NO
NO
NO
YES
NO: library support not provided
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C99 support
Feature as described in the C99 standard
ISO 60559 Arithmetic support
Trailing comma allowed in enum declaration
%lf conversion allowed in printf
Inline functions
The snprintf family of functions in <stdio.h>
Boolean type in <stdbool.h>
Idempotent type qualifiers
Empty macro arguments
New struct type compatibility rules
Additional predefined macro names
_Pragma preprocessing operator
Standard pragmas
__func__ predefined identifier
VA_COPY macro
Additional strftime conversion specifiers
LIA compatibility annex
Deprecate ungetc at the beginning of a binary file
Remove deprecation of aliased array parameters
Table 15: C99 support in st200cc
Status
NO
YES
NO
YES but not fully ansi compliant in the extern inline case
NO: library support not provided
NO bool native type but
<stdbool.h> header provided
YES, but still emits warnings
YES
NO
MOST
YES
NO
YES
NO
NO: library support not provided
NO
NO
NO
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st200c++
3
3.1 Introduction to st200c++
The toolset features a GNU C++ 3.3.3 front-end with full library support, but no exceptions support.
It is therefore recommended that you always compile with the -fno-exceptions option.
The current known limitations are documented, they will be removed in future releases.
This C++ compiler aims at improving performance and compliance with industrial standards (ISO/IEC 14882, Common Vendor ABI, SYSV ELF).
Note: Using the st200c++ compiler is very similar to using st200cc, therefore only the new options, definitions and usage rules are documented in this chapter.
3.1.1 References
The ISO/IEC 14882:1998 C++ standard is available from your local ISO or ANSI outlet.
A useful source of information concerning the C++ standard is http://www.open-std.org/JTC1/SC22/WG21 .
The GNU Compiler documentation can be found at http://gcc.gnu.org
.
The Common vendor ABI is described at http://www.codesourcery.com/cxx-abi .
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32 Getting started
3.2 Getting started
The st200c++ compiler is similar to any command-line compiler. It is either invoked from a command line interpreter or from a Makefile and implicitly recognizes files by their extension. It recognizes all st200cc compiler options, therefore this chapter only lists the specific C++ additions.
3.2.1 Invoking the compiler
The C++ compiler is invoked using the st200c++ command: st200c++ {<argument>} where:
<argument> = <option> | <input_file>
Examples: st200c++ -S -fno-exceptions file.cxx
# produces file.s
st200c++ -c -fno-exceptions file.cxx
# produces file.o
Conflicting options are resolved by using the last option on the command line. Any string that is not recognized as an option or a recognized input file is passed down to the link phase.
3.2.2 Input and output
File extension naming conventions are summarized in
Extension
.cxx
, .cpp, .c++,
.C
, .CC, .CPP, .CXX
.ii
Convention
C++ language source file to be pre-processed and compiled.
C++ language source file already pre-processed.
Table 16: Input names conventions
The final executable file does not need to have a specific file extension. If no output file name is specified through the -o option, the executable generated is a.out.
Examples: st200c++ file.cxx
# generates the executable a.out
st200c++ file.cxx -o file.u
# generates the executable file.u
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C++ specific command-line options
3.3 C++ specific command-line options
3.3.1 C++ dialect options
The option -std=value instructs the compiler front-end to select the appropriate
C++ language dialect to use. Possible value for -std are listed in
.
-std=c++98
-std=gnu++98
Option Description
Default if -fansi. Set language to ISO/IEC
14882.
Default. Set language to ISO/IEC 14882 and support GNU extensions.
Table 17: C++ dialect options
3.3.2 C++ specific warning options
Diagnostic messages can be requested from the compiler to notify potentially erroneous or dangerous C++ program constructions.
Table 18 lists a subset of the
GCC options.
-W[no-]abi
Option
-W[no-]ctor-dtor-privacy
-W[no-]effc++
-W[no-]old-style-cast
Description
Warn when compiler generates code that is probably not compatible with the vendor-neutral
C++ ABI.The default is -Wabi.
Warn when a class seems unusable because all the constructors or destructors in that class are private. The default is -Wctor-dtor-privacy
Warn about violations of style guidelines from
Scott Meyer’s ‘Effective C++ book’. The default is
-Wno-effc++ .
Warn if an old-style cast to a non-void type is used within a C++ program. The default is
-Wno-old-style-cast.
Table 18: C++ warning options
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C++ specific command-line options
Option Description
-W[no-]overloaded-virtual
-W[no-]deprecated
-W[no-]pmf-conversions
-W[no-]reorder
Warn when a function declaration hides virtual functions from a base class. The default is
-Wno-overloaded-virtual.
Do not warn about usage of deprecated features.
The default is -Wdeprecated.
Disable the diagnostic for converting a bound pointer to a member function to a plain pointer.
The default is -Wpmf-conversions.
Warn when the order of member initializers given in the code does not match the order in which they must be executed. The default is -Wno-reorder
Table 18: C++ warning options
The options in
Table 18 take a positive form; a negative form can be systematically
constructed by changing the -W prefix to a -Wno prefix, for example -Wnoformat.
3.3.3 Code generation options
The options in
Table 19 control various aspects of the code generation.
-f[no-]rtti
Option
-f[no-]exceptions
-f[no-]check-new
-f[no-]implicit-templates
-f[no-]access-control
Description
Enable (or disable) generation of rtti information.
Enable (or disable) exception handling.
Check that the pointer returned by operator new is non null before attempting to modify the storage allocated or calling the object’s constructor.
Emit code (or never emit code) for non-inline templates which are instantiated implicitly.
Turn on (or off) all access checking.
Table 19: C++ code generation options
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C++ specific command-line options
Option
-f[no-]elide-constructors
-f[no]-for-scope
-f[no]-gnu-keywords
-f[no]-permissive
-ftemplate-depth=n
-fmessage-length=n
-fdiagnostics-show-location=
[once|every-line]
Description
Omit creating a temporary which is only used to initialize another object of the same type.
-fno-elide-constructors forces the compiler to call the copy constructor even when it could be optimized out.
Under -fno-for-scope, the scope of variables declared in a for-init-statement extends to the end of the enclosing scope. Under
-ffor-scope , the scope of variables declared in a for-init-statement is limited to the for loop itself, as specified by the C++ standard. The default is
-ffor-scope.
When using -fno-gnu-keywords, typeof is not recognized as a keyword, so that code can still use this word as an identifier. The keyword
__typeof__ can still be used instead. The default is -fgnu-keywords.-ansi implies
-fno-gnu-keywords .
-fpermissive downgrades some diagnostics about non-conformant code, allowing some non-conformant code to compile. The default is
-fno-permissive .
Set the maximum instantiation depth for template classes to n.
Try to format error messages so that they fit on lines of about n characters.
Instructs the diagnostic messages reporter to emit
‘once’ or ‘every-line’ source location information.
Table 19: C++ code generation options
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36 Predefined macros
3.3.4 Directory and library options
lists the options that select header files, libraries and compiler executables.
-nostdinc ++
Option Description
No predefined include search path.
Table 20: Directory options
3.4 Predefined macros
Predefined macros are described in
.
Note: 1 The macros defined by the C compiler are still relevant unless otherwise stated (for example the macros defining the endianness and the core)
2 The list of macros currently defined can be obtained by typing: st200c++ -E -dM filename.cxx
where filename.cxx can be any .cxx file including an empty file.
3 Do not rely on a macro that is not documented, even if it is currently defined.
4 Some macros values are subject to change in future compiler versions, for instance front-end identification values.
__GNUC__
Name
__GNUC_MINOR__
__GNUC_PATCHLEVEL__
Default definition Purpose
3
3
3
Front end major release identification
Front end minor release identification
Front end minor release identification
Table 21: Predefined macros
See also
-no-gcc
-no-gcc
-no-gcc
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Predefined macros
__GNUG__
Name
_LANGUAGE_C_PLUS_PLUS
__cplusplus
3
Default definition
Defined only for C++ source
Defined for C++ source
Purpose
C++ Front end major release identification
Language currently processed is
C++ language.
Language currently processed is
C++ language.
See also
-no-gcc
__STRICT_ANSI__
__EXCEPTIONS
__DEPRECATED
__GXX_ABI_VERSION
Defined when
-std=c++98 -ansi
Defined, unless
-fno-exceptions is used defined, unless
-Wno-deprecated is used
102
Compiler is in strict ansi mode.
Compiler supports exceptions
Compiler supports legacy code
C++ abi version
-std
-f[no-]exceptions
-Wno-deprecated
Table 21: Predefined macros
Note: The C standard guarantees that the __cplusplus symbol is never defined when
compiling C source code, and the C++ standard guarantees that __cplusplus is defined when compiling C++ source code.
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GNU C++ language extensions
3.5 GNU C++ language extensions
3.5.1 Extensions to the C++ language
3.5.1.1 C language extensions supported in C++
The following C language extensions are supported in C++:
• alignof,
• typeof,
• case ranges,
• label as values.
3.5.1.2 C++ specific extensions
Minimum and maximum operators a <? b is the minimum, returning the smaller of the numeric values a and b.
a >? b is the maximum, returning the larger of the numeric values a and b.
3.5.2 Variable, functions and type attributes
3.5.2.1 C language attributes supported in C++
The following C++ attributes are supported in the C++ context.
Placement and layout attributes
• section,
• weak,
• alias,
• align,
• pack,
• constructor,
• destructor.
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GNU C++ asm statement 39
Optimization attributes
• malloc,
• noreturn.
3.5.2.2 C++ specific attributes
Initialization priority
Note: This feature is unsupported in the current release.
Support ‘__attribute__ ((init_priority (n)))’ for controlling the order of initialization of file-scope objects.
Where n is a number representing the priority of that object. File-scope objects are initialized in order from the lowest number (highest priority) to the highest (lowest priority).
3.5.3 Built-ins
The following C language built-ins are supported in C++:
• __builtin_constant_p,
• __builtin_expect.
3.6 GNU C++ asm statement
The GNU asm statement is supported as in the C compiler.
3.7 Standard Template Libraries (STL) support
This support is based on libstdc++-v3, you can consult the documentation online at the following site: http://gcc.gnu.org/onlinedocs/libstdc++/documentation.html
The very latest user documentation is available through: http://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-3.4/index.html
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The full C++ standard library is supported, excepted for the <complex> header and limited to the underlying C library support limitations (no wide char support), but augmented with the long long support.
The library includes the GNU STL extensions and the backward compatibility headers.
3.7.1 Standard headers status
All the following standard headers are supported, unless otherwise stated:
<algorithm>
<bits>
<bitset>
<cassert>
<cctype>
<cerrno>
<cfloat>
<ciso646>
<climits>
<clocale>
<cmath>
<complex>
<csetjmp>
<csignal>
<cstdarg>
<cstddef>
<cstdio>
Header files
<iomanip>
<ios>
<iosfwd>
<iostream>
<istream>
<iterator>
<limits>
<list>
<locale>
<map>
<memory>
<new>
<numeric>
<ostream>
<queue>
<set>
<sstream>
Table 22: STL standard header files
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<cstdlib>
<cstring>
<ctime>
<cwchar>
<cwctype>
<deque>
<exception>
<fstream>
<functional>
Header files
<stack>
<stdexcept>
<streambuf>
<string>
<typeinfo>
<utility>
<valarray>
<vector>
Table 22: STL standard header files
3.7.2 Supported extension headers:
All these extensions live in the ‘__gnu_cxx’ namespace (compared with the ‘std’ namespace of the standard library functions).
Note: Using these extensions can be useful, however, it may restrict portability, because not all compilers or libraries support these extensions.
<ext/algorithm>
<ext/functional>
<ext/hash_map>
<ext/hash_set>
<ext/iterator>
Header files
<ext/memory>
<ext/numeric>
<ext/rope>
<ext/slist>
Table 23: Supported extension headers
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3.7.3 Supported backward compatibility headers:
All the following backward compatibility headers are supported, unless otherwise stated.
Using these files directly is not recommended, the compiler warns about this usage as being deprecated. To remove this warning, use the -Wno-deprecated compiler option.
<algo.h>
<algobase.h>
<alloc.h>
<bvector.h>
<complex.h>
<defalloc.h>
<deque.h>
<fstream.h>
<function.h>
<hash_map.h>
<hash_set.h>
<hashtable.h>
<heap.h>
<iomanip.h>
<iostream.h>
<istream.h>
<iterator.h>
<list.h>
Header files
<map.h>
<multimap.h>
<multiset.h>
<new.h>
<ostream.h>
<pair.h>
<queue.h>
<rope.h>
<set.h>
<slist.h>
<stack.h>
<stream.h>
<streambuf.h>
<strstream.h>
<tempbuf.h>
<tree.h>
<vector.h>
Table 24: Backward compatibility headers
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RTTI notes 43
3.7.4 Identifying the STL version
The version supported is defined numerically through __GLIBCPP__ and as a string through _GLIBCPP_VERSION. With the current release, the value of __GLIBCPP__ is 20040214 and the value of _GLIBCPP_VERSION is 3.3.3. These versions are consistent with the C++ front-end used.
A simple way to show it is to compile and run the following program:
#include <cstdio> int main(int argc, char *argv[])
{
::printf("__GLIBCPP__ is %d __GLIBCPP_VERSION__ %is s\n",
__GLIBCPP__, _GLIBCPP_VERSION) ; return 0 ;
}
Note: In future releases, this preprocessor definition might be deprecated and replaced by
__GLIBCXX__ (and _GLIBCXX_VERSION).
3.8 RTTI notes
The rtti support is enabled by default, to disable it, use the -fno-rtti compiler option.
When rtti is enabled there is:
• support for the <typeinfo> header and the typeid operator.
• support for dynamic_cast.
If rtti is disabled:
• the compiler refuses to compile code containing calls to the typeid operator,
• code containing calls to dynamic_cast has undefined behavior (and typically aborts at run-time).
The limitation is, that as exceptions are not supported, the exception std::bad_cast is never thrown in case dynamic_cast fails, this is replaced by a call to the abort() library function.
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3.9 Exceptions notes
Exceptions are not supported. This implies that all C++ code must be compiled using the -fno-exceptions option. Be aware that even if your code does not use exceptions, the standard headers do, so you must use -fno-exceptions.
Note: The STL generally relies on exceptions. In the absence of exception implementation, the run-time support simply calls abort() when an exception is encountered. For
instance, a failure to allocate memory through the ‘new’ operator goes through a
__throw_bad_alloc function that in turns calls ‘abort’ when the exception support is disabled. The usual stack-unwinding process is not activated, so the objects that would be destroyed automatically through this process are not.
It is not possible to write exception specifications in function prototypes when the exception support is disabled, except for the empty specification clause throw().
It is possible to have a more explicit abort diagnostic by establishing a signal handler on the SIGABRT signal.
The compiler defines the preprocessor macro __EXCEPTIONS only when exceptions are enabled. When writing code to work both with and without exception handling support, use this macro to select the appropriate statements.
3.10 Checking operator new return value
When operator new is overloaded with a definition that may return 0, it must be declared throw() or the user must use the -fcheck-new compiler option, otherwise, unexpected control flow will be generated.
Consider the following example where operator new is declared throw():
#include <cstdio> namespace Tspace
{ class T
{ public: void *operator new(size_t n)
{ return 0; }
T() { ++ctor; } static int ctor;
}; int T::ctor = 0;
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} void surprise() { ::puts("UNEXPECTED\n") ; } int main(int argc, char *argv[])
{ using namespace Tspace ;
T *p = new T(); if (p != 0) surprise(); else { if(T::ctor != 0)
::puts("FAILURE") ; else
::puts("SUCCESS") ;
}
}
Using this example the following command:
$ st200c++ example.cxx outputs the warning: warning: ‘operator new’ must not return NULL unless it is declared
-output ‘throw()’ (or -fcheck-new is in effect) -output
Running the executable:
$ st220run -tsim a.out produces the following message:
FAILURE
Compiling the example with the -fcheck-new option and running the resulting executable:
$ st200c++ -fcheck-new example.cxx
$ st220run -tsim a.out produces the message:
SUCCESS
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3.11 Known limitations
3.11.1 Status of the known limitations
The known limitations are:
• no exception support,
• performance #pragmas are not recognized,
• no intrinsics support,
Known limitations
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Pragmas
4
4.1 Pragmas short description and syntax
Syntax
#pragma unroll
(unroll_amount)
#pragma ivdep
#pragma loopdep
PARALLEL|VECTOR|LIBERAL
#pragma loopmod(q,r)
#pragma looptrip(n)
#pragma pipeline(p,r)
#pragma loopseq READ |
WRITE
#pragma frequency_hint
NEVER|FREQUENT
Scope Description
Optimization level a
Start of a loop body
Start of a loop body
Start of a loop body
Start of a loop body
Start of a loop body
Start of a loop body
Start of a loop body
Unroll the loop unroll_amount times
Liberalizes dependence analysis
Liberalizes dependence analysis
Provides trip count modularity information
-O2
-O3
-O3
-O2
Provide trip count estimation information
-O2
Controls pipeliner
Ordering of the READ
(or WRITE) accesses
-O3
-O2
Execution frequency hint Any Applies to the function or statement that follows the pragma
Table 25: st200cc pragmas
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Pragmas short description and syntax
Syntax
#pragma ident “string” -
Scope Description
Optimization level a
Adds a .comment section to an assembly file.
Any
Marks a symbol as weak Any
Inlining b -O1
#pragma weak symbol
#pragma inline_next
(function)
#pragma noinline_next
(function)
#pragma inline_function
(function)
#pragma noinline_function
(function)
#pragma inline_file
(function)
#pragma noinline_file
(function)
#pragma defaultinline
(function)
-
-
Function call site
Function call site
Function
Function
File
File
Table 25: st200cc pragmas a. This column denotes the lowest optimization level for which the pragma has an effect.
For example -O0 means the pragma is applicable even when optimization is switched off. A list of optimization levels is given in
Section 2.2.9: Optimization options on page 16
.
b. All inlining pragmas are described in
Section 5.2.4: Inlining pragmas on page 63 .
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4.2 Loop optimization pragmas
4.2.1 #pragma unroll
This pragma suggests to the compiler the type of loop unrolling that should be done.
The pragma is a recommendation to the compiler to add n-1 copies of the loop body to the inner loop. The value of n must be at least 1. If it is 1, then unrolling is not performed.
If the loop that this pragma immediately precedes is an inner loop, then it implies standard inner loop unrolling.
Inner loop unrolling example: for (i=0; i < 10; i++)
#pragma unroll (2)
for (j=0; j < 10; j++)
a[i][j] = a[i][j]+b[i][j]; becomes: for (i=0; i < 10; i++)
for (j=0; j < 10; j+=2) {
a[i][j] = a[i][j] +b[i][j];
a[i][j+1] = a[i][j+1]+b[i][j+1];
}
If the loop that this pragma immediately precedes is an outer loop that contains only an inner loop, then outer loop unrolling, followed by the loop fusion of the resulting inner loops, is attempted. This transformation, known as unroll-and-jam, is especially useful to create parallel execution opportunities when the innermost loop alone does not present such opportunities.
Unroll-and-jam example:
// Ensure ad[] and sd[] do not alias.
#pragma unroll(2) for (i=0; i<16; i++) {
int sum = 0;
for (k=M; k<8+M; k++) {
sum += sd[k]*sd[k-i];
}
ad[i] = sum;
}
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50 Loop optimization pragmas becomes: for (i=0; i<16; i+=2) {
int sum0 = 0;
int sum1 = 0;
for (k=M; k<8+M; k++) {
sum0 += sd[k]*sd[k-i];
sum1 += sd[k]*sd[k-i-1];
}
ad[i] = sum0;
ad[i+1] = sum1;
}
The following tips provide information on how to control the desired inner loop unrolling with the pragma unroll value.
• A counted loop with a compile-time constant trip count is always fully unrolled if a pragma unroll with a value greater or equal to the loop trip count is specified.
• When a counted loop is not fully unrolled, the pragma unroll value is rounded to the greatest power of two lower than the specified unrolling value.
• The maximum size of a loop after unrolling is controlled by the command line option -OPT:unroll_size=<n>.
4.2.2 #pragma ivdep
This pragma instructs the compiler to liberalize dependence analysis between memory accesses. The #pragma ivdep applies only to inner loops. If it is used on a loop that has an inner loop, the compiler ignores it. By default, this pragma allows the compiler to assume there are no memory dependences between loop iterations.
The following command line options modify the ivdep semantic.
• -OPT:cray_ivdep=TRUE
Only ignore backward memory dependences (Cray semantics).
• -OPT:liberal_ivdep=TRUE
Also ignore all memory dependences in the same loop iteration.
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Example:
#pragma ivdep for (i = 0; i < n; i++) {
a[b[i]] = a[b[i]]+3; // These dependences cannot be computed by
// the compiler
}
4.2.3 #pragma loopdep
This pragma instructs the compiler to liberalize dependence analysis between memory accesses, based on the specified type of loop dependences. Contrary to the pragma ivdep described above, the semantics cannot be modified by command line options.
The loopdep pragma takes an argument to tell the compiler which kind of loop dependencies it can ignore, VECTOR, PARALLEL or LIBERAL.
#pragma loopdep VECTOR
#pragma loopdep VECTOR allows the compiler to assume there are no backward memory dependences between loop iterations. This pragma is equivalent to
#pragma ivdep, -OPT:cray_ivdep=TRUE .
Example:
#pragma loopdep VECTOR for (i = 0; i < n; i++) {
a[i] = a[i+k]+3;
}
In this example, the compiler cannot tell when a[i+k] does not depend on a[i], but this is in fact the case if k is always > 0 in the program. The pragma allows the compiler to assume there are no dependences between the read of a[i+k] in the current loop iteration, and the write of a[i] in the following loop iterations. The compiler could rewrite the loop as: for (i = 0; i < n; i+=2) {
t0 = a[i+k]+3;
t1 = a[i+1+k]+3;
a[i] = t0;
a[i+1] = t1;
}
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Loop optimization pragmas
#pragma loopdep PARALLEL
#pragma loopdep PARALLEL allows the compiler to assume there are no dependences between any two memory accesses that are in different loop iterations.
This pragma is equivalent to:
#pragma ivdep, -OPT:cray_ivdep=FALSE -OPT:liberal_ivdep=FALSE
Example:
#pragma loopdep PARALLEL for (i = 0; i < n; i++)
a[b[i]] = a[b[i]] + 3;
In this example, the compiler cannot tell that either the load or store of a[b[i]] in the current loop iteration does not depend on the load or store of a[b[i]] in a following loop iteration. This is in fact the case if b[i] != b[j] for all i != j. The compiler could rewrite the loop as: for (i = 0; i < n; i+=2) {
t1 = a[b[i+1]] + 3;
t0 = a[b[i]] + 3;
a[b[i+1]] = t1;
a[b[i]] = t0;
}
#pragma loopdep LIBERAL
#pragma loopdep LIBERAL allows the compiler to assume there are no dependences between any two memory accesses that are either in the same, or different, loop iterations. This pragma is equivalent to:
#pragma ivdep, -OPT:liberal_ivdep=TRUE
Example:
#pragma loopdep liberal for (i = 0; i < n; i++) {
a[j] = b[i];
c[i] = a[i] + 3;
}
In this example, the compiler cannot tell that the load of a[i] does not depend on the store of a[j]. This is in fact the case if i != j for all values of i and j in the loop iterations.
