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small doc update

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This commit is contained in:
Denis Vlasenko 2007-03-20 15:53:11 +00:00
parent c86e052b81
commit f363065021
1 changed files with 13 additions and 7 deletions
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20
docs/keep_data_small.txt
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@ -163,14 +163,20 @@ ridiculously large values. asm output diff for above example:
gcc doesn't seem to have options for altering this behaviour. gcc doesn't seem to have options for altering this behaviour.
gcc 3.4.3: gcc 3.4.3 and 4.1.1 tested:
char c = 1;
// gcc aligns to 32 bytes if sizeof(struct) >= 32 // gcc aligns to 32 bytes if sizeof(struct) >= 32
struct st { struct {
int c_iflag,c_oflag,c_cflag,c_lflag; int a,b,c,d;
int i1,i2,i3; // struct will be aligned to 4 bytes int i1,i2,i3;
// int i1,i2,i3,i4; // struct will be aligned to 32 bytes } s28 = { 1 }; // struct will be aligned to 4 bytes
}; struct {
struct st t = { 1 }; int a,b,c,d;
int i1,i2,i3,i4;
} s32 = { 1 }; // struct will be aligned to 32 bytes
// same for arrays // same for arrays
char vc31[31] = { 1 }; // unaligned char vc31[31] = { 1 }; // unaligned
char vc32[32] = { 1 }; // aligned to 32 bytes char vc32[32] = { 1 }; // aligned to 32 bytes
-fpack-struct=1 reduces alignment of s28 to 1 (but probably will break layout
of many libc structs) but s32 and vc32 are still aligned to 32 bytes.