Aligning Data. NXP CodeWarrior Development Studio

Aligning Data

The code in Listing 21-4 on page 356 has a zero small data threshold.

Listing 21-4. Zero Small Data Threshold

lis r3,...bss.0@ha addi r3,r3,...bss.0@l li r0,1 stw r0,0(r3) li r0,2 stw r0,4(r3)

When pooled data is implemented, the first used variable of either the

.data

,

.bss

or

.rodata

section gets a two-instruction fetch of the first variable in that section.

Subsequent fetches in that function use the register containing the already-loaded section address with a calculated offset.

NOTE

You can access small data in assembly files with the twoinstruction fetch used with large data, because any data on your board can be accessed as if it were large data. The opposite is not true; large data can never be accessed with small data relocations (the linker issues an error if you try to do so).

External declarations of empty arrays (for example, extern int red []

;) are always treated as if they were large data. If you know that the size of the array fits into a small data section, specify the size in the brackets.

21.4 Aligning Data

This section contains these topics:

•

Alignment Attribute Syntax

•

Aligning a Variable Declaration

•

Alignment in a Structure Definition

•

Typedef Declaration

•

Structure Member

•

Bitfields

21.4.1 Alignment Attribute Syntax

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Chapter 21 Power Architecture Code Generation

Use

__attribute__ ((aligned(...))) directive to specify to the compiler on what memory boundary to store data objects. This directive specifies which multiple of bytes to store an object.

The format of this directive is

__attribute__ ((aligned(

x )) where x is a decimal number of a power of 2 from 1 to 8192.

21.4.2 Aligning a Variable Declaration

Use the alignment attribute to specify a variable's alignment. For example, the following variable declaration aligns

V1

on a 16-byte boundary.

int V1[4] __attribute__ ((aligned (16)));

The following variable declaration aligns

V2

on a 2-byte boundary.

int V2[4] __attribute__ ((aligned (2)));

21.4.3 Alignment in a Structure Definition

Use the alignment attribute to specify how instances of a structure should be aligned. You must specify a minimum alignment of at least 4 bytes for structures. Specifying a lower number might cause alignment exceptions at runtime.

For example, this definition aligns all definitions of struct S1

on an 8-byte boundary.

struct S1 { short f[3]; }

__attribute__ ((aligned (8)));

struct S1 s1;

The following definition aligns all definitions of struct S2

on a 4-byte boundary.

struct S2 { short f[3]; }

__attribute__ ((aligned (1)));

struct S2 s2;

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Aligning Data

21.4.4 Structure Member

Use the alignment attribute to specify how to align a member in a structure.

For example, the following structure member definition aligns all definitions of struct S3 on an 8-byte boundary, where a

is at offset 0 and b

is at offset 8.

struct S3 {

char a;

int b __attribute__ ((aligned (8)));

};

struct S3 s3;

The following struct member definition aligns all definitions of struct S4

on a 4-byte boundary, where a

is at offset 0 and b

is at offset 4.

struct S4 {

char a;

int b __attribute__ ((aligned (2)));

};

struct S4 s4;

NOTE

Specifying

__attribute__ ((aligned (2)))

does not affect the alignment of

S4

because 2 is less than the natural alignment of int

.

21.4.5 Typedef Declaration

Use the alignment attribute to specify how objects of a specific type should be aligned.

For example, the following typedef declaration aligns all definitions of

T1

on an 8-byte boundary.

typedef int T1 __attribute__ ((aligned (8)));

T1 t1;

The following typedef declaration aligns all definitions of

T2

on an 1-byte boundary.

typedef int T2 __attribute__ ((aligned (1)));

T2 t2;

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