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4.2.4 #pragma loopmod
This pragma tells the compiler the number of times a loop is taken in terms of a multiple q and a residual r.
The syntax of this pragma is:
#pragma loopmod(q,r) where q is strictly a positive integer, r is a positive integer, r < q.
For example:
#pragma loopmod (4,0)
This tells the compiler that the loop is taken 4, 8, 12 .... times.
#pragma loopmod (4,1)
This tells the compiler that the loop is taken 5, 9, 13 .... times.
When applied to an inner loop, this pragma indicates that the trip count tc, that is the number of iterations that are executed by any execution of the loop can be written as: tc = p q + r with p > 0, r >= 0
Where p is strictly a positive integer. This information helps the compiler in loop unrolling optimization, and in software pipelining.
When unrolling loops, the compiler creates multiple loop bodies (the unrolling factor specifies the number of loop bodies created). However, the compiler cannot always statically determine the trip count. When it cannot determine the trip count, the compiler must also create residual code in case the unrolling factor is not a divisor of the loop trip count.
However, it is possible for application writers to know the modular properties of some of the loops in their own code. Bringing this accurate information to the compiler, the residual code can be largely removed or better optimized.
Note: Bringing inexact information on the trip count may lead to inexact code. Be careful that the property asserted is valid in all cases.
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The following example shows the use of the #pragma loopmod.
void copychar(unsigned char* __restrict p, unsigned char * q, unsigned int sz)
{
int i ;
assert(sz % 4 == 0) ;
#pragma loopmod(4,0)
for(i=0; i<sz; i++)
p[i] = q[i];
}
The function copychar duplicates a byte stream, whose size must be a multiple of
4. During unrolling, and without the pragma, the compiler would create a residual loop. This is totally removed when the pragma information is asserted. In this example, the pragma does not provide the compiler with any information about the memory alignment of p or q, which the compiler would need to generate word accesses after unrolling.
4.2.5 #pragma looptrip
This pragma instructs the compiler that the estimate of the number of the iterations of the loop (the loop trip count estimate) is n. This is not an assertion that the loop effectively iterates n times.
A number of optimizations are affected by the #pragma, when the compiler has not already determined the exact trip count:
• basic block frequency estimation uses this information as an approximation of the loop trip count,
• unrolling and cross-iteration optimizations are reduced if the given loop trip count estimate is low,
• software pipelining is limited if the estimate is low,
• automatic data prefetch generation is limited if the estimate is low.
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One scenario of usage is for ‘for’ loops with trip counts of unknown values where the user knows that the approximate effective value is low:
#pragma looptrip(4) for (i=0; i<n; i++) a[i] = b[i] ;
This example avoids non-beneficial optimizations. On such loops the compiler trip count estimate without the pragma is 100.
A second scenario is for ‘while’ loops where the user knows that the approximate effective trip count is high:
#pragma looptrip(100) while (*p++=*s++)
This example gives a better approximation of the weight of the loop. Generally the compiler trip count estimate for a ‘while’ loop is very low.
Note: Possible error messages are:
• Warning : pragma ‘LOOPTRIP’ : inconsistent with computed value, ignored
• Warning : pragma ‘LOOPTRIP’ : not followed by a loop, ignored
• Warning : malformed ‘#pragma looptrip (n)’
4.2.6 #pragma pipeline
This pragma is used to override pipelining and renaming defaults on a particular loop. This is a -O3 optimization and the defaults are: pipelining=3 and renaming=2 . With pipelining=3, the maximum performance software pipeline is built, with possible code increase. Using pipelining=1 and renaming=1 may reduce code size.
The meaning for pipelining is as follows:
• pipelining=0, no loop iteration overlap,
• pipelining=1, maximum overlap over 1 iteration,
• pipelining=2, maximum overlap over 3 iterations,
• pipelining=3, maximum overlap over 7 iterations.
The meaning for renaming is as follows:
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• renaming=0, no register renaming,
• renaming=1, register renaming enabled,
• renaming=2, local register modulo renaming.
4.2.7 Code generation pragmas
#pragma loopseq READ
#pragma loopseq WRITE
This pragma instructs the compiler that the memory READ accesses (or respectively the memory WRITE accesses) as they appear in the loop should be sequenced. This is not an assertion that the accesses must be kept in sequence, for instance, this is not a replacement for volatile accesses where it is mandatory to keep them in order.
The effect of this pragma is that the scheduler will serialize all load prefetch operations (or respectively all stores) in the loop. Thus the memory read (or write) accesses, as written in the C code are kept in order, as long as no aggressive transformation occurs in the loop.
A scenario is when the user wants to keep memory writes in order to take advantage of a combining write buffer:
#pragma loopseq WRITE for(i=0; i<n; i++) { a[i] = ...; a[i+1] = ... ; a[i+2] = ... ; a[i+4] = ... ;
}
The pragma hints that the compiler should keep writes to the array in order. If the loop is unrolled, generating a large number of stores, this improves locality and may take advantage of combining write buffers. By default the compiler does not put restrictions on the ordering of non-overlapping store operations.
A second scenario is when the user has scheduled by hand prefetch and load operations and wants to ensure that the compiler does not reorder them.
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#pragma loopseq READ for(i=0; i<n; i+=S) {
... = a[i] ;
__builtin_prefetch(&a[i+S]) ;
}
The pragma hints that the compiler should keep the load and prefetch in order. In this example, the prefetch is not placed before it is effectively used in the next iteration by the load.
4.2.8 Heuristic pragmas
#pragma frequency_hint
This pragma allows the user to specify information about the execution frequency for certain regions of code with the following frequency specifications:
NEVER
FREQUENT
This region of code is never or rarely executed. The compiler might move this region of the code away from the normal path.
This movement might either be at the end of the procedure or at some point to an entirely separate section.
This region of code is frequently executed. The compiler might try to put this region in the fall through path.
Example: if (debug) {
#pragma frequency_hint NEVER
trace();
}
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4.3 Miscellaneous pragmas
4.3.1 #pragma ident
Adds a .comment section in an assembly file.
4.3.2 #pragma weak
Marks a symbol as weak.
Instructs the link editor to not issue a warning if it does not find a defining declaration of the specified weak symbol. In which case the symbol is set to 0.
Allow the overriding of the current definition by a non-weak definition.
#pragma weak opt_handler extern void opt_handler (void); int main(int argc, char *argv[])
{
/ * If opt_handler has not been defined, the linker does not complain and the condition is false.*/
/* If opt_handler has been defined, the opt_handler is invoked.*/
if (opt_handler)
opt_handler();
}
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Optimization guide
5
5.1 Introduction
This chapter describes specific compiler options and techniques that can be used to gain maximum performance in your application.
5.2 Inlining
Inline function expansion is performed for function calls that the compiler estimates to be frequently executed. These estimations are based on a set of heuristics. The compiler might decide to replace the instructions of the call with code for the function itself (inline the call). The compiler supports both the single file inlining mode as described in
and cross file inlining through the IPA optimization described in
Section 5.9: Interprocedural analysis optimization (IPA) on page 86 .
5.2.1 Single file inlining
The purpose of this section is to make users aware of the underlying algorithms used to select functions to inline. First, it describes how possible candidates are selected for inlining, and how the selection is finalized, taking size conditions into account. Then, user-level compiler switches are listed, to show how the inlining process can be controlled.
The inlining decisions of the compiler can be observed with the -INLINE:list option, it is recommended that this option should be used when tuning inlining decisions.
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Inlining
There are two kinds of candidates for inlining: may-inline and must-inline functions.
May-inline functions are selected by the compiler according to the following conditions:
• function is declared with the inline C keyword,
• the functions not declared inline are may-inline candidates only if the
-INLINE:only_inline=off option is specified. In this case, the following categories are also may-inline candidates:
function is declared with the static C keyword, whose name is not weak, and whose address is neither passed nor saved,
finally, if the generated code does not follow the GP-relative model, the non-weak condition is sufficient. This large-span condition is included to add inlining opportunities in the current programming model of the ST200, where there is currently no support for shared objects. As a result, no function is preemptible, except by using the weak attribute.
Must-inline functions are specified by the user, through the command line option:
-INLINE:must=fn1,fn2,...
May-inline and must-inline functions are then checked against several criteria to decide whether to inline them or not.
Inlining criteria
Each candidate function is checked against inlining-exclusion cases which include:
• requires no-inlining by the user (-INLINE:never=fn, -INLINE:none command line options),
• recursive function,
• vararg function,
• exception handler.
After this preliminary test, each candidate function is inlined regardless of cost if it is marked must-inline, or if the -INLINE:all option has been specified by the user.
Otherwise, cost evaluation is used to decide whether to inline or not, and the candidate function is rejected if its estimated cost is above a given threshold set by the compiler. The -INLINE:list=on option can be used to list what is inlined.
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Inlining 61
Changing the compiler limits is not recommended, since this can lead to longer compilation times or increased memory usage or both, with no noticeable performance benefit.
Finally:
• the function to be inlined must be defined and visible in the same source file as the function using it,
• a static function that is inlined can be in specific circumstances considered
“dead”, and removed from the final object file.
5.2.2 st200cc inlining options
Table 26 specifies the options to control the stand-alone inlining.
More than one sub-option can be specified to the -INLINE: option either by using colons to separate each sub-option or by specifying multiple options on the command line. Some -INLINE: sub-options are specified with a setting that either enables or disables the feature. To disable a feature, specify the sub-option with either =OFF,
=FALSE or =0 (all these strings are case insensitive, for example
-INLINE:list=OFF ). To enable a feature, either use the option name alone (for example -INLINE:list) or any other string can be used on the right of the '=' sign
(as in -INLINE:list=all). It is generally recommended to use =ON, =TRUE, =1 for the sake of clarity (for example -INLINE:list=ON).
-inline
-noinline
-INLINE:(on|off)
-INLINE:all
Option
-INLINE:all_inline
Description
Enable inlining on inline functions. This is activated by default at optimization levels > 1.
Disable inlining.
Enable/disable inlining. Use of other -INLINE options implicitly set this to on.
Bypass cost evaluation when inlining 'may inline' functions. This option conflicts with none, and takes precedence if both are specified. Default is off.
Inline all functions marked by the C language inline keyword.
Table 26: Standalone inlining options
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Option Description
-INLINE:only_inline=(on|off)
-INLINE:static=(on|off)
-INLINE:aggressive=(on|off)
-INLINE:list=(on|off)
Default is on. Inline only functions marked by the
C language inline keyword. The
-INLINE:only_inline=off option is mandatory to allow inlining of non "inline" functions.
Default is off. Allow static functions to be candidates for inlining.
Inline even non-leaf, out-of-loop calls. Default is off.
List compiler actions. Default is off.
-INLINE:must=name1[,name2...]
-INLINE:specfile=filename
Always attempt to inline the names subroutines in addition to the default heuristic.
-INLINE:never=name1[,name2...] Never attempt to inline the names subroutines.
-INLINE:dfe Allow dead function elimination. Default is on.
Specifies a filename containing inlining options.
Default is none.
Table 26: Standalone inlining options
In addition to these options, the following option may be of interest when building a large body of inline functions (which is not recommended and may adversely affect performance).
Option Description
-OPT:0limit=[0..n] Functions larger than size n are not optimized.
Default is 3000. Specifying 0 removes any limit but may lead to a very long compile time.
Table 27: Option changing inlining behavior
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5.2.3 Extern inline functions
If both ‘inline’ and ‘extern’ are specified in a function definition, then the definition is used only for inlining. The function is never compiled on its own, not even if its address is referred to explicitly. The address becomes an external reference, as if the function had only been declared but not defined
This combination of ‘inline’ and ‘extern’ has almost the same effect as a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking ‘inline’ and ‘extern’) in a library file. The definition in the header file will cause most calls to the function to be inlined. If any instances of the function remain, they will refer to the single copy in the library.
5.2.4 Inlining pragmas
5.2.4.1 Introduction
The inlining process can be controlled within the C source code using #pragmas.
The st200cc compiler already supports several command-line options to configure its behavior, but it is not flexible enough. For instance, with the option
-INLINE:never=foo the user can disable the inlining of foo everywhere it is called; conversely, with -INLINE:must=foo the user can force inlining of foo everywhere.
The user has the ability to force inlining or non-inlining at call sites through the use of pragmas. In addition, the noinline and always_inline attributes can be used at function declaration.
5.2.4.2 Pragmas
To force inlining or non-inlining of a function in the scope of a call site, the following two pragmas are introduced:
• #pragma inline_next (foo,...) forces inlining of function foo in the next statement,
• #pragma noinline_next (foo,...) prevents inlining of function foo in the next statement.
The ... denotes that it is possible to provide several function names with the same pragma. It is equivalent to several pragma lines.
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Two similar pragmas are provided that can be used within the scope of a function:
• #pragma inline_function (foo,...) forces inlining of function foo every time it is called until the end of the current function,
• #pragma noinline_function (foo,...) prevents inlining of function foo every time it is called until the end of the current function.
The two call site scope pragmas take precedence over these two function scope pragmas.
Two lower priority pragma are provided, with file scope:
• #pragma inline_file (foo,...) to force inlining of function foo every time it is called until the end of the current source file,
• #pragma noinline_file (foo,...) to prevent inlining of function foo every time it is called until the end of the current file.
Finally, to revert inlining policy to the default one (that is, rely on the inliner’s evaluation of callee weight), the following pragma is introduced:
#pragma defaultinline (foo,...)
5.2.4.3 Function naming
As a special case, if the user does not provide any function name, the corresponding pragma applies to all functions called in the scope of the pragma. In this case, parentheses around the function names are optional.
5.2.4.4 User diagnostics
Several warning messages are provided to the user to help track errors.
If two conflicting pragmas are provided only the later is taken into account. For instance,
#pragma inline_next (foo)
#pragma noinline_next (foo) foo(); leads to: warning: #pragma noinline_next (foo) overrides previous #pragma inline_next (foo)
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If pragmas are provided at an invalid scope (that is outside of a function), the following message is displayed: warning: #pragma noinline_function (foo) ignored (incorrect scope)
To help track misspelling, a warning is also displayed if a pragma could not be applied to any function call:
#pragma noinline_next (bar) foo(i); leads to: warning: #pragma noinline_next (bar) matched no call
5.2.4.5 noinline and always_inline attributes
In order to enable the user to inhibit inlining of one function wherever it is called, the noinline attribute is introduced, and is used at the function declaration level.
Conversely, to enable the user to force inlining of one function wherever it is called, the always_inline attribute is introduced.
5.2.4.6 Precedence
Command-line options -INLINE:must=foo and -INLINE:never=foo take precedence over both pragmas and attributes.
Attributes take precedence over pragmas. That is, a function declared with
__attribute__((noinline)) is never inlined, regardless of pragma inline_xxx statements. However, the user can override this behavior with the
-INLINE:must=foo command-line option.
If several contradictory pragmas with same scope apply to the same function, the last one overrides the other ones.
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5.2.4.7 Examples
Example 1
This example illustrates the use of the #pragma noinline_next directive. All calls to f1() are candidates for inlining, except the one directly following #pragma noinline_next .
int ig = 0; inline void f1(int i) {ig += i;} void main()
{ f1(1); // f1 is candidate for inlining
#pragma noinline_next (f1) f1(2); // f1 is not marked for inlining f1(3); // f1 is candidate for inlining printf("result is %d\n", ig);
}
Example 2
This example illustrates the use of the #pragma inline_function directive. All calls to f1() following the #pragma inline_function (f1) directive are forced to be inlined, except the one directly following #pragma noinline_next (f1).
The call to f2() following the #pragma inline_next (f2) is also forced to be inlined, while the first call to f2() is only a candidate for inlining (inlining depends on the respective weights of f2() and its caller).
int ig = 0; int jg = 0; inline void f1(int i) {ig += i ;} inline void f2(int i) {jg += i ;} void main()
{
#pragma inline_function (f1) f1(1); f2(1);
// f1 is forced to be inlined
// f2 is candidate to inlining
#pragma noinline_next (f1) f1(2); // f1 is not marked for inlining
#pragma inline_next (f2) f2(3); // f2 is forced to be inlined f1(3); // f1 is forced to be inlined printf("result is %d %d\n", ig, jg);
}
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Example 3
This example illustrates the use of the #pragma defaultinline directive.
int ig = 0; int jg = 0; inline void f1(int i) {ig += i ;} inline void f2(int i) {jg += j ;} void main()
{
#pragma noinline_function (f1) f1(1); f2(1);
// f1 is not marked for inlining
// f2 is candidate to inlining
#pragma inline_next (f1) f1(2); // f1 is forced to be inlined
#pragma noinline_next (f2) f2(3); // f2 is not marked for inlining
#pragma defaultinline (f1) f1(4); // f1 is candidate to inlining printf("result is %d %d\n", ig, jg );
}
Example 4
This example illustrates the use of several function names or an empty name list with #pragma directives.
#pragma noinline_file () int f(int i) { return i+1; } int g(int i) { int j=i+f(i); // f is not marked for inlining
#pragma inline_next (f,g) j += f(i); j += f(i) + f(i); return j;
// f is forced to be inlined, g is ignored
// f is not marked for inlining
} int h(int i) {
#pragma noinline_next () int j=i + f(i) + g(i);
#pragma inline_next (f,g) j+=i + f(i) + g(i); return j;
}
// f and g are not marked for inlining
// f and g are forced to be inlined
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Inlining void main()
{
// g and h are not marked for inlining printf("result is %d %d\n", g(0), h(0));
}
Example 5
This example illustrates the use of the noinline attribute and shows how the attribute has precedence over #pragma.
#pragma inline_file(f3) int ig = 0; void __attribute__ ((noinline)) f3(int i) { ig += i ; } int main()
{ f3(1); // f3 is not marked for inlining
#pragma inline_next(f3) f3(2); // f3 is not marked for inlining
#pragma defaultinline (f3) f3(3); // f3 is not marked for inlining printf("result is %d\n", ig);
}
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Memory dependences in C programs 69
5.3 Memory dependences in C programs
Precise memory dependencies analysis is key to compilation optimization, since it enables the compiler to more freely schedule load instructions above store instructions. By default, a C compiler assumes that any pair of memory accesses which reference distinct types are not aliased (that is, memory dependent).
However, real world cases almost always involves pointers to the same types that are actually un-aliased: the compiler cannot generally deduce this property and must rely on additional information. This effect can be achieved either through the
C language restrict keyword, or with compiler option:
-OPT:alias=value where possible values are listed in
any typed value unnamed restrict disjoint
Description
The default. Any pair of memory accesses may be aliased.
Any pair of memory accesses that reference distinct types are not aliased.
Assume pointers never point to global objects.
Assume that different pointers never point to the same area
Assume multiple pointer indirection never overlap.
Table 28: Possible value to the -OPT:alias option
Although the compiler is able to compute precise memory dependences in many cases, this is not possible when complex memory accesses are involved, such as in the following example: for (i = 1; i < n; i ++) {
a[i-1] = a[i] + b[i];
} for (i = 1; i < n; i ++) {
c[d[i]] = c[i] + 1;
}
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Memory dependences in C programs
On the first loop, the compiler can fully determine the dependences between memory accesses, provided that it knows that a and b point to distinct memory locations (see the C language restrict qualifier). On the second loop however, without information on values in d, the compiler assumes that all memory accesses in the loop are dependent. In particular, the sequence of load and store memory accesses in the iterations of the loop must be strictly respected, resulting in a poor instruction schedule if the loop is unrolled or software pipelined.
A useful property for loop optimizations is when a loop is vectorizable. This property can be enforced on a loop by using the #pragma loopdep VECTOR. A vectorizable loop is such that it can be decomposed into a sequence of loops, one per statement of the original loop, without changing the program results. Moreover, for each loop resulting from that decomposition (that contains only one statement), all load memory accesses can be performed before all store memory accesses, which means that a vector version of the loop can be written. In practice, unless the target processor is a real vector processor, the compiler does not decompose vectorizable loops as described. Rather, it uses the vectorizable property of the original loop to remove dependences between memory accesses.
In the example above, the first loop is vectorizable, provided that a and b do not overlap. The second loop is also vectorizable if the assertion (d[i]<=i) holds for all i.
Another useful property for loop optimizations is when a loop is parallelizable. This property can be enforced on a loop by using the #pragma loopdep PARALLEL. A parallelizable loop is one for which, memory accesses that reference a given memory location may occur only in the same iteration of the loop. As a result, the sequence of memory accesses of the original loop can be changed in any way that preserves the relative order of memory accesses originating from the same loop iteration. Note that a parallelizable loop is always vectorizable, so the #pragma loopdep PARALLEL is stronger (but less generally applicable) than the #pragma loopdep VECTOR .
In the example above, the first loop is not parallelizable. The second loop is parallelizable if the assertion (d[i]==i) holds for all i.
The last useful property for loop optimizations is when a loop is liberal. This property can be enforced on a loop by using the #pragma loopdep LIBERAL. A liberal loop is one where all its memory accesses reference unique memory locations.
As a result, all the memory accesses in the loop can be freely reordered. Note that a liberal loop is always parallelizable, so the #pragma loopdep LIBERAL is stronger
(but less generally applicable) than the #pragma loopdep PARALLEL.
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Aliasing rules in C/C++ programs 71
In the example above, the second loop is liberal if the assertion:
(d[i]<1 || d[i]>=n) holds for all i. (For clarity, we omitted this case for the
VECTOR and PARALLEL pragmas.)
The restrict qualifier, which applies to pointers or arrays in a C program, is also highly useful to remove dependences between memory accesses inside and outside loops. The restrict property states that two memory accesses originating from different pointers or arrays cannot reference the same memory location, when at least one of the pointers or array has the restrict qualifier. Please note that all memory accesses based on a given restrict pointer or array are still assumed dependent, unless it is obvious to the compiler that they are not, or there is a
#pragma loopdep on the loop that applies to these dependences.
5.4 Aliasing rules in C/C++ programs
The -fstrict-aliasing option allows the compiler to assume the strictest aliasing rules applicable to the language being compiled (the aliasing rules are stated in clause 6.5 (7) of the ISO/IEC Standard (Expressions)).
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 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.
By default, the -fno-strict-aliasing option is used, for legacy reasons.
