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| author | Denis Vlasenko <vda.linux@googlemail.com> | 2007-03-21 10:07:01 +0000 |
|---|---|---|
| committer | Denis Vlasenko <vda.linux@googlemail.com> | 2007-03-21 10:07:01 +0000 |
| commit | b8e72fdde1186f41963697d57383c08402513ca8 (patch) | |
| tree | 1bd32dec2457680003e451239c4fb3ce03b068dc | |
| parent | afa37cf508dedfec778322381b0cc1a7bf73497c (diff) | |
| download | busybox-b8e72fdde1186f41963697d57383c08402513ca8.tar.gz | |
add info about gcc's sadistic alignment - and how to disable it
| -rw-r--r-- | docs/keep_data_small.txt | 28 |
1 files changed, 26 insertions, 2 deletions
diff --git a/docs/keep_data_small.txt b/docs/keep_data_small.txt index 15e41aefd..3c3d27396 100644 --- a/docs/keep_data_small.txt +++ b/docs/keep_data_small.txt @@ -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 |