On Wed, Jan 7, 2009 at 10:22 AM, Ye, Joey <joey...@intel.com> wrote: > This is about http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38736 and I'd > rather discuss it in gcc mail list. Basicly the problem is shown as following > example: > > Case 1 (on x86 or x86_64): > $ cat i.h > struct s > { > char dummy0; > // align at maxmiun aligned boundary supported by this target. > char dummy __attribute__((aligned)); > int data; > }; > extern void foo(struct s*); > $ cat foo.c > #include "i.h" > void foo(struct s* input) > { input->data = 1; } > $ cat main.c > #include "i.h" > extern void abort(void); > struct s g_s; > int main() > { > foo(&g_s); > if (g_s.data != 1) abort(); > } > > $ gcc -S foo.c > $ gcc -S main.c -mavx > $ gcc -o foo.exe foo.s main.s > $ ./foo.exe > Aborted > > The reason is that AVX target defines BIGGEST_ALIGNMENT to 32 bytes and > non-AVX x86 target does as 16 bytes. Since __attribute__((aligned)) aligns > struct memory according to BIGGEST_ALIGNMENT, objects built by avx/non-avx > GCC will result in different struct layout. > > There are options to solve this problem so far I can think of: > Option 1: Leave BIGGEST_ALIGNMENT as it is nowaday and modify all libraries > and header files using __attribute__((aligned)) similar to i.h > Option 2: Define BIGGEST_ALIGNMENT as a fixed value 16 bytes, for all x86 > target. > Option 3: Define BIGGEST_ALIGNMENT as a fixed value 32 bytes, for all x86 > target, and extend to 64 or more bytes in future. > > Option 1 follows the definition of __attribute__((aligned)) in GCC manual, > and it works as expected to provide a way to align at maxium target > alignment. However, fixing all libraries will be tidious and easy to miss. > Also documentation should mention the potiential issue using this feature. > > Option 2 and option 3 seems to be a quick solution, but their draw back is > obvious. Firstly it doesn't follow the definition of __attribute__((aligned)) > and can leave confusion to users. Secondly it eliminates a convenient way for > user utilize the maxium alignment supported in x86 family. Also very > importantly they won't solve all problem, for example if i.h is like this: > Case 2: > $ cat i2.h > #ifdef __AVX__ > #define aligned __aligned__(32) > #else > #define aligned __aligned__(16) > #endif > struct s > { > char dummy0; > char dummy __attribute__((aligned)); > int data; > }; > extern void foo(struct s*); > > Furthermore option 3 will result different behavior for GCC 4.3- and GCC > 4.4+, case 1 will still fail if foo.c is built by GCC 4.3- and main.c by 4.4+. > > In summary, I don't see an obvious best way to solve in PR38736. But IMHO > option 1 is more reasonable.
IMHO Option 2 is the most reasonable thing for GCC 4.4. Richard. > Thanks - Joey >