However, in future releases, the -fstrict-aliasing will be used by default for -O optimization levels.
Note: Particular attention is required before reporting any compiler issue when using
-fstrict-aliasing , specifically when code runs correctly with the default
compiler, but diverges when -fstrict-aliasing is used. This is often caused by a violation of aliasing rules, which are part of the ISO C/C++ standard. These rules say that a program is invalid if you try to access a variable through a pointer of an incompatible type.
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Aliasing rules in C/C++ programs
The example shown in
Figure 2 demonstrates this violation, where a float is
accessed through a pointer to integer:
#include <stdio.h> int main(int argc, char *argv[])
{
float a = 0.0f ;
int *pa = (int *)&a ;
*pa = 0x40000000; /* violation of aliasing rules */
if (a != 0.0f)
puts("LEGACY BEHAVIOUR") ;
else
puts("STRICT ALIASING BEHAVIOUR") ;
return 0;
}
Figure 2: Aliasing example source
The aliasing rules were designed to allow compilers to perform more aggressive optimization. Basically, a compiler can assume that all changes to variables happen through pointers or references to variables of a type compatible to the accessed variable. De-referencing a pointer that violates the aliasing rules results in undefined behavior.
In the case above, the compiler may assume that no access through an integer pointer can change the float a. Thus, the actual value of a may be unaffected by the writing through pa. What really happens is up to the compiler and may change with architecture and optimization level.
To disable optimizations based on alias-analysis for ‘faulty legacy code’, the option
-fno-strict-aliasing must be used as a work-around.
Note: Because the practice of reading from a different union member other 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, to fix the code above, you can use a union instead of a cast, as shown in
Note: This is a GCC extension which might not work with other compilers.
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Aliasing rules in C/C++ programs 73
#include <stdio.h>
/*
According to GNU documentation, this code should work in
both strict and non-strict aliasing rules
*/ int main(int argc, char *argv[])
{ union { float f ; int i;
} u; u.f = 0.0f ; u.i = 0x40000000 ; /* is 2.0f */ if (u.f != 2.0f) puts("NON-GNU BEHAVIOUR") ; else puts("GNU ALIASING BEHAVIOUR”) ;
return 0;
}
Figure 3: Aliasing example, using a union
Now the result is always "GNU ALIASING BEHAVIOUR".
Finally, to fully respect the ANSI C/C++ aliasing rules, it is necessary to write the data through a character type before reading it again, see
and
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Aliasing rules in C/C++ programs
The drawback of this standard conforming solution is that it has to account for endianness, and that it is less efficient than simply writing through an integer:
#include <stdio.h>
/*
According to ANSI standard, this code should work in
both strict and non-strict aliasing rules
*/
#include <stdio.h>
#define EXTRACTBYTE(val, pos) (((val) >> (pos*8)) & 0xff) int main(int argc, char *argv[])
{
union
{
float f ;
char c[4] ;
} u;
const unsigned int twoasint = 0x40000000 ;
u.f = 0.0f ;
#if defined(__BIG_ENDIAN__)
u.c[0] = EXTRACTBYTE(twoasint, 3) ;
u.c[1] = EXTRACTBYTE(twoasint, 2) ;
u.c[2] = EXTRACTBYTE(twoasint, 1) ;
u.c[3] = EXTRACTBYTE(twoasint, 0) ;
#elif defined(__LITTLE_ENDIAN__)
u.c[0] = EXTRACTBYTE(twoasint, 0) ;
u.c[1] = EXTRACTBYTE(twoasint, 1) ;
u.c[2] = EXTRACTBYTE(twoasint, 2) ;
u.c[3] = EXTRACTBYTE(twoasint, 3) ;
Figure 4: Aliasing example, writing through a character type, page 1 of 2
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#else
#error "Unknown endianness : please define either
__BIG_ENDIAN__ or __LITTLE_ENDIAN__"
#endif
if (u.f != 2.0f)
puts("UNEXPECTED BEHAVIOUR") ;
else
puts("ANSI ALIASING BEHAVIOUR") ;
return 0;
}
Figure 5: Aliasing example, writing through a character type, page 2 of 2
In this case, the program always prints “ANSI ALIASING BEHAVIOR” regardless of the compiler and its optimization options.
5.5 Data prefetching
The ST200 processor includes a data prefetching mechanism through a PFT instruction. Prefetching data ahead of its actual use, enables some of the data cache miss penalties to be hidden. Data cache miss cycles often account for a significant fraction of execution cycles.
The st200cc/st200c++ compiler includes support for user and automatic data prefetch instructions:
• User data prefetch instructions are inserted in the C/C++ code using a
__builtin__prefetch builtin.
• Automatic data prefetch is generated by the compiler at optimization level -O3, using specific command line options described in the following sections.
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5.5.1 Manual data prefetching void __builtin_prefetch (const void *addr)
You can insert calls to __builtin_prefetch into code, for which you know addresses of data in memory are likely to be accessed soon, in order to generate a data prefetch instruction.
The value of addr is the address of the memory to prefetch.
Data prefetch does not generate faults if addr is invalid, but the address expression itself must be valid. For example, a prefetch of p->next does not fault if p->next is not a valid address, but evaluation does fault if p is not a valid address.
The following option controls whether the __builtin_prefetch is accounted for, this can be useful to measure the effect of the insertion of the builtin:
The -m[no-]builtin-prefetch option disables or enables the
__builtin_prefetch builtin throughout the compilation. For example, with
-mno-builtin_prefetch the compiler allows calls to __builtin_prefetch in the source code, however, they are ignored.
5.5.2 Automatic data prefetching
Automatic prefetching may be enabled by the command line option:
-m[no-]auto-prefetch
Note: 1 This option is only effective when optimization level -O3 is selected.
2 Automatic prefetching is disabled by default.
Automatic prefetching inserts prefetch instructions for array references inside loops. In order to generate a prefetch for a memory reference the compiler may need to generate the prefetch on an earlier iteration of the loop to the memory reference.
To calculate the address that is used in the later iteration the compiler requires the difference between the load address and the prefetch address to be an integer constant.
For example, given the following code: for (i=0; i < 10; i++) {
....
load array [i]
}
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In
, on the i th iteration we want to load array[i] and eight iterations later we will want to load array[i+8]. To speed up the load of array[i+8] the compiler prefetches array[i+8] while it loads array[i]. This is possible because array[i+8] is a known constant distance from array[i].
array i+8 i th
+8 iteration, load [i+8] k i i th
iteration, load i, prefetch i+8
[i+8]=[i]+k , where k is a constant
Figure 6: Pre-fetching on earlier iteration of loop
The resulting compiled code has the prefetch instruction added: for (i=0; i < 10; i++) {
....
load array [i] prefetch array [i+8]
}
The compiler can handle more complex cases than this but in all cases the distance between the load address and the prefetch address (distance k in
a known constant
1
.
1. Mathematically the load address must be a linear function of the enclosing loop iteration variables.
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The following code gives a further example of prefetching: extern int A[160]; int foo(void)
{ int i; int sum = 0; for (i = 0; i < 160; i++) sum += A[i]; return sum;
}
The compiler’s loop unrolling optimization first unrolls this by a factor of 8 to give: extern int A[160]; int foo(void)
{ int i; int sum = 0; for (i = 0; i < 160; i+=8) { sum += A[i]; sum += A[i+1]; sum += A[i+2]; sum += A[i+3]; sum += A[i+4]; sum += A[i+5]; sum += A[i+6]; sum += A[i+7];
} return sum;
}
After the unrolling, the compiler inserts prefetch operations to give code equivalent to: extern int A[160]; int foo(void)
{ int i; int sum = 0; for (i = 0; i < 160; i+=8) { sum += A[i]; sum += A[i+1]; sum += A[i+2];
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__builtin_prefetch(&A[i+16]);
} return sum;
}
5.5.3 Advanced data prefetching options
By default, the prefetch instructions inserted read two cache lines ahead of the current array reference. This behavior may be adjusted by using the command line option:
-LNO:prefetch_ahead=n where n is the number of cache lines to read ahead. For example:
-LNO:prefetch_ahead={0,50}[2] where {0,50}[2] means that the -LNO:prefetch_ahead option expects an integer argument whose range is in the [0,50] interval, for which the default value is 2.
Alternatively, it is possible to specify the number of loop iterations to prefetch ahead using the command line option:
-LNO:prefetch_iters_ahead=n.
which makes the compiler prefetch array references for the loop n iterations after the current iteration.
-LNO:prefetch_iters_ahead={0,50}[2]
Caution: The compiler may also automatically unroll the loop: the value of n specifies the number of loop iterations before this automatic unrolling.
Automatic prefetching is not performed upon arrays that have the built-in function
__builtin_prefetch applied to them.
For example, the option:
-LNO:prefetch_iters_ahead={0,50}[2]
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The option -LNO:clean_miss_penalty={0,1000000000}[140] controls the number of cycles between the issue of a prefetch instruction and the availability of the data in the prefetch buffer.
The option -LNO:dcache_prefetch_buffers={0,1000}[8] controls the number of prefetch buffers available on the processor. These prefetch buffers are used to store prefetched data before a memory access move it to the data cache.
5.6 Profiling
Before optimizing any application, it is recommended that you analyze the critical areas of your code.
Profiling creates an instrumented program from your source code. Each time this instrumented code is executed, the instrumented program generates an information file that can be later displayed with the st200gprof tool.
This section is not a complete guide to profiling, but a quick refresher on how to proceed with the compiler. More details about using gprof can be found at http://www.gnu.org/software/binutils/manual/gprof-2.9.1/gprof.html
.
Example:
#st200cc -O2 -pg *.c -o myexe
After first run, a file gmon.out.000 is generated. It can be viewed with st200gprof with the following command:
#st200gprof myexe gmon.out.000
After each run in the same directory, the file name suffix part of the gmon.out.000 is incremented, so that profile information for the new run is available as gmon.out.001
, and so on.
The symbolic information available in the profile information can be augmented by using the st200cc -g option when compiling the source code.
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5.7 Call trace instrumentation
This section describes the use of the -finstrument-functions and
-minstrument-calls options.
5.7.1 Instrumenting functions: -finstrument-functions
The -finstrument-functions option provides standard GCC functionality.
Using this option generates instrumentation calls for entry and exit to functions.
Just after function entry and just before function exit, the following profiling functions will be called with the address of the current function and its call site: 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 specifically in the symbol table.
• The second argument is the address of the call site from where the current function was invoked. It corresponds to an address in the range of the caller function addresses that may be found in the symbol table of the executable.
The functions that are inlined by the compiler are not instrumented.
To force instrumentation of all functions use the -fno-inline option to disable inlining.
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).
The program must be linked with an object file that implements the two functions above to link correctly.
5.7.2 Instrumenting call to functions: -minstrument-calls
The -minstrument-calls is not a standard GCC option.
Using this option generates instrumentation calls just before, and just after each function call.
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Call trace instrumentation
The following profiling function is called with the address of the caller function and the address of the callee function: void __profile_call ( void *caller_fn, void *callee_fn, const char *caller_name, const char *callee_name, int event);
• The first argument is the address of the start of the current function (the caller function), which may be looked up specifically in the symbol table.
• The second argument is the address of the start of the called function (the callee function), which may be looked up specifically in the symbol table.
• The third argument is the name of the caller function.
• The fourth argument is the name of the callee function, or NULL if the call is an indirect call.
The function names passed in the third and fourth arguments are pointers to static strings that have the lifetime of the instrumented executable or shared object.
The function names are the mangled names in C++.
• The fifth argument is 0 when this function is invoked just before a call, instrumenting a function entry. It is 1 when this function is invoked just after a call, instrumenting a function exit.
Function calls that are inlined by the compiler are not instrumented.
To force instrumentation of all functions use the -fno-inline option to disable inlining.
A function may be given the attribute no_instrument_function, in which case this instrumentation is not done if the caller or the callee function has the attribute no_instrument_function .
The program must be linked with an object file that implements the function above to link correctly.
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The main differences with the -finstrument-functions option are:
• this instrumentation tracks (caller, callee) address pairs instead of (call_site, callee) address pairs. If the call site information is required use the
-finstrument-functions option,
• this instumentation provides the caller and callee name when available, which avoids a specific post processing pass to retrieve the function names,
• this instrumentation is at the call site and not in the callee, thus for instance calls to top level library functions (which are not instrumented) are seen while
-finstrument-functions do not see them, to disable the instrumentation of the call to a particular library routine you must declare it with the no_instrument_function attribute,
• this instrumentation is not standard GCC functionality.
5.8 Profiling feedback optimization (PFO)
5.8.1 Principles
The st200cc profiling feedback model is a three step process.
• Instrumentation of C/C++ sources.
• Execution of the binary on sample input, generating files of feedback data. The run can be executed either on a simulator or on hardware.
• Feedback annotation of C/C++ at the same point as in the first compilation.
Sources are enhanced with feedback data that are used and refined during the compilation process.
st200cc feedback instruments the input sources early in the compiler back-end.
Consequently, feedback annotations are almost independent from optimization levels.
5.8.2 Command line
The st200cc command line option -fb_create <name> is used to generate an instrumented executable program. This can be used to produce one or more
name.instr0.##### files for subsequent feedback compilation.
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When the -c option is used to produce an object file (that is eventually linked to form an instrumented executable), the -fb_create option should also be specified at link time. If different names are specified, the one used to compile the main file
(where the main procedure is) will be used.
The executable is instrumented with special instructions that generate information that the compiler can use to better optimize your program.
When this instrumented executable is run (usually with a representative training input set) that information is stored in a unique file called name.instr0.#####.
An unique file is generated for each execution.
The st200cc command line option to specify the feedback files to be used is -fb
<name> . All files with the name name.instr0.###### will be combined.
5.8.3 Example
Step 1
Instrument the program to set it up for running a test set with profiling tests enabled. To do this, compile the program with the -fb_create name. This option inserts code into the program so it can track which spots of the code are used often when the program is run (for example, how many times a branch is taken or how many times a loop iterates). The library libinstrC.a is automatically linked with the program.
$ st200cc -fb_create fbtest -o ftest ftest.c
Step 2
Run the program with some sample input. When the program executes, statistics are generated and written to files for later use. The instrumented program runs significantly slower than usual. This creates a file of frequency data, named fbtest.instr0.###### . The 0 in instr0 indicates that instrumentation occurred before Very High Optimization (VHO) lowering which is the very first optimization of the back end compilation process. Eventually, it should be possible to instrument at a variety of stages, but this is not activated for the moment. Multiple runs can be performed using different input data to generate multiple frequency data files. Runs can be executed either on simulators or hardware, the only restriction is to have access to the host file system.
$ st200run -tsim -- ./ftest
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Step 3
Re-compile the program with the -fb name option. This tells the compiler that it has some sample statistics from which it can optimize code better. It then uses these statistics to compile a new program that is better tuned for the input. Feedback data will be read and combined from all files with names fbtest.instr0.######.
Note: It is necessary to delete obsolete feedback data files (for instance if the code is changed) or use a different feedback file name (for instance to evaluate enhancements on various data sets).
$ st200cc -fb fbtest -o ftest ftest.c
5.8.4 What feedback does
Feedback collects frequency counts on control flow such as branches, loops, and procedure calls. Memory accesses are not counted.
During the instrumentation step, the compiler inserts extra code into the original code. The inserted code consists of procedure calls to libinstrC.a routines which gather control flow data, such as the number of times a branch is taken or not taken, how many times a loop iterates, or how many times procedures are invoked.
When the instrumented binary is invoked on the test data, the program performs as normal, except that the libinstrC.a routines collect control flow frequencies and store the counts into a file when the program completes.
During the annotation step, the compiler reads the frequency data from any available feedback data files and attaches the data to the code. Annotation occurs at the same point of compilation that instrumentation occurred.
Later phases of the compiler (for instance instruction cache optimization, if-conversion and code generation) can use the data to guide optimization decisions.
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Interprocedural analysis optimization (IPA)
5.9 Interprocedural analysis optimization (IPA)
5.9.1 Introduction
The -ipa option enables interprocedural analysis. The compiler identifies optimization opportunities across module boundaries. It does this by extending the scope that is examined during optimization and inlining from a single module to multiple modules.
The -ipa option must be included in both the compile and link phases.
The major benefits of IPA are:
• interprocedural constant propagation,
• interprocedural alias analysis,
• inter-module inlining.
A more advanced use of IPA is function specialization (cloning).
5.9.2 Using IPA
The only necessary option to trigger the IPA compilation is -ipa.
No fundamental Makefile changes are necessary to the usual build process, as the compiler driver takes care of all the necessary phases.
The compilation time and the link time is longer as much of the optimization work is driven from the linker. This can be observed by using the -v compiler option.
The following steps are performed when building an executable in IPA mode:
• the .c files are translated into special .o files,
• the .o files are merged together (code, symbol table),
• the .o files are analyzed and optimized,
• the final link is performed.
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5.9.3 IPA command line options
The following table describes advanced IPA options.
Option Description
-IPA:aggr_cprop=ON|OFF
-IPA:cgi=ON|OFF
-IPA:cprop=ON|OFF
-IPA:depth=n
-IPA:dfe=ON|OFF
-IPA:dve=ON|OFF
-IPA:forcedepth=n
-IPA:inline=ON|OFF
-IPA:keeplight=ON|OFF
Enables or disables aggressive inter-procedural constant propagation. Attempts to avoid passing constant parameters, replacing the corresponding formal parameters by the constant values. The default in ON.
Enables or disables constant global variable identification.
This options marks non-scalar global variables which are never modified as constant, and propagates their constant values to all files. The default is ON.
Enables or disables inter-procedural constant propagation. This option identifies formal parameters which always have a specific constant value. The default is ON. See also -IPA:aggr_cprop.
This option is identical to maxdepth=n
Enables or disables dead function elimination. This option removes subprograms which are never called from the program. The default is ON.
Enables or disables dead variable elimination. This option removes variables which are never referenced from the program. The default is ON.
Sets inline depths. Instead of the default inlining heuristics, this option directs IPA to attempt to inline all functions at a depth of (at most) n in the call graph, where functions which make no call are at depth 0, those which call only depth 0 function are at depth 1, and so on. This ignores the default heuristic limits on inlining.
Performs inter-file subprogram inlining during main IPA processing. The default in ON.
Directs IPA to not send -keep to the compiler, in order to save space. The default is ON. Setting it to OFF will leave intermediate files in a directory which has the name of the final executable suffixed with .ipakeep.
Table 29: Advanced IPA options
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Option
-IPA:maxdepth=n
-IPA:multi_clone=n
-IPA:node_bloat=n
-IPA:plimit=n
-IPA:space=n
-IPA:specfile=filename
Description
Directs IPA to not attempt to inline functions at a depth of more than n in the call graph, where functions which make no call are at depth 0, those which call only depth 0 functions are at depth 1, and so on. Inlining remains subject to overriding limits on code expansion. See also forcedepth , space and plimit.
Specifies the maximum number of clones that can be created from a single procedure. By default, this value is 0. Aggressive procedure cloning may provide opportunities for inter-procedural optimization, but it also may significantly increase the code size.
When used in conjunction with -IPA:multi_clone, this specifies the maximum percentage growth of the total number of procedures relative to the original program.
Stops inlining into a particular subprogram once it reaches size n in the intermediate representation. The default is 2500.
Stops inlining when the program size has increased by n %. For example, n=20 limits code expansion due to inlining to approximately 20%. The default is 100%.
Opens filename to read more options. A spec file contains zero or more of the options allowed by IPA.
Table 29: Advanced IPA options
5.9.4 Limitations
The IPA optimization is not compatible with the -g compiler option.
The IPA optimization is currently not supported with the PFO optimization (see
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5.10 Symbol visibility specification
Command line options are provided to control symbol visibility.
These options are useful in the context of position independent code generation(-fpic), where symbol visibility has a significant impact on code performance and the dynamic symbol table size.
Two options are provided:
• -fvisibility=[default|protected|hidden|internal]: this option specifies the default visibility to apply to symbols defined in the current source file.
Note: This does not set the visibility of symbols declared externally.
• -mvisibility-decl=<file> : this option specifies a visibility specification file that the compiler can use to determine the visibility for a symbol declaration or defintion.
These options are useful to optimize a shared object DSO or a relocatable library as defined by the ST200 relocatable library model described in the ST200 Cross
Development Manual.
5.10.1 Introduction
In the context of -fpic code generation, symbol visibility has a significant impact on code generation.
In addition to GP relative addressing which is an invariant of PIC code, some other code generation overheads appear depending on the symbol visibility.
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These overheads are summarized in
:
Overhead
Default visibility
Yes
Protected visibility
Yes
Hidden visibility
Yes
Internal visibility
No f The GP value must be set up on entry to the function symbol as the function can be called from outside, this generates a specific sequence in the prologue of the function.
A pointer to the symbol (function or data) is taken from the GOT entry, generating a load instead of a single address computation and some additional data for the GOT entry.
Inlining of the function symbol is not possible as the symbol is preemptible
A call to the function symbol goes through a stub function in the PLT, generating one additional jump to the stub function and some additional code for the stub function.
The symbol appears in the dynamic symbol table, this augments the size of the loadable data segment.
Yes
Yes
Yes a
Yes
Yes
No b
Yes
Yes
No c
No
No d
No e
No
No
No
No
Table 30: Visibility overheads a. This overhead can be removed by some link time options and by the use of linker version scripts. These link time options are not used by the compiler to optimize the code.
b. Inlining of the function symbol is possible as the symbol is not preemptible.
c. A pointer to the symbol can be built from a GP relative address computation as the symbol is local to the library.
d. A call to the function symbol is direct as the symbol is local to the library.
e. The symbol does not appear in the dynamic symbol table as it is local to the library.
f. There is no need to set up the GP value as the function was called from another function inside the same library, and thus the GP value is already set.
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Internal visibility has no remaining overheads.
To support visibility declarations the current st200cc compiler implements:
• visibility attribute GNU extension
1
.
• linker option -Bsymbolic which has the same effect as protected visibility but at link time only.
• linker version scripts that allow the user to explicitly list the symbol that is required to be exported globally, thus reducing the dynamic symbol table size.
The following sections give example usage of the -fvisibility and
-mvisibility-decl options.
5.10.2 Usage of the -fvisibility option
This option is mainly used to declare by default all symbols as having hidden visibility.
The advantage of this is that the user has to declare by using attributes the functions or data that he explicitly wants to be global.
For instance if the interface for a library is reduced to three functions (initialize, process and finalize) the user can enable better code generation by including the config.h
file (see
Figure 7 ) in its sources and using the -fvisibility=hidden
option at compile time.
91
#define VISIBILITY(v) __attribute__((visibility(#v))) extern void initialize(void) VISIBILITY(protected); extern void process(void) VISIBILITY(protected); extern void finalize(void) VISIBILITY(protected);
Figure 7: config.h
1.The __attribute__((visibility(...))) GNU extension does not apply to static symbols as ELF defines visibility only for global symbols. This is unfortunate as the compiler can take advantage of the fact that a static function is internal.
