b8e72fdde
Denis Vlasenko
2007-03-21
Subject
add info about gcc's sadistic alignment - and how to disable it
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docs/keep_data_small.txt+26 -2
| @@ -178,5 +178,29 @@ struct { | |
| char vc31[31] = { 1 }; // unaligned | |
| 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. |
| + | -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. |
| + | |
| + | I will try to cook up a patch to add a gcc option for disabling it. |
| + | Meanwhile, this is where it can be disabled in gcc source: |
| + | |
| + | gcc/config/i386/i386.c |
| + | int |
| + | ix86_data_alignment (tree type, int align) |
| + | { |
| + | #if 0 |
| + | if (AGGREGATE_TYPE_P (type) |
| + | && TYPE_SIZE (type) |
| + | && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST |
| + | && (TREE_INT_CST_LOW (TYPE_SIZE (type)) >= 256 |
| + | || TREE_INT_CST_HIGH (TYPE_SIZE (type))) && align < 256) |
| + | return 256; |
| + | #endif |
| + | |
| + | Result (non-static busybox built against glibc): |
| + | |
| + | # size /usr/srcdevel/bbox/fix/busybox.t0/busybox busybox |
| + | text data bss dec hex filename |
| + | 634416 2736 23856 661008 a1610 busybox |
| + | 632580 2672 22944 658196 a0b14 busybox_noalign |