Re: Options of fixing biggest alignment in PR target/38736

["Richard Guenther" <richard dot guenther at gmail dot com>]

On Wed, Jan 7, 2009 at 10:22 AM, Ye, Joey <joey.ye@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
>