Though the extensions allow static symbols to be declared internal.
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This informs the compiler that all functions or data defined for the compilation of the library are hidden, except those explicitly declared protected in the config.h file.
Without the -fvisibility option the user would have to attribute all of the functions definition with the VISIBILITY(hidden) macro.
Note: The -fvisibility=hidden option has some limitations. It applies only to function definitions, not to declarations. Thus, if a file that is part of the library references a symbol in another file in the same library, the compiler still generates GOT accesses, as it does not have the information that the definition is effectively in the library.
To remove the above limitation, the user must use the -ipa option that enables the compiler to consider all files at once and thus effectively apply the code generation optimizations.
5.10.3 Usage of the -mvisibility-decl option
The main usage of the -mvisibility-decl option is to avoid modifying the source code by adding explicit visibility attributes to function and data, declarations and definitions.
For instance, this effect with the -fvisibility=hidden option can be achieved without declaring the visibility attribute in the header file. The file mylibrary.v
(see
) can replace the config.h file. The user then compiles with the options: -mvisibility-decl=mylibrary.v -fvisibility=hidden.
# Visibility specification for mylibrary
{ protected:
# symbols in the interface initialize; process; finalize;
}
Figure 8: mylibrary.v
Another more advanced usage is to explicitly define all the symbols that are outside of the library, all the symbols that are part of the library interface and let all other symbols be local to the library.
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For instance, if the user knows the list of symbols that are used by the library but are external to the library (for example, printf and malloc) the specification file mlibrary_all.v
shown in
can be used to compile with the command line option: -mvisibility-decl=mlibrary_all.v.
# Visibility specification for mylibrary
{ default:
# symbols usable outside of the library printf; malloc; protected:
# symbols in the interface initialize; process; finalize; hidden:
# all other symbols are kept local
*;
}
Figure 9: mylibrary_all.v
With the mylibrary_all.v visibility specification the library designer asserts the following:
1 No symbol other than (printf, malloc) can be referenced from the library without being defined in the library. If this assertion if violated a static link-time error will occur.
2 Only the symbols (initialize, process, finalize) are accessible to the other modules. There will be a dynamic link time error if this assertion is violated by another module.
3 All other symbols are local to the library.
The advantage of 1 is that the library dependencies on other modules are explicit. It can be useful for libraries, though in some contexts it may be time consuming to enumerate all such symbols.
The advantage of 2 is that the library interface is explicit and an error occurs if it is violated.
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The advantage of 3 is that the compiler can optimize all these hidden symbols and optimize all the references to these hidden symbols.
Note: 1 The -ipa option is not necessary in this case to optimize reference to symbols as
opposed to the -fvisibility=hidden option.
2 The -ipa option is still useful to further optimize the library by identifying from the set of hidden symbols which ones are internal. A sufficient condition for a symbol to
be internal is that it is hidden and the address is never taken. The -ipa option can compute the ‘address taken property’ and use the fact that the symbol has been declared hidden to set it internal.
The visibility specification file can be used to specify internal visibility for some symbols, though it must be done very carefully as in this case no link time (static or dynamic) error occurs if the symbol is passed to another module (thus violating the internal property). This must only be done in cases where the library designer is sure that an address is not passed outside of the library.
5.10.4 The visibility specification file
The visibility specification file resembles the version script files used by the GNU linker except that it specifies all levels of visibility instead of only the global and local properties.
The specification file has the following BNF grammar as shown in
, with the additional notion that the ‘-’ denotes a range (for instance, a-z represents all characters from a to z inclusive): visibility_spec ::= { { visibility_decls } } visibility_decls ::= { visibility_decl } visibility_decl ::= VISIBILITY : pattern_decls pattern_decls ::= { pattern_decl } pattern_decl ::= lang_decl | symbol_decl lang_decl ::= extern " LANGUAGE " { symbol_decls } symbol_decls ::= { symbol_decl } symbol_decl ::= PATTERN ;
VISIBILITY ::= default | protected | hidden | internal
LANGUAGE ::= "C" | "c" | "C++" | "c++"
PATTERN ::= {a-z | A-Z | 0-9 | _ | : | [ | ] | ? | * }
Figure 10: Visibility specification file BNF
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The PATTERN is a simplified regular expression suitable for ‘globbing’ as generally found in shell command line expansion. The implementation uses the ‘globbing’ function fnmatch() of the libiberty.a GNU library.
The LANGUAGE is one of C or C++ and specifies the demangling scheme that must be used to match a symbol.
If no language declaration is specified the pattern must match the assembly name of the symbol.
For example,
Figure 9 is a commented visibility specification file dummy.v:
# A sample visibility specification file
{ protected:
# will match the symbol named my_func my_func;
# will match the symbol named my_data my_data;
# will match all symbols prefixed by protected_ protected_*; default: extern "C++" {
# will match all C++ symbols of std:: namespace std::*;
# will match all typeinfo information of std:: namespace typeinfo*std::*;
# will match all vtable of std:: namespace
} hidden: extern C++ {
# will match all methods, data of the mycomplex class mycomplex::*; #
} vtable*std::*;
# will match all vtable tables of std:: namespace
VTT*std::*;
# will match the vtable for complex using the mangled name
_ZTV9mycomplex;
}
Figure 11: dummy.v
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GNU C extensions supported by st200cc
6
6.1 Introduction
GNU cc provides a large set of extensions that are widely used in the GNU Linux community. These extensions can be used to:
• describe embedded features, for example, data section placement,
• provide guidance to the compiler for optimization, for example, the noreturn function,
• provide language extensions, for example, conditional lvalue or C99 features,
• instruct the compiler to modify the Application Binary Interface, for example using the packed attribute.
The GNU extensions are sometimes the only way to access ELF features that are not directly available in the C language, for example, to declare a symbol weak.
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Extensions to the C language family
6.2 Extensions to the C language family
st200cc provides several language features not found in ANSI standard C. (The
“-pedantic” option directs st200cc to print a warning message if any of these features are used.) To test for the availability of these features in conditional compilation, check for a predefined macro __GNUC__, which is always defined under
st200cc.
It is recommended to always put code containing st200cc extensions under the C pre-processor macro __GNUC__.
#if __GNUC__
/* Original GNU code */
#else
/* Work-around code */
#endif
6.2.1 Statements and declarations in expressions
Statements and declarations in expressions allow complicated C statements to be written and used as if they were a simple C expression, optionally returning a result value. Local declarations and labels may be embedded.
This provides a way to construct a safe preprocessor macro that comprises several statements, without using the do { } while(0) trick that swallows the semi-colon.
#define cfoo() \
( { int y = foo (); int z; \
if (y > 0) z = y; \
else z = - y; \
z; })
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6.2.2 Locally declared labels
When GNU extensions are used in conjunction with expression statements and macros, they enable service labels to be used, that is labels whose scope is limited to the current statement. For example:
#define SEARCH(array, max, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \ found: \
value; \
})
6.2.3 Labels as values
The address of a label defined in the current function, or a containing function, can be obtained with the extended && unary operator that has type void*. For example: const char * cgoto(int i)
{
void *ptr = &&foo;
static void *array[] = { &&foo, &&bar, &&hack };
goto *array[i]; foo: bar: return "foo" ;
} return "bar" ; hack: return "hack" ;
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6.2.4 Naming an expression's type
A name can be given to the type of an expression using a typedef declaration with an initializer. To define name as a type name for the type of expression, do: typedef name = expression;
This can be used in conjunction with the statements-within-expressions feature described in
. For example, to define a safe “maximum” macro that
operates on any arithmetic type:
#define max(a,b) \
({typedef _ta = (a), _tb = (b); \
_ta _a = (a); _tb _b = (b); \
_a > _b ? _a : _b; })
The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for a and b.
Note: In the future the GNU language may include a new form of declaration syntax that allows the declaration of variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts.
6.2.5 Referring to a type with typeof typeof allows you to refer to an object data type by referring to an object of that type. It is particularly useful to write generic and safe macro-definitions, which can then be applied to various primitive types or user-defined data types. Without this extension, it is necessary to define as many specific macros as the number of different types used in calls to the generic macro.
#define max(a,b) ({ \ typeof (a) _a = (a); \ typeof (b) _b = (b); \
_a > _b? _a: _b; \
})
6.2.6 Generalized L-values
Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues. For example:
(a, b) += 5;
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6.2.7 Conditionals with omitted operands
The middle operand in a conditional expression may be omitted, for example: z = x? : y;
6.2.8 Double-word integers long long support (integer 64-bits) is supported by the ST200 Micro Toolset. It is now also an ISO C99 feature.
long long x;
6.2.9 Hexadecimal floats
Floating-point numbers are written in hexadecimal format: float f = 0x1.fp3;
6.2.10 Specifying a register for a local variable
A register may be specified for a local variable, for example: register long r15 asm (“r15”) = name;
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6.2.11 Array of length zero
Zero length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable length object.
#include <stdio.h>
#include <stdlib.h> struct line {
int length;
char contents[0];
}; struct line *newline( unsigned int this_length)
{
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
return thisline ;
} void delline(struct line *thisline)
{
free(thisline) ;
} int main(int argc, char *argv[])
{
enum { __MAXL = 128 } ;
enum { __L = 16 } ;
struct line *lines[__MAXL] ;
int i ;
printf("sizeof(line) : %d\n", sizeof(struct line)) ;
for(i=0; i< __MAXL; i++) {
lines[i] = newline(__L) ;
}
for(i=0; i< __MAXL; i++) {
delline(lines[i]) ;
}
puts("Done.") ;
return 0 ;
}
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6.2.12 Array of variable length
An array of variable length is an automatic array defined with a length that is not a constant expression. Also known as a VLA.
#include <stdio.h>
#include <stdlib.h> void sadcat(char *s1, char *s2)
{
char str[strlen (s1) + strlen (s2) + 1];
strcpy (str, s1);
strcat (str, s2);
printf("%s + %s == %s\n", s1, s2, str) ;
printf ("sizeof(str) = %d\n", sizeof(str));
} void tester (int len, char buffer[len][len]) {
int i=0, j=0;
char tt[len][len];
for (i=0; i<len; i++)
for (j=0; j<len; j++)
buffer [i][j] = i*j;
printf ("sizeof(tt) = %d\n", sizeof(tt));
}
printf ("sizeof(buffer) = %d\n", sizeof(buffer)); char data[10][10]; int main(int argc, char *argv[])
{
sadcat("Foo", "Bar") ;
tester (4, data);
tester (10, data);
}
return 0 ;
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6.2.13 Macro with variable number of arguments
This extension enables a macro to be defined that can safely be expanded into a function with a variable number of arguments. These macros are also called CPP vararg macros.
For example, the following C program:
#define eprintf(format, args...) fprintf (stderr, format, ##args)
eprintf ("success!\n");
eprintf ("%s%d: ", input_file_name, line_number); gets expanded into: fprintf ((&__iob[2]), "success\n!"); fprintf ((&__iob[2]), "%s%d: ", input_file_name, line_number);
Note: GNU C supports two types of “variable number of arguments” syntax. The ISO C99
format, which uses __VA_ARGS__ and the GNU format that uses ##args. The ISO
C99 format does not support the case where the number of parameters passed as part of the ellipsis is zero. GNU C reuses the ## trick to absorb the comma in this case. For example:
#include <stdio.h>
#define gnu_eprintf(format, args...) \
fprintf (stdout, "gnu_eprintf " format, ## args)
#define isoc99_eprintf(format, ...) \
fprintf (stdout, "isoc99_eprintf " format, __VA_ARGS__)
#define extended_isoc99_eprintf(format, ...) \ fprintf (stdout, "extended_isoc99_eprintf " format, ## __VA_ARGS__)
#define errprintf(args...) \
gnu_eprintf ("errprintf " "%s\n", ## args) int main(int argc, char *argv[]) {
/* Try 1, 2, 3 arguments */
gnu_eprintf ("One argument: %s. Done.\n", __FILE__);
gnu_eprintf ("Two arguments: %s:%d. Done.\n", __FILE__, __LINE__);
isoc99_eprintf ("One argument: %s. Done.\n", __FILE__);
isoc99_eprintf ("Two arguments: %s:%d. Done.\n", __FILE__, __LINE__);
extended_isoc99_eprintf ("One argument: %s. Done.\n", __FILE__);
extended_isoc99_eprintf ("Two arguments: %s:%d. Done.\n", __FILE__,
__LINE__);
extended_isoc99_eprintf ("Three arguments: %s:%s:%d. Done.\n",
__FUNCTION__, __FILE__, __LINE__);
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Extensions to the C language family 105
/* The case with no arguments ... */
gnu_eprintf ("No arguments. Done.\n");
/* The line below causes a syntax error */
isoc99_eprintf ("No arguments. Done.\n");
extended_isoc99_eprintf ("No arguments. Done.\n");
}
/* Cascade of macros with variable number of arguments */
errprintf (__FILE__);
return 0 ;
6.2.14 Strings literals with embedded newlines
GNU cpp permits string literals to cross multiple lines without escaping the embedded newlines. Each embedded newline is replaced with a single newline character in the resulting string literal, regardless of what form the newline took originally.
The macro definition:
#define MESSAGE \
"Hello, good brave new World!
" would be written under ISO:
#define MESSAGE \
"Hello,\n" \
"good brave new World!\n"
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6.2.15 Non-Lvalue arrays may have subscripts
In ISO C99, arrays that are not lvalues still decay to pointers, and may be subscripted. However, they may not be modified or used after the next sequence point and the unary operator “&” may not be applied to them.
struct foo {int a[4];}; struct foo f() {
static const struct foo f = { 2, 4, 8, 16 };
return f ;
} void bar (void)
{
int i;
for (i=0; i<4; i++)
printf ("f().a[%d] == %d\n", i, f().a[i]) ;
} int main(int argc, char *argv[])
{
bar ();
f().a[0] = 15;
bar ();
}
return 0 ;
6.2.16 Arithmetic on void and function pointers
In GNU C, addition and subtraction are supported by pointers to void and by pointers to functions. size used for a void or of a function is 1. sizeof is allowed for void and for a function: it returns 1.
void f0(void) {} void *p = 0; void (*pf)(void) = 0; bar (void) {
p++;
pf++;
printf ("sizeof(void) = %d\n", sizeof(void));
}
printf ("sizeof(func) = %d\n", sizeof(f0));
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Extensions to the C language family 107
6.2.17 Non-constant initializers
As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions. For example: int foo (int f, int g)
{
int beat_freqs[2] = { f-g, f+g };
return beat_freqs[0] * beat_freqs[1] ;
}
6.2.18 Compound literals
Compound literals used to be called “Constructor Expressions” before ISO C99 normalized them under the term “Compound Literals”. A compound literal looks like a cast containing an initializer:
#include <stdio.h>
#include <malloc.h> struct foo {int a; char b[2];} ; struct foo * givefoo(int x, int y, char a, char b) {
}
struct foo * sfoo = (struct foo *) malloc(sizeof (struct foo));
/* Fill in the anonymous struct at once with a Compound Literal */
*sfoo = (struct foo) {x + y, a, b};
return sfoo;
GNU C allows initialization of objects with static storage duration by compound literals, whereas ISO C99 does not.
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6.2.19 Designated initializers
This extension was called “GNU Style Labeled Elements in Initializers”. It is now an ISO C99 feature. It allows the initialization of particular elements of an aggregate, a structure or an array, by specifying the member name or the indices of the elements to initialize, in any order.
const int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 }; int a[6] = { [4] 29, [2] = 15 } ; enum { v1 = 1, v2 = 2 , v4 = 4 } ; int b[6] = { [1] = v1, v2, [4] = v4 } ; struct point { int x, y; }; struct point makep(int xvalue, int yvalue )
{
struct point p = { y: yvalue, x: xvalue };
return p ;
} struct point makepp(int xvalue, int yvalue )
{
struct point p = { .y = yvalue, .x = xvalue };
return p ;
}
With GNU C the = character can be omitted after the [index] indication.
6.2.20 Case ranges
Case ranges may be specified with integer value intervals in switch statements.
const char * which (int v) {
switch (v) {
case 0 ... 31: return "Control";
case 'A' ... 'Z': return "Upper";
case 'a' ... 'z': return "Lower";
default: return "None";
}
}
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Extensions to the C language family 109
6.2.21 Cast to a union type
A cast to union type is similar to other casts, except that the type specified is a union type. The type is specified either with the union tag or with a typedef name.
union foo { int i; double d; } u, v; makefoo (int i, double f) {
u = (union foo) i;
}
v = (union foo) f;
6.2.22 Dollar signs in identifier names
Dollar signs are allowed in identifier names.
int $a;
6.2.23 Prototypes and old-style function definitions
GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition.
int isroot (uid_t); int isroot (x) /* ??? lossage here ??? */
uid_t x;
{
return x == 0;
}
6.2.24 C++ comments
// C++ comment
C++ comments are not recognized by the st200cc options -ansi or -traditional.
This is to avoid problems with constructs that contain the forward slash character
“//”. For example: x = a //**/b;
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6.2.25 Character ESC in constants
The sequence “\e” is recognized in string or character constants as an ASCII
<escape> character.
char escape = '\e'; char s[] = "\e\e";
6.2.26 Inquiring on alignment of types or variables
__alignof__ allows enquiries about how an object is aligned, or the minimum alignment required by a type or variable.
struct foo { int x; char y; } f; int x = __alignof__ (double); int b = __alignof__ (f.y);
6.2.27 Incomplete enum type
An enum type can be defined without specifying its possible values.
typedef enum _e e; struct _s {
e* p;
} s; enum _e { red, green, blue, black }; e x;
6.2.28 Function names as strings
GNU cc predefines two magic identifiers to hold the name of the current function.
The identifier __FUNCTION__ holds the name of the function as it appears in the source. The identifier __PRETTY_FUNCTION__ holds the name of the function pretty printed in a language specific fashion.
char here[] = "Function " __FUNCTION__ " in file " __FILE__;
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Attributes 111
6.3 Attributes
Generally, attributes are a much better design than a #pragma directive for a number of reasons. Firstly, an attribute specification is a piece of C language that can be generated by use of a cpp macro definition, whereas a #pragma directive generation is generally not supported by non-GNU C preprocessors. Secondly, it avoids the scoping issues of the #pragma directive.
Several attributes can be applied to the same object by using a comma to separate them. For example, to declare a symbol that is both weak and aliased: void useful (void) __attribute__ ((weak, alias("useful_func")));
6.3.1 Placement and layout section
Applied to function: Place a function in a user-defined section.
void myfunc (void) __attribute__ ((section(".mytext"))); void myfunc (void) {
printf ("From myfunc in .mytext section.\n");
}
Applied to a data object: Place the data in a user-defined section.
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
Support must be explicitly added in the startup file or system loader to load the newly created section.
aligned
Applied to a variable or a structure field: Specifies a minimum alignment for a variable or structure field, measured in bytes. The aligned attribute can only increase the alignment; it can be decreased by specifying packed as well.
int x __attribute__ ((aligned (16))) = 0; struct _s { int x[2] __attribute ((aligned (8))); }; short array [3] __attribute ((aligned));
Applied to a type: typedef int more_aligned_int __attribute__ ((aligned(8)));
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Attributes weak
Applied to a function: Causes the function to be emitted as a weak symbol. Set to 0 if the symbol is not defined at link time. This is primarily of use in defining library functions which can be overridden in user code: void d_stub (void) __attribute__ ((weak)); if (d_stub) { d_stub();
}
Applied to data: Causes the declaration to be emitted as a weak symbol rather than a global symbol. This is primarily of use in defining variables which can be overridden in user code: int debug __attribute__ ((weak)) = 0; alias
Applies only to functions: The required functionality is to provide an alias name for a given function. Often used in conjunction with the weak requirement to define an alternate weak name for a given function.
void useful_func (void) {
/* ... Do something ... */
} void useful (void) __attribute__ ((alias("useful_func"))); packed
Applies only to data: Specifies that a variable or structure should have the smallest possible alignment - one byte for a variable, and one bit for a field, unless a larger value with the aligned attribute is specified.
The specified data alignment is applied during data layout, and the code generator emits safe sequence of instructions to avoid causing a misalign trap.
struct foo { char a; int x __attribute__ ((packed)); };
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Attributes 113 used
The GCC manual specifies that the used attribute may only apply to functions. For
st200cc it may also apply to variables:
• The used attribute, attached to a function, means that the code must be emitted for this function, even if this function appears never to be referenced.
• This attribute, attached to a variable, means that the definition must be emitted for the variable even if it appears that the variable is not referenced.
The used attribute follows the same syntax as any GCC attribute.
For a procedure: static int Foo() __attribute__ ((used)) ;
For uninitialized data: static foo __attribute__((used)) ;
For initialized data: static foo __attribute__((used)) = 2 ;
Note: The assembly has been specifically extended to support this attribute:
.type Foo, @function, used
.type foo, @object, used
A motivation for using this attribute is to avoid the deletion of an unreferenced symbol by the dead code, dead data or IPA optimization. This can be useful for debugging purposes (for instance a function dumping a specific data structure that is called only interactively from debugging sessions is removed if not marked ‘used’, since the compiler does not find any reference to it).
constructor and destructor
Applies only to functions: The constructor attribute causes the function to be called automatically before execution enters main(). Similarly, the destructor attribute causes the function to be called automatically after exit().
void initdata (void) __attribute__ ((constructor)); void terminatedata (void) __attribute__ ((destructor));
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6.3.2 Optimization
This section applies only to functions.
noreturn
Enables a function to be declared that cannot return, such as abort or exit.
Useful indication to optimizers.
void byebye () __attribute__ ((noreturn)); malloc
Used to tell the compiler that a function returns a pointer that cannot alias anything. Useful indication to optimizers.
void * get_block (int) __attribute__ ((malloc));
6.3.3 Visibility attributes
The visibility attributes are supported as follows:
__attribute__((__visibility__(“visibility-type”)))
__attribute__((visibility(“visibility-type”))) where visibility-type can be “default”, “hidden”, “protected”,
“internal” default
Default visibility is the normal case for ELF. This value is available for the visibility attribute to override other options that may change the assumed visibility of symbols.
hidden
Hidden visibility indicates that the symbol will not be placed into the dynamic symbol table, so that no other module (executable or shared library) can reference it directly.
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Attributes 115 protected
Protected visibility indicates that the symbol will be placed in the dynamic symbol table, but that references within the defining module will bind the local symbol.
This means that the symbol cannot be overridden by another module.
internal
Internal visibility is like hidden visibility, but with additional processor-specific semantics. For the ST200, this means that the function is never called from another module.
Note: That hidden symbols, though they cannot be referenced directly by other modules can be referenced indirectly by function pointers. By indicating that a symbol cannot be called from outside the module, the compiler may for instance omit the load of a PIC register since it is known that the calling function has already defined the correct value.
6.3.4 Miscellaneous attributes format
The format attribute specifies that a function takes printf, scanf, strftime or strfmon style arguments which should be type-checked against a format string.
extern int my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format(printf, 2, 3))); format_arg
The format_arg attribute specifies that a function takes a format string for a printf , scanf, strftime or strfmon style function and modifies it, so that the result can be passed to a printf, scanf, strftime or strfmon style function.
extern char * my_dgettextprintf (void *my_domaint, const char
*my_format) __attribute__ ((format_arg(2))); mode
This attribute specifies the data type for the declaration - whichever type corresponds to the mode. Refer to the GNU Compiler Collection Internals document for the definitions of modes, see http://gcc.gnu.org/onlinedocs/gccint .
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Use the keywords __byte__, __word__ and __pointer__ to indicate the mode corresponding to these quantities.
unsigned int qi __attribute__ ((mode (QI))); unsigned int w __attribute__ ((mode (__word__)));
6.3.5 Built-ins
A built-in is used as a function call, but is expanded by the compiler very early in the intermediate representation, instead of doing a function call.
__builtin_constant_p
This built-in tests if a value is a constant at compile time.
int x;
#define C 1 int main () {
if (__builtin_constant_p (C) == 1)
}
printf ("c is proved to be a constant\n");
if (__builtin_constant_p (x) == 0)
printf ("x is a not proved to be a constant\n");
return 0;
__builtin_return_address
__builtin_return_address gets the return address of the currently executing function.
void bar () { printf ("RA = 0x%08x\n", (int)__builtin_return_address (0));
}
__builtin_expect long __builtin_expect (long exp, long c)
__builtin_expect provides the compiler with branch prediction information.
The return value is the value of exp, which should be an integral expression. The value of c must be a compile-time constant. The semantics of the built-in are that it is expected that exp == c.
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For example: if (__builtin_expect (exp, 0)) foo (); would indicate that we do not expect to call foo, since we expect exp to be zero.
__builtin_classify_type
__builtin_classify_type ignores the value of the object, considering only its data type. It returns an enum describing what kind of type object is.
enum type_class __builtin_classify_type(object) enum type_class
{ no_type_class = -1, void_type_class, integer_type_class, char_type_class, enumeral_type_class, boolean_type_class, pointer_type_class, reference_type_class, offset_type_class, real_type_class, complex_type_class, function_type_class, method_type_class, record_type_class, union_type_class, array_type_class, string_type_class, set_type_class, file_type_class, lang_type_class
};
__builtin_prefetch void __builtin_prefetch(const void *addr, ...)
This function causes the compiler to generate a pft instruction for address addr.
There are two optional arguments: rw and locality. These arguments are currently ignored by the ST200 compiler.
Automatic prefetching is not performed upon arrays that have the built-in function
__builtin_prefetch applied to them.
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GNU ASM
7
7.1 Introduction
st200cc compiler accepts “extended inline assembly” asm, as part of C programs.
This chapter only summarizes the main features of the asm implementation and describes its limitations, it is not a substitute for the GNU documentation.
7.1.1 Syntax asm (template : output operands : input operands : clobber list) ; or
__asm__ (template : output operands : input operands : clobber list) ;
• template : the assembler instruction, defined as a string constant,
• output operands : a list of comma separated output operands,
• input operands : a list of comma separated input operands,
• clobber list : a list of comma separated clobbered operands.
The template section contains plain assembler, and uses ordinary ST200 assembler syntax, with the notable exception of the %i (i is a positive integer) notation that refers to the i th
output operand or input operand.
Note: Multiple consecutive strings are automatically concatenated and enable a readable and correct template input. Multiple assembler instructions can be put together in a
single asm template, separated by explicit newline characters ‘\n’.
If there are no output operands but there are input operands, two consecutive colons must be placed where the output operands would go.
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In the output and input list:
• each operand is described by an “operand constraint string” followed by a C expression in parentheses,
• the available constraints are the following:
r general purpose register operand,
b boolean register operand,
i immediate integer operand, including symbolic constants only known at assembly time,
n immediate integer operand, known at compile time,
g general purpose register or immediate integer,
• an operand constraint can be prefixed by the following modifiers:
= write-only operand, used for output operands,
& early clobber operand, does not prevent the use of =.
In the clobber list:
• general registers are referred to by ri (where i has the range [0,63]), they map to the corresponding Ri hardware registers,
• branch registers are referred to by bi (where i has the range [0,7]), they map to the corresponding Bi hardware boolean registers.
7.1.2 Assumptions
• Output operand expressions must be lvalues.
• The compiler assumes that the input is consumed before the outputs are produced, unless an output operand has the ‘&’ constraint modifier (also called
“early clobber”). The compiler will not assign the same register to an input operand and an early-clobber operand. However, the compiler may assign the same register to an input operand and to a non-early-clobber output operand.
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Introduction 121
7.1.3 Volatile
The volatile syntax is as follows: asm volatile (template : output operands : input operands : clobber list); or
__asm__ volatile (template : output operands : input operands : clobber list);
The volatile keyword indicates that an instruction has side effects. A volatile statement will not be deleted if it is reachable. The order of volatile asm statements and, or other volatile accesses will be preserved. A consecutive sequence of volatile asm statements may not stay perfectly consecutive, since some other instructions may be scheduled in between.To achieve the effect of keeping instructions perfectly consecutive, use a single asm instruction.
An asm statement without any operand or clobbers will be treated identically to a volatile asm statement, the same as for an asm statement without an output operand.
7.1.4 Scheduling considerations
On the ST220 processor, scheduling considerations must account for the absence of hardware interlocks. The programmer must make sure that all asm outputs are available in the next cycle following the asm, for instance by adding explicit
(possibly empty) bundles.
On the ST231 processor, this is not necessary as this processor is interlocked.
Generating different schedules for these different processors can be handled by using the core symbol defined by the C preprocessor (__st220__, __st231__).
In any case, correct instruction bundling must occur (for example, bundling constraints and multiplication alignments).
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7.1.5 Restrictions
• The compiler does not parse the assembler instruction template, this means that it does not check if it is valid assembler input.
• Up to 10 operands, results and clobbered registers are allowed.
• Multiple alternative constraints are not supported.
• At -O3 optimization level, the loop nest optimizer is disabled for loops containing asm statements.
7.1.6 Example
The following function adds two unsigned integers, completes a carry and obtains the 33-bit result through the addcg ST200 instruction. The output is represented by 64 bits. It uses both ordinary registers and boolean registers as input and output, and shows that struct members can be used as C expressions to receive the computed result.
inline unsigned long long addcarry(unsigned int t, unsigned int s, unsigned int c) { typedef union { unsigned long long v_ ;
#if defined(__LITTLE_ENDIAN__) struct { unsigned int l_ ; unsigned int h_ ; } lh ;
#elif defined(__BIG_ENDIAN__) struct { unsigned int h_ ; unsigned int l_ ; } lh ;
#else
#error "addcarry : Unknown endianness"
#endif
} __ui64 ;
__ui64 ui64v_ ;
} asm("addcg %0, %1 = %2, %3, %4"
: "=r" (ui64v_.lh.l_) , "=b" (ui64v_.lh.h_)
: "r" (t), "r" (s), "b" (c)
) ; return ui64v_.v_ ;
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Intrinsic functions
8
8.1 Introduction
8.1.1 Rationale
The st200cc compiler recognizes a number of intrinsic operators which can be used to produce assembly language statements that otherwise could not be expressed through standard ANSI C/C++.
These intrinsics are specified and called just like standard ANSI C/C++ functions using standard types, however, they are treated specially by the compiler. The
ST200 intrinsics apply to the ST220 and ST231 parts.
8.1.2 Models
ST200 intrinsics have been modelled as C functions, which act as executable specifications. This has the benefit that models can be used to develop DSP algorithms on a workstation, that are immediately and safely ported to the ST220 and ST231.
The implementation of the models imodels.c is delivered with the current compiler distribution.
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8.2 Naming intrinsics
8.2.1 Naming scheme
STMicroelectronics intrinsics may be used for several families of DSP and micro controllers whose natural integral types are not always equivalent.
To accommodate a variety of targets, a naming scheme has been designed which takes into account this variety. An intrinsic has basically a generic name containing its signature:
__{return_type}_{operation}_{arguments_types}+
Intrinsics are given shorter and more mnemonic alias names for each target, taking into account the natural names of types on the specific target.
For instance, the natural name of the 32-bit integer on the ST200 is word, so the intrinsic that realizes the clamped addition of two 32 bit integers is named
__int32_addc_int32_int32 in its generic form and __addcw in its ST200 form.
Where the addc part denotes the clamped addition operator, and w the fact that the result is of word type.
Note: The presence of the two leading underscores on each name denotes (according to the
ISO/IEC 9899 C Standard) that no such name should be defined by the user.
More specifically:
“All identifiers that begin with an underscore and either an upper case letter or another underscore are always reserved for any use.”
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Naming intrinsics 125
Finally the International Telecommunication Union (ITU) and European
Telecommunications Standard Institute (ETSI)
1
basic operators have their own naming scheme, thus they must be explicitly mapped onto ST200 intrinsics. This is explained further in
.
8.2.2 Building intrinsic names
This section elaborates on the rules for building intrinsic names. It is important to understand the principles behind the naming conventions, because of the number of intrinsic functions that are available. Understanding these principles relieves the need to remember specific names, and enables the name of an intrinsic to be deduced from its specific operation.
The following sections expand on the type mapping and name conventions used in the intrinsics.
1. Many of the standard algorithms in Wireless and Wireline communications such as GSM (Global System for Mobile Telecommunications) or AMR-NB for
UMTS (Adaptive Multi Rate Narrow Band for UMTS) encoders, are provided by the ITU and ETSI, as ANSI C code using 16-bits fractional arithmetic.
For the ITU refer to http://www.itu.int
or for the ETSI, refer to http://www.etsi.org
.
To specify the fractional arithmetic model which is foreign to ANSI C, a set of subroutines are used implementing basic fractional operations. As these basic fractional operations are provided as ANSI C source codes, an algorithm complying with the ITU or ETSI compiles on any ANSI C compiler.
It therefore requires little effort to compile and run the algorithm on any workstation or DSP. However, an efficient implementation on a DSP requires some modifications of the original code. The st200cc compiler provides a set of intrinsics that help make these modifications as minor as possible.
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Types
Table 31 summarizes the type names use in the intrinsics description.
Description
-
-
Fractional q15
-
-
Fractional q31
Fractional q63
-
-
Name int16 uint16
Models mapping short unsigned short fract16 short int32 uint32 int unsigned int fract32 int fract64 long long int64 long long uint64 unsigned long long
ST200 mapping (size) short (16) unsigned short (16) short (16) int (32) unsigned int (32) int (32) long long (64) long long (64) unsigned long long (64)
Table 31: ST220 intrinsic type names
ST200 notation l l h uh h (equivalent to signed short) w uw w (equivalent to signed int) ul
Note: 1 These types are defined in the st220types.h files, which is part of the compiler distribution. This file applies to both the ST220 and ST231.
2 Operations on fractions are specifically denoted by the operator name with the addition of the f suffix to the standard name.
3 Using models requires a full 64-bit implementation of the long long and unsigned long long types.
The model requires the standard C99 long long type. The standard PC compiler uses __int64 to support a full 64-bit type. Other compilers can be obtained that do support the long long type, for example a gcc based compiler. Refer to http://sources.redhat.com/cygwin .
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Operations
Table 32 summarizes the operator names used in the intrinsics description.
The reader must be familiar with the concepts of modular, saturating, rounding, and fractional arithmetic. Pay particular attention to the Remarks column in
Model stem
_add_
_addc_
_sub_
_subc_
_neg_
_negc_
_min_
_max_
_abs_
_absc_
_dist_
_shr_
_shrr_
_shlc_
_mul_
_mulc_
_mulrc_
Description Remarks
Modular addition
Saturating addition
Modular subtraction
Saturating subtraction
Modular negate
Saturating negate
Minimum
Maximum
Modular absolute value
Saturating absolute value
Distance
Right shift Positive shift amount, shifting more than integral operand size in bits is implementation defined
Rounded right shift
Saturating left shift Positive shift amount, shifting more than integral operand size in bits is implementation defined
Modular multiplication
Saturating multiplication
Rounded saturating multiplication
Table 32: ST200 intrinsic operator names
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Naming intrinsics
Model stem
_mulh_
_div_
_divc_
_mod_
_modc_
_bitcnt_
_bitrev_
_rotl_
_xshl_
_xshr_
_edges_
_lzcnt_
_prior_
_norm_
_clamp_
_round_
_get_
_put_
_bitclr_
_bitset_
_bitnot_
_bitval_
Description Remarks
High multiplication
Division
High part of classical multiplication
Sign of Quotient is sign of algebraic quotient, if inexact |q| < |D/d|
Saturating fractional division
Modulus When Quotient q is defined, q * d + Modulus =
D
Saturating fractional modulus
Population count
Bit reverse
Rotate left
Cross-shift left
Cross-shift right
Edges
Leading zero count
Get left shifts to normalize
ETSI get left shifts to normalize
Saturation
Rounding
Get high/low part
Put in high/low part
Bit clear
Bit set
Bit complement
Bit test
Table 32: ST200 intrinsic operator names
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Model stem
_inseq_
_insne_
_insgt_
_insge_
_inlts_
_insle_
Description Remarks
Compare for equal and insert
Compare for not equal and insert
Compare for greater and insert
Compare for greater or equal and insert
Compare for less and insert
Compare for less or equal and insert
Table 32: ST200 intrinsic operator names
For example, searching for an operator that computes the maximum value of two
32-bit integers is quite simple.
1 Operator stem is max, and there is no specific ST200 name.
2 Operands are both of int32 type, so the suffix is int32_int32.
3 Result type is int32, so the prefix is int32.
4 Therefore, the generic model name of the operator is
__int32_max_int32_int32 .
To get the specific ST200 operator name is also quite simple.
1 Operator stem is max (there is no specific ST200 name).
2 Operand and result type are identical, int32 type, therefore the suffix w is used.
3 Therefore, the generic model name of the operator is __maxw.
Note: All operators have been quite thoroughly defined, to avoid redundancies, and are limited to useful combinations. There is no need, for example, to have a fractional addition operator, because addition of fractional types using the same representation is equivalent to addition on ordinary integral types.
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130 Using intrinsics from C/C++
8.3 Using intrinsics from C/C++
8.3.1 Include files
This section explains the usage of include files that are peculiar to intrinsics.
Note: All techniques described here apply to both C and C++, and the headers are both C and C++ compatible.
Using intrinsics on an ST200 platform
ST200 intrinsics prototypes are available in the st220.h include file.
Note: All header files mentioned in this chapter apply to both the ST220 and ST231.
To include intrinsics in an application use:
#include <st220.h>
If ETSI names are required use the following as well:
#include <etsitost220.h>
This injects a set of macros that renames original ETSI/ITU basic operators.
Most of these macros are just name defines, with the exception of shift operators that are macros with parameters. This means that it is not possible to take the address of shift operators.
Note: Including st220.h implicitly includes st220types.h which defines the appropriate basic types.
Using intrinsics models
Prototypes for the intrinsic models are provided in imodels.h and their implementation is in imodels.c
To include an intrinsics model in an application use:
#include <imodels.h>
If ETSI names are required use the following as well:
#include <etsitom.h>
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Note: Including imodels.h implicitly includes st220types.h which defines the appropriate basic types.
Care must also be taken to link with the imodels.o file, produced on the specific platform.
Switching between ST200 intrinsics and modelled intrinsics
Is is often extremely useful to be able to keep the same source baseline, either running with models on a workstation, or on an ST200 platform.
For example, on an ST220 this is done using the implicitly defined __st220__ symbol:
#ifdef __st220__
#include <st220.h>
#include <etsitost220.h>
#else /* !__st220__ */
#include <imodels.h>
#include <etsitom.h>
#endif /* __st220__ */
Note: 1 It is possible to remap names in both directions.
2 For an ST231 platform, the symbol __st231__ would be used with the same ST220 header files, as these apply equally to ST231.
An application written exclusively using the model names can be retargeted immediately to ST200 by including:
#include <mtost220.h>
This has the effect of renaming all models to ST200 intrinsics names.
An application written exclusively using the ST200 names can easily be switched to using model names by including:
#include <st220tom.h>
This has the effect of renaming all ST200 intrinsic names to model names.
To allow the remapping of names, use one set of names exclusively, that is use either the model names or the ST200 intrinsic names.
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Using intrinsics from C/C++
Example
The following example demonstrates the usage of ST200 intrinsics in the application source code while maintaining the ability to compile on a generic target by switching automatically back to models.
#if defined(__st200__)
/* Include intrinsics definitions when target is ST200 */
#include <st220.h>
#else /* !__st200__ */
/* Include models and renaming scheme when target is unknown */
#include <imodels.h>
#include <st220tom.h>
#endif /* __st200__ */ unsigned int SwapBytes(unsigned int x)
{
/*
* On ST200, this will be emitted as a single instruction
* bswap $r16 = $r16
*
* On other targets, thanks to <st220tom.h> macros,
* this name will be replaced by the generic name
* and looked up in the models source files.
* Thus the generated code will be a call to
*__uint32_swapb_uint32
* To compile on a workstation with a generic compiler
* (for instance gcc on Solaris or Linux) you will have to
* include <tools-dir>/host/imodels
* in your include search path
* (use the -I compiler option )
* to add <tools-dir>/host/imodels/imodels.c to your source
* files */ return __swapbw(x) ;
}
When compiled with the st200cc compiler, the following code provides maximum efficiency for the ST200 target, it can be compiled for any other target and automatically guarantees the same execution semantics.
With the st200cc compiler, an application using this file swapb.c can be compiled without modification (apart from defining the toolset installation path).
$ st200cc swapb.c other-files.c -o main.u
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With a gnu gcc compiler on Unix:
$ gcc -I<tools-dir>/include -I<tools-dir>/host/imodels <tools-dir>/ host/imodels/imodels.c swapb.c other-files.c -o main.sol
Note: the installation path of the ST200 toolset must be defined, this example assumes that the $PATH environment variable has been updated with <tools-dir> .
Finally, if this example is to be used in real code, it would be better to define the
SwapBytes function as inline, and provide it through an appropriate header file.
8.4 Understanding intrinsic models
The inner workings of an intrinsic can be difficult to understand. In order to understand the precise operating semantics it helps to understand the model implementation that intrinsics are based on.
The following sections describe the fundamental operators that the models are based on, then gives an implementation example.
8.4.1 Understanding fundamentals operators
The following macro operators are defined:
• EXT(x, n) normalizes x considered signed on n bits in a long long,
• EXTU(x, n) normalizes x considered unsigned on n bits in a long long,
• CLAMP(x, n) saturates x on n bits in a long long.
All models are based on a systematic and consistent use of the operators which enables understanding of the inner workings of intrinsics.
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Intrinsic functions summary
8.4.2 Understanding models
Consider the example of the __int16_addc_int16_int16 operator: int16 __int16_addc_int16_int16(int16 r0, int16 r1) {
long long t0 = EXT(r0, 16);
long long t1 = EXT(r1, 16);
long long t2 = t0 + t1;
long long t3 = CLAMP(t2, 16);
return t3;
}
In this example, inputs are normalized to long long, through the EXT operations, keeping 16 bits signed precision. Then the addition is computed with full precision and finally the result is saturated to 16 bits by the CLAMP operator.
8.5 Intrinsic functions summary
The tables shown in
provide the names of all ST200 intrinsics
functions along with their model names and properties.
The tables contain:
• the model name of the intrinsic,
• the purpose of the intrinsic:
C operator: supports C arithmetic,
C runtime arithmetic helper: supports optimized C arithmetic,
DSP operator: saturating/fractional arithmetic,
ETSI/ITU operator: standard conformance operator,
• the ST200 specific name.
The intrinsic functions are generally categorized by operator. Within each category they are grouped by operand and result type. All the compiler-generated arithmetic intrinsics are held in the library libgcc.a.
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8.5.1 Functions
Intrinsic
__int32_addc_int32_int32
__int16_addc_int16_int16
Purpose
ETSI L_add
ETSI add
Table 33: Add intrinsics
Intrinsic
__int32_subc_int32_int32
__int16_subc_int16_int16
Purpose
ETSI L_sub
ETSI sub
Table 34: Subtract intrinsics
ST200 name
__addcw
__addch
ST200 name
__subcw
__subch
Intrinsic
__int32_negc_int32
__int16_negc_int16
Purpose
ETSI L_negate
ETSI negate
Table 35: Negate intrinsics
ST200 name
__negcw
__negch
Intrinsic
__int32_min_int32_int32
__int16_min_int16_int16
__uint32_min_uint32_uint32
__uint16_min_uint16_uint16
Purpose
C optimization
C optimization
C optimization
C optimization
Table 36: Minimum intrinsics
ST200 name
__minw
__minh
__minuw
__minuh
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Intrinsic
__int32_max_int32_int32
__int16_max_int16_int16
__uint32_max_uint32_uint32
__uint16_max_uint16_uint16
Purpose
C optimization
C optimization
C optimization
C optimization
Table 37: Maximum intrinsics
ST200 name
__maxw
__maxh
__maxuw
__maxuh
__int32_abs_int32
__int16_abs_int16
__int32_absc_int32
__int16_absc_int16
Intrinsic Purpose
C optimization
C optimization
ETSI L_abs
ETSI abs_s
Table 38: Absolute value intrinsics
ST200 name
__absw
__absh
__abscw
__absch
Intrinsic
__int32_dist_int32_int32
__int16_dist_int16_int16
__uint32_dist_uint32_uint32
__uint16_dist_uint16_uint16
Purpose
DSP operator
DSP operator
DSP operator
DSP operator
Table 39: Absolute distance intrinsics
ST200 name
__distw
__disth
__distuw
__distuh
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Intrinsic
__int32_shrr_int32_uint16
__int16_shrr_int16_uint16
Purpose a
ETSI L_shr_r
ETSI shr_r
ST200 name
__shrrw
__shrrh
Table 40: Right shift intrinsics a. To achieve the ETSI operator, a combination of the intrinsic functions are required. For example, L_shl in
is implemented in terms of __shlcw and __shrw.
137
Intrinsic
__int32_shlc_int32_uint16
__int16_shlc_int16_uint16
Purpose
ETSI L_shl
ETSI shl
Table 41: Left shift intrinsics
ST200 name
__shlcw
__shlch
Intrinsic
__int64_mul_int32_int32
__uint64_mul_uint32_uint32
__int32_mul_int32_int16
__int16_mul_int16_int16
__uint32_mul_uint32_uint16
__uint16_mul_uint16_uint16
__fract32_mulc_fract32_fract32
__fract32_mulc_fract32_fract16
__fract16_mulc_fract16_fract16
Purpose
C optimization
C optimization
C optimization
C optimization
C optimization
C optimization
DSP operator
DSP operator
ETSI mult
Table 42: Multiply intrinsics
ST200 name
__muln
__mulun
__mulm
__mulh
__mulum
__muluh
__mulfcw
__mulfcm
__mulfch
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Intrinsic functions summary
Intrinsic Purpose
__int32_div_int32_int16
__int16_div_int16_int16
__uint32_div_uint32_uint16
__uint16_div_uint16_uint16
__fract32_divc_fract32_fract32
__fract32_divc_fract32_fract16
__fract16_divc_fract16_fract16
C optimization
C optimization
C optimization
C optimization
DSP operator
DSP operator
ETSI div_s
Table 43: Divide intrinsics
ST200 name
__divm
__divh
__divum
__divuh
__divfcw
__divfcm
__divfch
Intrinsic
__int32_mod_int32_int16
__int16_mod_int16_int16
__uint32_mod_uint32_uint16
__uint16_mod_uint16_uint16
__fract32_modc_fract32_fract32
__fract32_modc_fract32_fract16
__fract16_modc_fract16_fract16
Purpose
C optimization
C optimization
C optimization
C optimization
DSP operator
DSP operator
ETSI mod_s
Table 44: Modulus intrinsics
ST200 name
__modm
__modh
__modum
__moduh
__modfcw
__modfcm
__modfch
Intrinsic
__int64_mul_int32_int16
__uint64_mul_uint32_uint16
__fract32_mulc_fract16_fract16
__fract16_mulrc_fract16_fract16
Purpose
C optimization
C optimization
ETSI L_mult
ETSI mult_r
Table 45: Multiply special intrinsics
ST200 name
__mpml
__mpuml
__mpfcw
__mpfrch
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Intrinsic
__int32_mulh_int32_int32
__int16_mulh_int16_int16
__uint32_mulh_uint32_uint32
__uint16_mulh_uint16_uint16
Purpose
C Runtime
Arithmetic helper
C Runtime
Arithmetic helper
C Runtime
Arithmetic helper
C Runtime
Arithmetic helper
Table 46: Multiply high intrinsics
ST200 name
__mulhw
__mulhh
__mulhuw
__mulhuh
Intrinsic Purpose
__uint32_edges_uint32_uint32
__uint16_edges_uint16_uint16
__uint32_rotl_uint32_uint16
__uint16_rotl_uint16_uint16
__uint32_xshl_uint32_uint32_uint16
__uint16_xshl_uint16_uint16_uint16
__uint32_xshr_uint32_uint32_uint16
__uint16_xshr_uint16_uint16_uint16
__uint16_xshr_uint16_uint16_uint16
__int16_prior_int32
__int16_prior_int16
__int16_norm_int64
__int16_norm_int32
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
ETSI norm_l
ETSI norm_l
Table 47: Miscellaneous intrinsics
ST200 name
__edgesw
__edgesh
__rotlw
__rotlh
__xshlw
__xshlh
__xshrw
__xshrh
__priorl
__priorw
__priorh
__norml
__normw
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Intrinsic functions summary
Intrinsic
__int16_norm_int16
__uint16_lzcnt_int64
__uint16_lzcnt_int32
__uint16_lzcnt_int16
Purpose
ETSI norm_s
DSP operator
DSP operator
DSP operator
Table 47: Miscellaneous intrinsics
ST200 name
__normh
__lzcntl
__lzcntw
__lzcnth
Intrinsic
__int16_clamp_int32
__int16_near_int32
__int16_nearc_int32
__int16_round_int32
__int16_roundc_int32
__int32_puth_int16
__int32_putl_int16
__int16_geth_int32
__int16_getl_int32
Purpose
ETSI saturate
IEEE round
IEEE round and clamp
DSP operator
ETSI round
DSP operator
DSP operator
ETSI extract_h
ETSI extract_l
ST200 name
__clampwh
__nearwh
__nearcwh
__roundwh
__roundcwh
__puthw
__putlw
__gethh
__getlh
Table 48: Clamp, round, insert and extract intrinsics
Intrinsic Purpose
__uint32_bitclr_uint32_uint16
__uint16_bitclr_uint16_uint16
__uint32_bitset_uint32_uint16
__uint16_bitset_uint16_uint16
__uint32_bitnot_uint32_uint16
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
Table 49: Bit manipulation intrinsics
ST200 name
__bitclrw
__bitclrh
__bitsetw
__bitseth
__bitnotw
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Intrinsic
__uint16_bitnot_uint16_uint16
__uint16_bitval_uint32_uint16
__uint16_bitval_uint16_uint16
__int32_bitrev_uint16_int32
Purpose
DSP operator
DSP operator
DSP operator
DSP operator
Table 49: Bit manipulation intrinsics
ST200 name
__bitnoth
__bitvalw
__bitvalh
__bitrevw
Intrinsic
__int32_inseq_int32_int32_int32
__int32_insne_int32_int32_int32
__int32_insgt_int32_int32_int32
__int32_insge_int32_int32_int32
__int32_inslt_int32_int32_int32
__int32_insle_int32_int32_int32
__uint32_inseq_uint32_uint32_uint32
__uint32_insne_uint32_uint32_uint32
__uint32_insgt_uint32_uint32_uint32
__uint32_insge_uint32_uint32_uint32
__uint32_inslt_uint32_uint32_uint32
__uint32_insle_uint32_uint32_uint32
Purpose
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
Table 50: Viterbi intrinsics
ST200 name
__inseqw
__insnew
__insgtw
__insgew
__insltw
__inslew
__insequw
__insneuw
__insgtuw
__insgeuw
__insltuw
__insleuw
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Intrinsic functions summary
Intrinsic
__int32_mul_int32_int32
__uint32_mul_uint32_uint32
__int32_div_int32_int32
__uint32_div_uint32_uint32
__int32_mod_int32_int32
__uint32_mod_uint32_uint32
Purpose
C operator
C operator
C operator
C operator
C operator
C operator
Table 51: Integer 32 bit support
Intrinsic
__int64_add_int64_int64
__uint64_add_uint64_uint64
__int64_sub_int64_int64
__uint64_sub_uint64_uint64
__int64_neg_int64
__uint64_neg_uint64
__int64_shr_int64_uint16
__uint64_shr_uint64_uint16
__int64_shl_int64_uint16
__uint64_shl_uint64_uint16
__int64_mul_int64_int64
__uint64_mul_uint64_uint64
__int64_div_int64_int64
__uint64_div_uint64_uint64
__int64_mod_int64_int64
__uint64_mod_uint64_uint64
Purpose
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
C operator
Table 52: Long long support
ST200 name
__mulw
__muluw
__divw
__divuw
__modw
__moduw
ST200 name
__shll
__shlul
__mull
__mulul
__divl
__divul
__modl
__modul
__addl
__addul
__subl
__subul
__negl
__negul
__shrl
__shrul
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Intrinsic Purpose ST200 name
__int64_addc_int64_int64
__int64_subc_int64_int64
__int64_negc_int64
__int64_absc_int64
__int64_max_int64_int64
__uint64_max_uint64_uint64
__int64_min_int64_int64
__uint64_min_uint64_uint64
__int32_clamp_int64
__int32_round_int64
__int32_roundc_int64
__int32_near_int64
__int32_nearc_int64
__fract64_mulc_fract32_fract32
__fract64_mul_fract32_fract16
__int64_puth_int32
__int64_putl_int32
__int32_geth_int64
__int32_getl_int64
DSP operator
[D_add]
DSP operator
[D_sub]
DSP operator
[D_neg]
DSP operator
[D_abs]
DSP operator
DSP operator
DSP operator
DSP operator
DSP operator
[D_sat]
DSP operator
[D_round]
DSP operator
[D_round]
IEEE operator
IEEE operator
DSP operator
[D_mult]
DSP operator
DSP operator
DSP operator
DSP operator
[D_extract_l]
DSP operator
[D_extract_h]
Table 53: High precision fractional arithmetic
__addcl
__subcl
__negcl
__abscl
__maxl
__maxul
__minl
__minul
__clamplw
__roundlw
__roundclw
__nearlw
__nearclw
__mpfcwl
__mpfml
__puthl
__putll
__gethw
__getlw
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Intrinsic functions summary
Intrinsic Purpose
__uint16_swapb_uint16
__uint32_swaph_uint32
__uint32_swapb_uint32
DSP operator
DSP operator
DSP operator
Table 54: Bit and byte ordering
ST200 Name
__swapbh
__swaphw
__swapbw
Intrinsic
__uint16_bitcnt_uint32
__uint16_bitcnt_uint16
Purpose
DSP operator
DSP operator
Table 55: Population count
ST200 Name
__bitcntw
__bitcnth
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8.6 ST231 intrinsics
This compiler supports the following new intrinsics: int __st200mul32(int, int); int __st200mul64h(int, int); unsigned int __st200mul64hu(unsigned int, unsigned int); int __st200mulfrac(int, int);
These intrinsics were originally developed to give quick access to new ST231 instructions, when they were not selected by the compiler. However, the three integer multiplication intrinsics are now almost useless since the compiler correctly selects the new instruction. The last intrinsic ( __st200mulfrac ) is never selected by the compiler, since its semantic is complex (multiply fractional with saturation and rounding).
To simplify development work, these intrinsics are available for any ST200 core.
They are expanded:
• as a single machine instruction when the target is st231 (for example, when
-mcore=st231 command-line is used),
• as a sequence of instructions that emulate the behavior of the st231 native instructions for other cores (st220).
Table 56 summarizes these intrinsics properties either when emulated or native:
Name
__st200mul32
__st200mul64h
__st200mul64hu
__st200mulfrac
Instructions / critical path for emulation
(ST220)
3 / 2
7 / 4
13 / 6
17 / 8
Table 56: ST231 intrinsics properties
Instructions / critical path for ST231
1 / 1
1 / 1
1 / 1
1 / 1
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Division and modulus built-ins
8.7 Division and modulus built-ins
The compiler supports several division and modulus built-ins that expand to a loopless sequence of instructions to compute the result of the integer division (or modulus) operands for signed or unsigned integers.
These built-ins only exist for the integer division (or modulus) operator:
__builtin__divw
__builtin__divuw
__builtin__modw
__builtin__moduw
The benefits are that the division operators do not suffer from instruction call penalties (and possible related ICache conflicts), and that other instructions can be scheduled at the same time to maximize resource usage.
The drawback is that contrary to the library version, these operators do not minimize the number of iterations, whereas the library version considers the respective magnitude of the operands to minimize the division cycles when beneficial.
Note: These division and modulus operators should not be used when the divisor is a constant since the compiler emits efficient inline code when the constant is a power of 2.
Table 57 summarizes the properties of these built-ins.
__builtin__divw
__builtin__divuw
__builtin__modw
__builtin__moduw
Name Instructions / critical path
88 / 41
76 / 39
77 / 40
78 / 39
Table 57: ST231 intrinsics properties
These built-ins incur a high cost in code size, and should probably be inlined only in critical inner loops.
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Compiler bugs
9
9.1 Introduction
This chapter describes the different categories of compiler bugs and how they should be reported to STMicroelectronics.
9.2 Identifying a compiler bug
9.2.1 Category 1
The following cases are compiler or toolset bugs:
• the compilation phase ends with an assertion message,
• the compilation phase ends with a system error message (core dump, bus error),
• the compilation phase produces an output that cannot be assembled,
• the compilation phase never ends, or at least does not end in a reasonable amount of time,
• the compiler produces an error message for code that is valid input,
• the compiler produces code that does not compute the expected results (but see
).
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9.2.2 Category 2
The following case is possibly not a compiler or toolset bug.
The code is functional under a specific optimization level, but not under another.
This may be due to an existing code bug that is only exposed by aggressive optimization.
9.3 Checks performed by user
The following checks should be performed on your code before reporting a bug:
• check that the code works correctly on at least one other compiler, on another host,
• check that the code does not access out-of-bound memory,
• check that the source code does not raise any warning when compiled with the
-Wall option,
• check that the source code does not make assumptions that may be false: specifically check restrict annotations, and optimization pragmas,
• check that the code does not exercise language edges or does not violate language standards: an example of undefined behavior is to assume a specific behavior of shift operators when the shift amount is negative or bigger than the size of the type shifted.
9.4 Work-around
The following can be carried out to temporarily work-around a compiler bug.
• Demote the optimization level to -O1 or -O0 when compiling the specific file creating the problem, either in category 1 or 2. (See
.)
• Remove the optimization pragmas or restrict annotations.
• Finally, check that you have an up-to-date compiler release.
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9.5 Reporting a compiler bug
Carry out the following if a compiler bug is encountered.
• Obtain your compiler version by running the command st200cc -version.
• If the compiler bug is in category 1 (see
Section 9.2.1 on page 147 ), prepare a
pre-processed input file that can reproduce the problem.
• If the compiler bug is in category 2 (see
Section 9.2.2 on page 148 ), prepare a
source set and Makefile that can reproduce the problem.
• Supply the full command line that generates the problem.
• Report the result of the following command in the shell that you use: uname -a.
• Prepare a description of the expected result and the actual result.
• Report all the above information through your local ST Field Applications
Engineer (FAE).
Finally, when in doubt, it is preferable that a possible bug is reported than forgotten.
9.6 Known bugs and limitations
Please refer to the Bug list supplied with the toolset on the CD for an up-to-date list of bugs and limitations.
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ICache/dead code optimization
10
10.1 Introduction
st200cc includes a binary phase optimization. This phase calls the binopt binary that performs ICache optimization, dead code and data removal.
st200cc provides a comprehensive set of options to perform ICache optimization.
These options call the binopt binary, which may also be invoked directly as described in
Section 10.4: binopt on page 166 .
The ICache optimization options of the st200cc driver control the final function layout of the program. Function reordering in the final executable is important for targets with instruction cache. Indeed, functions that have temporal locality
(determined by static analysis or profile based analysis), should also have spatial locality in order to optimize cache usage by reducing cache conflicts. Depending on the algorithm used for function placement, memory spatial locality or cache line spatial locality may be enforced.
Profile-based optimization (dynamic mode) is performed by first running the application, collecting profiling data and then recompiling and linking the application, passing in the profiling data to the ICache optimizer. Static analysis is performed by the optimizer using estimation instead of run-time data.
The binopt phase also performs dead code and unused data removal. st200cc provides a set of options to customize this phase.
Binary optimizations are performed in several phases, and it is important to pass the optimization options to both the compilation and the link phase. Several examples are given in
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10.1.1 How it works
Binary optimizations are based on studying relocation at the assembler level.
The ICache optimizer builds a call graph (estimated or not according to the mode used). The edges are weighted by the number of calls between procedures. This weight is estimated from compiler information contained in the .profile_info section. Then, some algorithms stemming from the references are applied and an optimal function layout is found (see
Section 10.6: References on page 174 ). Only
functions whose body is in the .text section are candidates to move.
Dead code and dead data elimination follow the same principles. However, the generated ‘relocation’ graph takes into account all symbols (data, read-only data and code). This graph is traversed, and un-reached nodes are removed.
Candidate symbols to these optimizations must:
• have the moveable assembler attribute:
procedures beginning with .proc and ending with .endp automatically have the moveable attribute,
data needs the moveable attribute to be set with the following syntax:
.type my_data, @object, moveable
Note: The compiler generates this attribute for symbols that it estimates to be moveable.
This generation is not linked to any optimization level.
• have a size,
• be assembled with --emit-all-relocs option to st200as. This option is set by
st200cc driver if dead code or ICache optimizations are required.
Such a symbol is called a moveable symbol in the rest of this section.
These conditions guarantee that all relocations within a moveable symbol are not resolved, and that all relocations pointing towards a moveable symbol are also not resolved. So, such a symbol can be safely removed or moved. Relocations will be resolved in a final link by st200ld.
Note: 1 ICache optimization and dead code removal are implicitly turned on by the st200cc
-Os and -O2 (and higher) optimization options.
2 Debug information is preserved by binary optimizations.
3 Mixing objects files compiled at different optimization levels is safe. Only symbols with the listed characteristics are candidates for binary optimization.
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10.1.2 Synopsis
Several options specifically relate to ICache optimization and may be used in conjunction with the --icache-opt option, they are summarized below and described in detail in
• Enable or disable ICache optimization: st200cc --icache-opt=[on|off]
• Enable ICache optimization using a user-defined ordering file to order functions in the final executable: st200cc --icache-opt=on --icache-mapping=file.ly
• Apply compiler generated static analysis to the optimization: st200cc --icache-opt=on --icache-static
[--icache-algo=algorithm]
• Apply profiling data (obtained during program execution) to the optimization: st200cc --icache-opt=on --icache-profile=file.dt
--icache-profile-exe=exe.dt [--icache-algo=algorithm]
• Enable dead code and data removal: st200cc --deadcode
• Disable dead code and data removal: st200cc --no-deadcode
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10.1.3 ICache files
Table 58 provides a synopsis of each of the files used in binopt optimization.
.dt
File
.ly
.po
.op
Description
Simulator trace file.
This gives start and destination addresses for each jump in the execution sequence. Use the TracePlugin simulator plug-in. This has the extension
.so
for Unix hosts or .dll for Windows Hosts.
a
This file is passed to st200cc via the --icache-profile option.
Function layout file.
This gives the order of functions in the final executable. This file is generated by the st200cc --icache-static or --icache-profile options when the -keep option of st200cc is used., or hand written and passed to st200cc with the --icache-mapping option.
Pre-optimized object file.
This file is a relocatable object file and is generated with the -r linker option.
It contains unresolved relocations. A relocatable object file is used by the reordering phase (in order to move code, relocations must not be resolved).
This file is kept when the -keep option of st200cc is used.
Post-optimized object file.
This file is a relocatable object. It contains reordered code. The final executable is obtained by resolving relocations in this file (simple link phase).
This file is kept when the -keep option of st200cc is used.
Table 58: st200cc binopt optimization files
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File Description
.icg
.rmlist
Instruction cache call-graph file.
This file is a user external file. Use this to add edges to the estimated static call-graph (see
Section 10.4.4: How to configure binopt on page 171
for more details).
Removed symbol file.
This file is generated by binopt when dead code and data removal is activated. It contains list of symbols (data and functions) removed. It also contains some statistics about saved space. This file is generated when the
-keep option of st200cc is used.
Table 58: st200cc binopt optimization files a. To use the TracePlugin plug-in, for example to invoke st200run to run the simulator on a Unix host, use a command similar to the following: st200run -d "st200sim 220 MODE ISS TRACING_ON true
TRACE_PLUGIN_MODULE
<tools-dir>/host/tplugins/TracePlugin
OUTPUT_TRACE_FILE file.dt" -- executable
<tools-dir> is the directory where the toolset is installed.
See the ST200 Cross Development Manual for further details about st200run.
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10.2 st200cc options
The following key options are interpreted by st200cc:
--icache-opt=value
Enables or disables ICache optimization. value may be set to on for enable or off for disable. This option is implicitly turned on by the st200cc optimization options -O2 or higher. All other
ICache optimization options are controlled by this option.
--icache-mapping
Turns on function reordering based on a user specified file.ly.
This is done in several steps:
• in the assembly phase the assembler is requested to emit all relocations,
• in link phase several steps are performed:
a pre-link phase generates a relocatable image of the executable (.po file),
a reordering phase [BINOPT] generates the optimized relocatable (.op file) from the input file (file.ly) and the relocatable image,
a final link phase maps the optimized relocatable file to an executable.
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--icache-static=value
Enables or disables static analysis based function reordering.
value may be set to on for enable or off for disable. When this option is set to on, a new elf section (.profile_info) is created.
This section contains frequency estimation of call counts. These sections are present only in .po and .op object file. The linker removes this section in the final link phase.
Static analysis is done in several steps.
• In the compiler back end, estimated block frequencies are emitted in a special section.
• In the assembly phase, the assembler is instructed to emit all relocations.
• The link phase is split into several phases.
A pre-link phase generates a relocatable image of the executable (.po file).
An optimization phase [BINOPT] generates the function layout file (.ly file) based on estimated block frequencies.
The algorithm used for the computation of function layout can be changed with the --icache-algo option. Then, it generates an optimized relocatable image from the relocatable image and the function layout file (.ly).
A final link phase maps the optimized relocatable file to an executable.
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--icache-profile=file.dt
Turns on profile based function layout. The file.dt drives the function placement algorithm. If file.dt does not exist, the optimization is skipped and the following message warning is emitted by the driver: profile file not found, profile and relink your application
Profile based function layout has the following steps.
• In assembly the phase the assembler is instructed to emit all relocations.
• The link phase is split into several phases.
A pre-link phase generates a relocatable image of the executable.
An optimization and trace analysis phase [BINOPT] generates the function layout file based on the file.dt.
This phase needs the original binary to build the symbol table and match addresses of the trace file with the function name. This binary is passed to this phase with the
--icache-profile-exe=exe option of the st200cc driver. (If this option is omitted, the file specified with the
-o =exe option to st200cc is used. In this case this file is overwritten by the final link phase.). If the exe file does not exist (or a.out if omitted), --icache-profile is ignored. Coherence between file.dt and the original binary exe is checked by the dates of these two files
(file.dt must be more recent than exe). The algorithm used for the function layout can be changed with the
--icache-algo option. Then, it generates an optimized relocatable image from the function layout file and the relocatable image.
A final link phase maps the optimized relocatable file to an executable.
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--icache-algo=algorithm
Controls the placement algorithms used by static or profile based for ordering. algorithm may take the values ph, col, trg or ltrg .
• algorithm=ph
This is the default. Enables a placement that optimizes main memory spatial locality of the code by implementing [PH90].
There is no code size impact with this algorithm.
• algorithm=col
Optimizes main memory spatial locality with the knowledge of cache configuration to avoid cache conflicts. The implemented algorithm is [HKC96]. This algorithm may increase code size. It should be used for applications that have a large code covering / cache size ratio (> 2).
• algorithm=trg
Enables a more optimal placement for minimization of cache conflicts at the cost of large code size increase. This implements the [GBSC97] algorithms and only works with profile based option. It uses temporal information from the profiling trace.
• algorithm=ltrg
Enables a near optimal placement for minimization of cache conflicts at the cost of large code size increase. It should be used with caution or for experiments. It implements variations of the [GBSC97] algorithms. This algorithm might take a very long time to execute.
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--deadcode/--no-deadcode
Activate/deactivate dead data and dead code removal.
Dead code and data removal is done in several steps.
• In the assembly phase, the assembler is instructed to emit all relocations.
• The link phase is split into several phases.
A pre-link phase generates a relocatable image of the executable (.po file).
An optimization phase [BINOPT] removes unused symbols and generates a lightened relocatable image.
A final link phase maps the optimized relocatable file to an executable.
10.2.1 Other options
All the binopt binary options may be passed to st200cc by prefixing them with
-Wo, ; see
Section 10.4: binopt on page 166 .
10.2.2 Default behavior
By default, only moveable procedures in the .text section are candidates to move.
Code in user defined sections are not taken into account by binopt, however, a layout file can be defined for these procedures with the --icache-mapping option to st200cc.
However, since dead code and data removal is based on an analysis of the ‘relocation’ graph of the whole application, moveable symbols in any section are candidates for removal. In addition, binopt requires knowledge of the whole application, so all symbols have to be defined in the relocatable entry object (.po file).
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10.2.3 Option combinations
If the --icache-static, --icache-mapping and --icache-profile options are used together on the same command line, each specified optimization is taken as far as possible while they do not compete. When they compete, --icache-mapping has first priority, --icache-profile has second priority and --icache-static has the lowest priority of the three options. If the file specified with
--icache-mapping or --icache-profile does not exist, the option is ignored.
When combining ICache optimization and dead code and data removal, [BINOPT] phase makes both optimizations at once. Dead code and data removal is first executed, then ICache optimization is completed on the lightened code. This means that, dead functions do not pollute ICache optimization.
10.2.4 Full optimization
For the most efficient function layout optimization, the run-time libraries must be compiled with the --icache-opt and --icache-static options. Alternatively the global optimization option -O2 (or higher), must be used. This applies to both the standard libraries provided with the ST200 toolchain and to user libraries.
Dead code and data optimization also needs to compile libraries with --deadcode or -O2 (or higher) options, because only code assembled by a special option of st200as (--emit-all-relocs) can be removed.
10.2.5 Relocatable files
The -r option to the linker, which can be used to generate a relocatable file is compatible with ICache optimizations. In this case, -r option is passed to the final link phase and the optimized relocatable object is generated.
The -r option deactivates the dead code and data removal optimization. Indeed, when building a relocatable object, binopt cannot see the whole application, some symbols may be undefined and resolved later.
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10.2.6 Shared libraries, call shared executable
The dynamic mode (-shared, -call-shared options), which can be used to generate a shared library or an executable dynamically linked to shared libraries, is not compatible with binary optimizations. Binary optimization requires an intermediate relocatable object to be built which could not be later transformed into a dynamic object.
Note: Shared libraries and call-shared executables are not supported by this release.
10.2.7 Passing other options to the optimization phase
The -Wo,[option] can be used to pass specific options to the optimization phase
[BINOPT].
For example:
• -Wo,--sizecacheline,64 -Wo,--cacheline,512
These options set the configuration cache to 64 bytes x 512 lines.
• -Wo,--begin,my_main
These options activate the instruction cache optimization for my_main function only.
It is possible to enhance the estimated static call-graph with an external (.icg) file.
This is only for optimizations based on a static analysis. To do this, use:
-Wo,--icg,myconfig.icg
A .icg file is a list of the following items:
source destination frequency
Where frequency is a float, and source and destination are function names.
A .icg file can be useful to enrich the estimated call graph with syscalls that are not detected via a simple relocation analysis. A default .icg file is taken. Use the following command to obtain the default file:
-Wo,--icgdump
See
Section 10.4.4: How to configure binopt on page 171
for more details.
The following option preserves a symbol and all its descendants during the dead code phase. More than one --preserved option may be given. Only one name may be given with each --preserved option. This symbol can be a function or data.
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-Wo,--preserved,my_symbol
The following option preserves all symbols defined in my_section and in all its descendants. More than one --section_preserved option may be given. Only one name may be given with each --section_preserved option.
-Wo,--section_preserved,my_section
Likewise the -Yo,path options can be used to define a specific path for the
[BINOPT] phase.
10.3 Examples
10.3.1 Compiling with static ICache optimizations st200cc -O2 -o opt_exe *.c
or st200cc --icache-opt=on --icache-static -o opt_exe *.c
or
# 2 phases
# compilation + frequency estimation (heuristic) st200cc -O2 -c *.c
# link (repeat CFLAGS) st200cc -O2 -o opt_exe *.o
or
# 2 phases
# compilation + frequency estimation (heuristic) st200cc --icache-opt=on --icache-static -c *.c
# link (repeat CFLAGS) st200cc --icache-opt=on --icache-static -o opt_exe *.o
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10.3.2 Compiling a relocatable object with static ICache optimizations
This is useful for libraries.
# lib_stat is the main function of the library (algorithms
# use this function as the root of the static call-graph) st200cc *.c --icache-opt=on --icache-static -o lib.o -v -Wl,-r
-Wo,--begin -Wo,lib_start
# use the archiver to produce the lib.a from lib.o
st200-ar -rc lib.a lib.o
or st200cc *.c --icache-opt=on --icache-static -c st200cc *.o --icache-opt=on --icache-static -o lib.o -Wl,-r -Wo,--begin
-Wo,lib_start st200-ar -rc lib.a lib.o
10.3.3 To keep a generated layout file st200cc --icache-opt=on --icache-static -o opt_exe -keep *.c -Wo,-v,-v
The generated file opt_exe.ly contains the computed layout file.
This layout can then be forced with: st200cc --icache-opt=on --icache-mapping=opt_exe.ly -o opt_exe *.c
-Wo,-v,-v
10.3.4 Using profile driven ICache optimizations st200cc --icache-opt=on -c *.c
First link: st200cc --icache-opt=on -o exe *.o
Profile: st200run -d "st200sim 220 MODE ISS TRACING_ON true TRACE_PLUGIN_MODULE
<tools-dir>/host/tplugins/TracePlugin OUTPUT_TRACE_FILE trace.dt" -- exe
Re-link: exe must still exit st200cc --icache-opt=on --icache-profile=trace.dt
--icache-profile-exe=exe -o opt_exe *.o -Wo,-v,-v
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10.3.5 Using profile driven ICache optimizations using -O2 or higher
The compiler optimization options -O2 or higher are used.
# Build first executable with instruction cache information
# (default behavior with -O2 or upper) st200cc *.c -O2 -c st200cc *.o -O2 -o exe --no-deadcode
Profile: st200run -d "st200sim 220 MODE ISS TRACING_ON true TRACE_PLUGIN_MODULE
<tools-dir>/host/tplugins/TracePlugin OUTPUT_TRACE_FILE trace.dt" -- exe
Re-link with the profile file: st200cc *.o -O2 --icache-profile=trace.dt --icache-profile-exe=exe -o opt_exe -Wo,-v,-v
10.3.6 Compiling with st200gprof file-driven ICache optimizations
Compile using profiling: st200cc *.c -O2 -keep -o exe -pg --no-deadcode
Execute and create the gmon.out.000 file: st200run -d "st200sim 220 MODE ISS" -- exe
Recompile source file without profiling: st200cc *.c -O2 --icache-profile=gmon.out.000 --icache-profile-exe=exe
-keep -o opt_exe -Wo,-v,-v
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10.3.7 Linker options
The majority of arguments intended for linker phase (such as -Wl,-[option]) are passed to the first relocatable link phase (when this phase is needed). However some specific options are only passed to the final link phase (after [BINOPT] phase):
-T , -Wl,-T, -Wl,-r, -r, -Wl,--relax, --relax, -Wl,-Map, -Wl,-M.
Some others linker options deactivate binary optimization:
-Wl,-shared, -Wl,--shared, -Wl,-dy, -Wl,-Bdynamic,
-Wl,-call_shared, -Wl,-export-dynamic, -Wl,-E,
-Wl,-dynamic-linker, -Wl,--dy, -Wl,--Bdynamic,
-Wl,--call_shared, -Wl,--export-dynamic, -Wl,--dynamic-linker,
-call_shared, -shared.
10.4 binopt
This section describes how to invoke and use the binopt binary to perform instruction cache optimization.
Note: The options to binopt are accessible through the st200cc driver, by prefixing them
with the -Wo option.
10.4.1 Synopsis
<tools-dir>/bin/binopt [options] --input <file-in> --output
<file-out> where <tools-dir> is the directory the toolset is installed in, file-in is an input executable (relocatable binary for static mode, real executable for dynamic mode).
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10.4.2 Options
Global binopt options
--icacheopt
--deadcode
--codereorder
Perform instruction cache optimizations.
Perform dead code elimination.
Perform code reordering from a hand-written layout file.
--input
--output
Input object file (relocatable object).
Output object file (relocatable object).
--input_exe Input executable (real executable for dynamic
ICache optimization).
--deadcodeobject <file> Keep intermediary output object file after dead code elimination.
--layoutfile <file> Keep intermediary output layout file of functions
(.ly file).
--deadcodefile <file> Keep intermediary list of removed symbols.
--preserved <name> Preserve a symbol and all its descendants. More than one --preserved option may be given. Only one name may be given with each --preserved option.
--section_preserved <my_section>
Preserve all symbols defined in my_section and all its descendants. More than one
--section_preserved option may be given. Only one name may be given with each
--section_preserved option.
--scan_end_proc (-E) Use old $endproc label to find function size (default is false, function size is now deduced from elf attributes).
User command file
@<file> Give a user command file. See
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Input and output options
--icg (-g) <file> ICache call-graph user configuration (user defined edges). See
Section 10.4.4: How to configure binopt on page 171 .
--outlayout (-o) <file> Specify file where layout is written (if omitted output is sent to stdout).
Trace options
--davinci (-y) <file> Dump call-graph into davinci syntax. See
.
--dump (-d)
--icgdump (-D)
--help (-h)
--version (-V)
--verbose (-v)
Dump call-graph to stdout in xvcg format. See
Dump ICache call-graph user configuration.
Displays a help page of options.
Show version.
Increase verbose level by one (default is 0).
Type of optimization
--coloring (-C)
--ltrg (-L)
--ph (-P)
--ph_col (-O)
--profile (-X)
Change the placement algorithm to coloring algorithm [COL].
Change the placement algorithm to Large Temporal
Relationship Graph [LTRG] (only applicable in profiling mode and may be very long).
Change the placement algorithm to Pettis & Hansen
[PH90] (default algorithm).
Change the placement algorithm to a mix of
[PH90]and coloring [PH_COL]. Use [PH90] call-graph merge with [COL] node merge. Node merge attempts to minimize cache conflicts using the set of the available colors for each procedure.
Perform dynamic optimization from a profile file
(default mode).
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--trg (-T)
-- static (-S)
Change the placement algorithm to Temporal
Relationship Graph [TRG] (only applicable in profiling mode).
Perform static optimization from .profile_info section contents.
call-graph configuration
--begin (-b) <proc:weight>
Start symbol with its weight (default is main:1.0).
Several -b options are allowed. Weight is only taken into account in static mode.
--ignore (-i) <proc> Ignore symbol (default is empty). Several -i options are allowed.
See
Section 10.4.4: How to configure binopt on page 171
.
Algorithm specific options
--cacheline (-c) <nb> Number of lines in the cache (default is 512), for all algorithms except for [PH90].
--chunksize (-s) <size> Chunk size (default is 64). Each function is split into chunks of size <size>, only for [TRG] and [LTRG].
--fetch (-f) <size> Size of “addend to size” in cache line, to take into account prefetch (default is 1), only for [COL] and
[PH_COL].
--sizecacheline (-z) <size>
Size of a cache line in bytes (default is 64), for all algorithms except for [PH90].
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--deffreq (-e) Default call site frequency (default 1.000000e+00), for non instrumented part of code, only for static mode.
--prune (-p) <float> Percentage of the sum of edge-weights kept for pruning graph (default 100 - 1.000000e-07), only for static mode.
--trace (-t) <file> Branch sampling trace file
(syntax: src dest nb_of_calls), only for profile mode.
10.4.3 binopt dump options
binopt enables a call-graph to be generated either a static call-graph, or a profile call-graph. Within the call-graph “nodes” are procedures and “weight of the edges” is the number of calls (estimated for a static call-graph or based on the profile trace).
VCG/XVCG dump
By using the --dump option, binopt generates a XVCG (Visualization of Compiler
Graphs) file which is sent to stdout.
Example: binopt -o myexe.ly --static --icacheopt --input myexe.po --dump
--output myexe.op > myexe.dump
or st200cc *.c -O2 -Wo,--dump -o myexe > myexe.dump
then xvcg myexe.dump&
VCG/XVCG - USAAR Visualization Tool V.1.3 can be downloaded at: http://rw4.cs.uni-sb.de/~sander/html/gsvcg1.html#availability daVinci V2.1 dump
When the --davinci <file> option is used, binopt generates a daVinci file in
<file> .
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Example: binopt -o myexe.ly --static --icacheopt --input myexe.po --davinci myexe.davinci --output myexe.op
or st200cc *.c -O2 -Wo,--davinci,myexe.davinci -o myexe then daVinci myexe.davinci& daVinci V2.1 can be downloaded at: http://www.tzi.de/~davinci
10.4.4 How to configure binopt
To help binopt to better estimate the call-graph of your final application, a set of options are provided.
• -icg <file>, ICache call-graph user configuration file.
In this file, it is possible to add or remove edges of the estimated static call-graphs. Use binopt --icgdump to see an example of such a file. It can be useful to add edges which are not caught by a simple relocation analysis
(indirect call or syscalls), or to remove polluting calls which you know are not taken. This option is only valid for static mode.
• -ignore <proc>, all calls to proc and its descendants are ignored.
It may be useful to ignore calls to a library procedure. This option may be used in both static and profile mode. For example:
--ignore printf --ignore fprintf
• -begin <proc[:weight]>, any procedure specified with this option is interpreted as an entry point in your application.
Relative weight can be specified to give more importance to one procedure in relation to another. This option can be used in both static and profile mode. In profile mode, weight is ignored. For example:
--begin myapp_init:0.3 --begin myapp_main:0.5 --begin myapp_finish:0.2
or in profile mode:
--begin myapp_init --begin myapp_main --begin myapp_finish
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10.4.5 User command file
A command file containing options to pass to binopt is specified by using the
@<filename> option. Options can be separated by space, tabulation or newline.
For example: cat cmd.binopt
>--begin init:0.2 --begin main:0.8
>--ignore printf
>--ignore fprintf
>--davinci myapp.davinci
binopt --layoutfile myexe.ly --static --input myexe.po @cmd.binopt
--output myexe.op --icacheopt or st200cc *.c -O2 -Wo,@cmd.binopt -o myexe
10.5 Warning and error messages
10.5.1 binopt
• “symbol %s is not a function: ignored”: warning: Symbol defined in an .icg file is not a function.
• “no need of trace file in static mode”: error: use --trace option with --static.
• “No .profile_info section present. BB frequency is 1.”: warning: there are not any
.profile_info
section in the input object file whereas --static is used.
binopt estimated all call counts to 1.
• “Sorry too many start functions max is %d”: error: user gives more than 128 start functions.
• “Sorry too many ignored functions max is %d”: error: user gives more than 128 ignored functions.
• “unknown argument %s”: error: wrong option.
• “Too many arguments”: error: too many arguments on the command line.
• “need executable”: error: input object file is not given to binopt.
• “fatal error opening file %s”: error: cannot access input object file.
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• “Not a bfd_object !”: error: input object file is not recognized.
• “ERROR:Unable to find any calls in the trace file %s!!!”: error: in profile mode, cannot find any calls.
• “need trace file”: error: trace file has been omitted.
• “Can't build graph for daVinci with this algorithm”: in profile mode, daVinci dump is not accessible for TRG and LTRG algorithms.
• “start symbol %s is not a function: ignored”: warning: start symbol given with
--begin option is not a function.
• “start symbol %s is not global: ignored”: warning: start symbol given with
--begin option is static.
• “ignored symbol %s is not a function: ignored”: warning: ignored symbol given with --ignore option is not a function.
• “ignored symbol %s is not global: ignored”: warning: ignored symbol given with
--ignore option is static.
• “overflow during normalization of static call-graph”: warning: during the normalization of static call-graph, overflow has been detected. A re-normalization is done to avoid overflow in next phases.
• “Can't apply this algorithm placement for static estimation, take PH.”: warning:
TRG or LTRG algorithm are not accessible in static mode.
• “Cannot find %s in symbol table\n”: warning: a symbol given in a .ly input file cannot be found in the symbol table.
• “Cannot find %s with index %d”: warning: warning: a symbol given in a .ly input file cannot be found in the symbol table with this index (case of several static functions with the same name).
• “Can't open link order file %s”: warning: input ordering file (.ly) cannot be accessed. No reordering is done, just copying.
• “bfd error %s”: error: input object file is not a bfd object.
• “%s: No symbols”: warning: cannot find any symbol in the input object file.
• “Cannot remap symbol %s as requested in the link mapping: symbol not found, with unknown size or already placed”: warning: a function cannot be moved.
• “input mapping file %s is in wrong format”: warning: syntax error in the reordering input file (.ly). Ignore this file, output object file is a copy of input.
• “no relocation found just copying”: warning: cannot find any relocation, so reordering is impossible. Output object file is a copy of input.
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10.6 References
[COL]: “Efficient Procedure Mapping Using Cache Line Coloring”, Hashemi, Kaeli
& Calder (1996) WRL Research Report 96/3.
[GBSC97]: “Procedure Placement Using Temporal Ordering Information”, NiKolas
Gloy, Trevor Blackwell, Michael D. Smith, and Brad Calder, IEEE 1997.
[HKL96]: “Efficient Procedure Mapping using Cache Line Coloring”, Air H,
Hashemi, David R. Kaeli and Brad Calder, WRL Research Report 96/3, 1996.
[BINOPT]: Optimization at binary level. Binary name is binopt. This utility performs instruction cache optimization, dead code and data removal. It includes old codereorder and icacheopt tools.
[LTRG] An algorithm inspired from [TRG], using a multi-graph that represents exactly cache conflicts.
[PH90]: “Profile Guided Code Positioning”, Karl Pettis and Robert c. Hansen, ACM
1990.
[PH_COL]: An algorithm inspired from [PH90] for graph merge and [COL] for graph node union (take into account colors to minimize conflict, insert blank when needed).
[TRG] “Procedure Placement Using Temporal Ordering Information”, Gloy,
Blackwell, Smith, Calder (1997) published in the Proceedings of Micro-30.
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11.1 Introduction
This chapter documents the assembler and the assembly language for the ST200 processor family.
The ST200 assembler is based upon gas, the GNU assembler. Much of the information about assembler directives and expression syntax is repeated, in abbreviated form, from the standard gas documentation. The primary areas in which the ST200 assembler deviates from gas are in the syntax for instructions and in the restrictions placed upon the standard directives. In addition, the ST200 assembler accepts a number of ST200 specific directives and options. The description of ST200 instruction syntax assumes some familiarity with the architecture of the ST200 processor family.
details the syntax of assembler source programs.
describes how the assembler is run: the command line options, input and output file formats.
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11.2 Assembler syntax
11.2.1 General description
An ST200 assembly file consists of three types of fields: comments, directives and instructions. Comments are treated by the assembler as white space and, other than syntax, have little bearing on the subsequent description.
Directives include constant declarations, memory allocation commands, and commands to guide both the assembler and linker.
Instructions in the ST200 architecture may consist of multiple operations that are to be executed simultaneously. In the following discussion, we use the term bundle to refer to a group of operations that are defined to be executed simultaneously.
An assembly file may consist of an arbitrary interleaving of directives, comments, and bundles (with a few restrictions on specific directives). A bundle consists of a sequence of operations and comments terminated by a pair of semicolons “;;” on a separate line.
11.2.2 Comments
There are two types of comments supported by the assembler. In both cases the comment is equivalent to one space. The input routines for the assembler, filter out all comments and replace them with a single space.
• C-style comments consist of an text beginning /* and ending with the next */.
C-style comments may not be nested.
• Line comments consist of any text on a single line beginning with the first pound character, #. A line comment can appear on a line by itself or as the last thing on any input line.
To be compatible with past assemblers, a special interpretation is given to lines that begin with # followed by an absolute expression. In this case the expression is interpreted as the logical line number of the next line. An optional string following such an expression is interpreted as a new logical file name. The rest of the line, if any, should be whitespace.
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Example:
# This is an ordinary comment.
# 42-6 new_file_name # New logical file name
# This is logical line #36.
Comments beginning with the strings #APP or #NO_APP also have a special interpretation and should not be used.
11.2.3 Lexical categories
11.2.3.1 Whitespace
Whitespace consists of one or more blanks or tabs, in any order. Unless whitespace occurs within character constants, any whitespace sequence is treated as exactly one space.
11.2.3.2 Symbols
A symbol is one or more characters chosen from the set of all letters (both upper and lower case), digits and the four characters _.$?. No symbol may begin with a digit.
Case is significant. All characters are significant. Symbols are delimited by characters not in the set described above, or by the beginning of a file (since the source program must end with a newline, the end of a file is not a possible symbol delimiter).
11.2.3.3 Constants
A constant is a number, written so that its value is known by inspection, without knowing any context.
Example:
.byte 74, 0112, 0x4A, 0X4a, 'J, 'J' # All the same value
• Character constants. There are two kinds of character constants. A
character stands for one character in one byte and its value may be used in numeric expressions. String constants (properly called string literals) are potentially many bytes and their values may not be used in arithmetic expressions.
• Strings. A string is written between double-quotes. It may not contain double-quotes or null characters. Special characters may be included in a string by using escape sequences as with C-language programs. Strings may be automatically null terminated in some contexts (.string, .asciiz).
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Assembler syntax
• Numerical constants. The assembler distinguishes three types of numbers according to their storage requirements in the target machine. Integers are numbers that fit into 32 bits (an “int” in the C language). Bignums are integers, but they are stored in more than 32 bits.
Integers.
A binary integer is “0b” or “0B” followed by zero or more of the binary digits, either 0 or 1.
An octal integer is “0” followed by zero or more of the octal digits chosen from
01234567.
A decimal integer starts with a non-zero digit followed by zero or more digits chosen from 1234567890.
A hexadecimal integer is “0x” or “0X” followed by one or more hexadecimal digits chosen from 0123456789abcdefABCDEF.
Bignums.
A bignum has the same syntax and semantics as an integer except that the number (or its negative) may require more than 32 bits. Bignums are not necessarily permitted in all places where integers are permitted.
Floats.
The ST200 floating-point runtime model defines two of the IEEE-754 standard data type formats, single and double. The IEEE single format has a precision of 24 bits (24 significant bits), and 32 bits overall. The IEEE double format has a precision of 53 bits, and 64 bits overall.
.float <expression>
.double <expression>
Where <expression> should have the format:
0e:<hex digits>
The <hex digits> are the hexadecimal form of the IEEE representation
(32-bit for .float, 64-bit for .double).
It is not possible to use true floating-point format (for example: +1.0e+10), because this is not currently implemented in the assembler.
.real4 <expression>:<number>
.real8 <expression>:<number>
Where <expression> is an integer or hexadecimal constant, just like
.data4
or .data8. The integer or hexadecimal constant is the binary representation of the IEEE format (32-bit for .real4, 64-bit for .real8).
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11.2.3.4 Statements
A statement begins with zero or more labels, optionally followed by a key symbol which determines the type of the statement. The key symbol determines the syntax of the rest of the statement. Assembler directives begin with a “.”. The directives accepted by the assembler are described in
Section 11.2.9: Assembler directives on page 187
. Any statement that begins with a letter is part of an ST200 instruction bundle. The bundle terminator (;;) is also a statement.
A statement is terminated by a newline character. Newlines within character constants are an exception, they do not terminate a statement. It is an error to end any statement with end-of-file; the last character of any input file must be a newline. Statements may be written on multiple lines provided that each line other than the last ends with a backslash (\).
11.2.3.5 Labels
A label is a symbol immediately followed by one or two colons (“:” or “::”).
Whitespace before a label or after the colon(s) is permitted, but whitespace between the symbol and colon(s) of a label is not permitted. Labels followed by a single colon are treated as local symbols, labels followed by a double colon are global symbols they are visible to other object modules linked with the output of the assembler.
Symbols can also be marked as global with the .global directive.
11.2.4 Symbols
The lexical specification for symbols has already been described. This section defines the semantics of symbols.
11.2.4.1 Giving symbols other values
A symbol can be given an arbitrary value by writing a symbol followed by a double equals sign (==), followed by an expression. This is equivalent to using the .equ or
.set
directives discussed later. Symbols are not required to be defined before use; however, the current implementation of the assembler allocates space for long immediates whenever a symbol is undefined even if the symbol is eventually resolved to a value that can fit in a short immediate.
Example: foo == 3
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11.2.4.2 Local and global labels
Labels defined with a double colon are global, that is they are visible to all the subprograms being linked. All other labels are local, that is they are only visible in the source file where they are declared.
Example: local_label: global_label::
11.2.4.3 Symbol names
Symbol names are required to begin with a letter or one of “_?”. The initial character may be followed by any string of digits, letters and characters from the set
“._?$”. The case of letters is significant: foo is a different symbol from Foo.
• Local symbol names. Local symbols may be defined for use in a given source file, but are not represented in the object file for subsequent use by a linker or debugger. By convention, local symbol names begin. “.L” (upper case), “L?”,“$”,
“?”, or “_?”. The assembler writes these symbols to the object file if the “-L” option is used.
• The dot symbol. The special symbol “.” refers to the current address that is being assembled. Therefore the expression “here: .long .” defines the location here to contain its own address. Assigning a value to “.” is treated the same as a “.org” directive. This means that the statement “. == . + 4” is the equivalent to “.skip 4” discussed later.
• Value. The value of a symbol is defined as a 32 bit 2's complement number. For a symbol which labels a location in the text, data, or bss sections, the value is the offset (in bytes) from the start of the section to the label.
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11.2.5 Expressions
An expression specifies an address or numeric value. An expression consists of one or more arguments delimited by operators. The arguments of an expression are symbols, numbers or sub-expressions.
11.2.5.1 Sub-expressions
A sub-expression is either an expression within parentheses or a prefix operator followed by an argument.
Example:
( 3 + foo )
-3
11.2.5.2 Absolute expressions
An expression that can be evaluated to a value without knowledge of addresses (one that is not relative to the start of some object section) is called absolute. Expressions that are not absolute have the form (section + offset) where section is typically one of text, data, bss or undefined, and offset is an offset into the section (a signed,
2’s complement, 32 bit integer). Some expression operators may only be applied to absolute sub-expressions.
11.2.5.3 Operators
• Prefix operators. The assembler supports the following unary prefix operators.
The argument must be absolute.
Negation. Two’s complement negation.
~ Complement. Bitwise not.
• Infix operators. The assembler provides a number of binary infix operators.
Apart from “+” or “-”, both arguments to the operator must be absolute; the result is absolute. Within a precedence class, operators are applied left to right.
Highest precedence.
*
/
Multiplication.
Division: truncation is the same as the C operator “/”.
%
<
Remainder.
Relational less.
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<< Shift left, same as the C operator “<<”.
Relational greater.
>
>> Shift right, same as the C operator “>>”.
Intermediate precedence.
|
&
Bitwise inclusive OR.
Bitwise AND.
^ Bitwise exclusive OR.
!
Bitwise NOT.
Lowest precedence.
+
-
Addition.
Subtraction.
11.2.6 Bundles
An ST200 processor may issue multiple conventional instructions (operations) in a single cycle; these operations are statically scheduled. A group of simultaneously issued operations is called a bundle. The end of a bundle is denoted by “;;”. Each operation of a bundle, as well as the bundle terminator must reside on a separate line in the source file (each operation is treated as a separate “statement”). The bundle may be interleaved with comments. Each operation of a bundle is a separate syntactic “statement”.
Example: operation0 # a comment operation1 /* another comment */ operation2
;;
Labels may not appear within a bundle; they must occur before the first operation of the bundle. The destination of a branch must be the start of a bundle.
The ST200 processors are implemented with a single cluster called c0. The assembler uses the operand syntax to determine the target cluster for a particular operation; however, as an aid to readability, a cluster identifier may optionally precede each operation of a bundle. These identifiers have the form (cx) where x is a cluster number.
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Example: c0 operation0 operation1
;;
The assembler does not interpret the cluster identifiers, they are purely an aid to reading complex code. In order to determine where a syllable is scheduled (and hence to generate the bits corresponding to a bundle) the assembler uses cluster information embedded in register names. The syntax for operands is discussed in
11.2.7 Operands
There are two basic types of operands used in defining the ST200 operations: register identifiers and immediates.
11.2.7.1 Registers
The syntax for a register descriptor is:
$ register_type cluster.register_number
where:
• the register type is a string (usually one character) that defines the bank of registers within a cluster,
• the cluster equals 0,
• the register number denotes the register within that bank.
Example:
$r0.1
$b0.3
general purpose register 1 of cluster 0 branch register 3 of cluster 0 c0 add $r0.3 = $r0.3, 42 # r0.3 <- r0.3 + 42 c0 add $r0.11 = $r0.12, 1 # r0.11 <- r0.12 + 1
;;
In the preceding example, two adds are scheduled to be issued in a single cycle. As can be seen, the cluster identifiers are redundant.
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In the currently planned ST200 processors, c0 has the following register banks:
$r A bank of 64 general purpose 32 bit registers.
$b A bank of 8 single bit branch (or condition).
The branch registers are used as condition bits for branch and select operations as well as operations that generate or consume carry bits.
There are a few general purpose registers with defined functions:
$r0.0
These registers always return 0 when read.
$r0.63
This register, known as the link register, is used to store the return address for all call operations. In addition, it is read for the target address by some call and goto operations.
To simplify the ST200 assembly syntax, starting from the R3.2 release of the toolchain, the cluster number is no longer required in the register syntax. By default, the cluster ‘0’ will be used by the assembler. The two following instructions are then equivalent: add $r11 = $r12 , $r2 add $r0.11 = $r0.12, $r0.2
11.2.7.2 Immediates
There are two basic types of immediates – absolute and PC relative. In the ST200 family, PC relative immediates are used for branch, call, and goto operations while all other immediates are absolute.
All immediates are encoded as 2's complement numbers. Immediates that can be represented with a small number of bits, are encoded directly in the machine word containing the operation, while longer immediates require an extension word.
The ST200 assembler allows all immediates to be defined by a number, symbol, or expression. In the case of PC relative immediates defined by labels, the assembler calculates the value relative to the PC.
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11.2.8 Operations
There are two basic types of operation statements accepted by the assembler: integer operations and control flow operations such as branches. The syntax of each class of operation is described in detail below. Case is significant in operation names: all operation names are lower case.
11.2.8.1 Integer operations
As a general rule, operands that are written by an operation appear to the left of any
“=”, while operands that are read appear to the right. When more than one operand is read (written), the operands are separated by “,”. There are a small number of operations (branch and prefetch operations) that do not write an operand and hence have no “=”. The optional prefix c0 denotes cluster 0.
Example: c0 slct $r11 = $b2, $r3 , 3 sub $r2 = 3, $r4
11.2.8.2 Conditional branches
Conditional branch operations read a branch register ($b0.y) and conditionally branch to a destination.
Example: c0 br $b0.1, dest_label
In this example, the branch is taken when $b0.1 contains 1.
Branch registers, which are read by conditional branch operations, are written by other operations including compares and arithmetic operations that generate a carry. These branch registers are also read by “select” operations and arithmetic operations that read a carry bit.
11.2.8.3 Call, goto and return
call and goto operations are similar to branches, except that they are unconditional. Each of these operations has two forms, an immediate, PC relative, form, and an indirect form that uses the system link register ($r63). Furthermore, the call operations write the link register with the address of the bundle following the current bundle. The return operation is a pseudo-operation for the indirect form of the goto. In order to make all the state used by an operation explicit, these operations explicitly name the link register even when it is implicit.
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186 Assembler syntax call $r63 = $r63 goto dest_label return $r63
# call through the link register
# goto a destination
# same as goto $r63
Only one branch, goto, or call operation may be issued in a single bundle.
Furthermore, the bundle sequentially following a taken branch, goto, or call operation is quashed.
11.2.8.4 Memory operations
The ST200 processors support three basic categories of memory operations: loads, stores, and prefetches. In the current processors, a bundle may include, at most, one memory operation.
Memory addressing
The ST200 architecture only supports one addressing mode. All memory accesses are made via a pointer in a register plus an immediate offset. The assembler syntax for this is: offset [ register ]
Example: stw 4[$r1] = $r23 # *($r1 + 4) <- $r23
Dismissible modifiers
The ST200 processors support speculative (dismissible) load operations. In particular it is possible to provide code that dereferences a pointer prior to the test to determine that the pointer is valid. A memory operation may be defined by software to be dismissible, in the event that a non-recoverable virtual memory fault occurs by performing a dismissible load, the value returned is defined to be 0. A dismissible load is denoted by the operation modifier “.d”. In the following example, probe(a) evaluates to true if a can legally be dereferenced.
Example: ldw.d $r2 = 0[$r1] # $r2 <- probe($r1) ? *($r1): 0
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Prefetch
The ST200 processors provide a group of prefetch operations that bring data into the appropriate cache, but do not modify a register and do not stall the machine on a cache miss.
Example: pft 8[$r8] # fetch the cache line containing * (8+$r8)
11.2.9 Assembler directives
The ST200 assembler supports most of the standard directives provided by the GNU assembler. Only a subset of the directives accepted by the assembler is described here.
11.2.9.1 Alignment directives
At present, the directives that insert values into memory, or change the location counter, are only valid in the data section. Using them in the text section is reported as an error.
.align abs-expr, abs-expr
This pads the location counter, in the current section, to the specified boundary. The first expression (which must be absolute) is the alignment request in bytes. The alignment request must be a power of 2. The second, optional, expression (also absolute) gives the value to be stored in the padding bytes. The default case is to pad by 0.
Example:
.align 8
The preceding advances the location counter until it is a multiple of 8. If the location counter is already a multiple of 8, no change is made.
11.2.9.2 Section directives
.section name Assemble the following statements into section name.
.data
.text
Assemble the following statements into the data section.
Assemble the following statements into the text section.
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11.2.9.3 Data directives
.comm symbol, length, alignment
Declare a named common section in the bss section. The length and alignment parameters must be absolute expressions.
.skip size, fill
Fill the next size bytes with the optional fill value. The default case is to fill with 0.
.byte
expression:number, ...
.data1 expression:number, ...
These directives, which are equivalent, take one or more pairs of parameters separated by commas. The first value of the first expression is assembled into the next byte location. The second, optional, parameter indicates the number of duplicate bytes to generate. Any expression that evaluates to a value that cannot be represented by a byte is truncated with a warning.
.hword expression:number, ...
.data2 expression:number, ...
These directives, which are equivalent, take one or more pairs of parameters separated by commas. They assemble the value of the first expression into the next two bytes. If the expression evaluates to a value that requires more than 16 bits to represent it then it is truncated and a warning issued. The second parameter in each pair is optional; if present it specifies the number of consecutive 16 bit locations to fill with the value of the first parameter.
.word
expression:number, ...
.data4 expression:number, ...
.real4 expression:number, ...
These directives, which are equivalent, take one or more pairs of parameters separated by commas. They assemble the value of the first expression into the next four bytes. The second parameter in each pair is optional; if present it specifies the number of consecutive 32 bit words to fill with the value of the first parameter. It is expected that when .real is used, the expression represents a floating point number.
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Assembler syntax 189
.quad
expression:number, ...
.data8 expression:number, ...
.real8 expression:number, ...
These directives, which are equivalent, take one or more pairs of parameters separated by commas. They assemble the value of the first expression into the next eight bytes. The second parameter in each pair is optional; if present it specifies the number of consecutive 64 bit words to fill with the value of the first parameter.
11.2.9.4 Symbol directives
.equ symbol, expression
.set symbol, expression
These directives, which are equivalent, set the value of symbol to expression. This changes the symbol’s value and type to conform to that of expression. If the symbol was flagged as external (see .global) then it remains external. The value of a symbol can be set many times in the assembly file. If the symbol is external, then the value stored in the object file is the last value assigned to it.
.global symbol
This directive defines a symbol as external (that is, visible to the linker). A label can also be marked as global directly by defining it with a double colon,“::”.
11.2.9.5 Include directive
.include file
This directive provides a way to include supporting files at specified points in your source program. The code from “file” is assembled as if it followed the point of the .include; when the end of the included file is reached, assembly of the original file continues.
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190 Assembler syntax
11.2.9.6 ST200 machine directives
.assume parameters
The .assume directive expects one or more parameters, separated by commas. It is also possible to use several consecutive .assume directives.
The parameters of the .assume directive are encoded in the elf flags of the object file to identify the basic code generation assumptions taken by the compiler. This allows the linker to detect attempts to link code assembled with different incompatible assumptions.
The .assume directive is generated by the C compiler. The user should only consider writing a .assume directive when mixing assembly code with C code.
The .assume directive accepts five types of parameters, listed in
Table 59 . Values within the parameter types are exclusive.
Parameter type
Core identification
Silicon implementation identification
Software ABI convention
OS ABI identification
Values st220 , st231
Description cut0 , cut1, cut2, cut3 , cut4, cut5 no-abi , old-multiflow-abi , lx-embedded-abi , pic-abi , gcc-abi bare-machine os21 , linux
,
Defaults to st220 if not already specified in the assembly file or by the -mcore command line option.
Note: this directive does not impact the encoding table used by the assembler, only the ELF flags.
Defaults to cut0 if not already specified in the assembly file.
Defaults to no-abi if not already specified in the assembly file.
Defaults to bare-machine if not already specified in the assembly file.
Table 59: .assume directive parameters
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Assembler syntax 191
.proc parameters
This directive is supported for compatibility reasons with existing code. The parameters are ignored.
.endproc parameters
This directive is supported for compatibility reasons with existing code. The parameters are ignored.
.type symbol, [@object | @function], moveable
Declare the symbol to be either the name of an object or a function. If moveable is specified, it can be moved around by the
ICache optimizer.
11.2.9.7 ST200 special directives
The following directives are generated by the compiler, but are ignored by the assembler. In most cases they are used by the compiled simulator.
.__longjmp
, .__setjmp, ._longjmp, ._setjmp, .call, .comment, .entry,
.import
, .return, .sversion, .type
11.2.9.8 Restrictions on standard directives
A number of restrictions currently apply to standard directives.
The following directives are not currently permitted in the text section:
.ascii
, .asciz, .byte, .data1, .data2, .short, .string
The following directives are permitted in the bss section, but they only have an impact on the size of the bss section, not its content (because the bss section has no content in the binary file):
.ascii
, .asciz, .byte, .data1, .data2, .short, .string, .data4, .data8,
.hword
, .long, .octa, .quad, .real4, .word, .single, .double, .float
The following directives are currently ignored:
.dc
, .dc.b, .dc.d, .dc.l, .dc.s, .dc.w, .dc.x, .dcb, .dcb.b, .dcb.d, .dcd.l,
.dcb.s
, .dcb.w, .dcb.x, .ds, .ds.b, .ds.d, .ds.l, .ds.p, .ds.s,
.ds.w,.ds.x
, .fill, .lflags, .mri, .org, .p2align, .p2alignw, .p2alignl
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192 Invoking the assembler
11.3 Invoking the assembler
11.3.1 Assembler command line
Here is a a brief summary of how to invoke st200as: st200as [ -a[hls][=file] ] [ -EB ] [ -EL ] [ -I dir ] [ -J ]
[ -L ] [ --mcore=st220|st231 ] [ -o objectfile ] [ -R ]
[ --statistics ] [ -v|--verbose ] [ --version ] [ -version ]
[ -W ] [ -Z ] [ -- | files... ]
The assembler can also be invoked by running the C compiler, with an input file that has a “.S” suffix. This allows the C pre-processor to be used. See
for details.
11.3.1.1 Command line options
This section gives a more detailed description of the command line options of the assembler.
Options may appear in any order and may be before, after, or between file names.
The order of file names is significant.
-a[hls] [= file ] Enable listings.
Enable listing output from the assembler. The option “-a” requests high-level, assembly and symbols listing. “-ah” requests a high-level language listing, “-al” requests an output-program assembly listing, “-as” requests a symbol table listing. The letters after “-a” may be combined into one option, for example “-als”. “-a=file” sends the listing output to file.
-EB
-EL
-I
Generate big endian object code.
Generate little endian object code. This is the default.
Include file search path.
Add a path to the list of directories that st200as searches for files specified in .include directives. “-I” may be used multiple times to include a variety of paths. The current working directory is always searched first; after that,
st200as searches any “-I” directories in the same order as they were specified (left to right) on the command line.
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-J Disable signed overflow warning.
Do not warn about signed overflow.
-L Include local labels.
Labels beginning with “L” (upper case only), “$”, “?”, or “_?” are called local labels. Normally these labels are not represented by symbols in the object file. This option instructs the assembler to create symbols representing local labels in the object file.
-mcore=st220|st231
Specifies whether the assembler should select the ST220, or the ST231 encoding table. The default, if no command line option is found, is ST220. It also encodes the corresponding information in the elf flags of the object file unless a .assume st2xx directive is found, in which case the elf flags will be the ones specified by the .assume st2xx directive.
-o objectfile
-R
Name the object file.
The default output file created by st200as is a.out. The output file name can be used with this option.
Join text and data sections.
Instructs st200as to merge the data section with the text section in the object file.
--statistics Display assembly statistics.
Use “--statistics” to display statistics about the resources used by st200as, such as the total execution time taken for assembly (in CPU seconds).
-v | --verbose Announce version.
To determine which version of st200as is executed, include the option “-v” on the command line.
-W
-Z
Suppress warnings.
By default all warnings are printed to the standard error file.
This option, disables the printing of warning messages.
Generate object file in spite of errors.
After an error message, st200as normally produces no output. With this option a output file is generated (with possibly erroneous data).
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11.3.1.2 Arguments
Except for “--”, any command line argument that begins with a hyphen is an option. Each option changes the behavior of st200as. No option changes the way another option works. An option is a “-” followed by one or more letters; the case of the letters is important.
Some options expect exactly one parameter to follow them. The parameter may either immediately follow the option letter or it may be the next command line argument.
In the following example the two command lines are equivalent: st200as -o my-object-file.o mumble.s
st200as -omy-object-file.o mumble.s
11.3.1.3 Input and output files
The phrase source program, abbreviated to source, is used to describe the program input to one run of st200as. The source program may be contained in one or more files; how the source is partitioned into files does not change the meaning of the source. By convention, assembler source files have a “.s” suffix.
The source program is a concatenation of the text in all the files, in the order specified.
Two hyphens “--” name the standard input explicitly as one of the files for st200as to assemble.
Each run of st200as produces an output file, the object file. If the source file is empty, st200as produces a small empty object file. The default output file name is a.out
, but it may be renamed using the -o option.
11.3.2 Error and warning messages
st200as may write warnings and error messages to the standard error file (usually the terminal). Warnings report assumptions made so that st200as can continue assembling a flawed program; errors report grave problems that prevents the source program from being assembled. Fatal error messages cause assembly to stop immediately.
Warning and error messages have the format: (where nnn is a line number).
file_name:nnn:Message text
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Revision history
Version
B
A
D
C
F
E
H
G
Date Comments
June ‘05
November ‘04
Revised to incoprporate the R4.1 Toolset release.
Revised to incorporate final comments for the R4.0 Toolset release.
September ‘04 Revised to include IPA, C++ and to reflect the R4.0 Toolset release.
June ‘04 Revised to include ST231 and to reflect the ST200 R3.2 Toolset release.
January ‘04
October ‘03
June ‘03
May ‘03
Minor update to clear review comments.
Revised to reflect ST200 R3.1 Toolset release.
Format changes to conform to style guide.
First complete version, submitted to ADCS
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Index
Symbols
indent 48, 58 inline_file 48, 64 inline_function 48, 64 inline_next 48, 63
ivdep 47, 50 loopdep 47, 51-52 loopmod 47, 53 loopseq 47, 56 looptrip 47, 54
noinline_file 48, 64 noinline_function 48, 64 noinline_next 48, 63
/* and */ comment characters 176
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A
aggregate
array
B
board.x 22 bootboard.o 22 bootcore.o 22 bootsoc.o 22
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C
C99 2, 13, 27-29, 97, 101, 104-108
codereorder
comment
constant
core 22, 25, 190, 193 core.x 22
D
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E
ELF 2, 97 endianness 2, 5, 22, 25
etsitom.h 130 etsitost220.h 130
F
-f[no]strict-aliasing 17, 71-75
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-f[no-]zero-initialized-in-bss 19
-fdiagnostics-show-location 35
-finstrument-functions 16, 81-83
function
G
style labeled elements in initializers 108
H
I
ICache
absolute distance 136 absolute value 136
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bit manipulation 140 clamp 140
high precision fractional arithmetic 143
switching model and ST220 names 131
ISO
ISO9899
K
L
label
load application. See st200run
M
macro
variable number of arguments 104
201
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N
O
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liberal_ivdep 50 unroll_size 50
optimization
call trace instrumentation 81-83
st200cc ICache options 156-159
st200cc options. See -O0 to -O3 and -Os
P
R
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libraries
S
standard template libraries 39-43
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203
ICache optimization examples 153, 163
ICache optimization 151, 154, 156-157,
standard template libraries. See st200c++
statement
static
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structure
symbol
T
U
V
visibility
W
-Werror-implicit-function-declaration 14
-Wimplicit-function-declaration 14
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205
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