Re: s390{,x} ABI incompatibility between gcc 4.0 and 4.1

[Joern RENNECKE <joern dot rennecke at st dot com>]

Jakub Jelinek wrote:

> I have looked just at one failure, but maybe all of them are the same thing.
> typedef char __attribute__((vector_size (16))) v16qi;
> int i = __alignof__ (v16qi);
>
> with GCC 4.0 sets i to 8 (s390{,x} have BIGGEST_ALIGNMENT 64), but
> GCC 4.1 sets i to 16.
> The changes that created this binary incompatibility are
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23467
> I think.  layout_type sets TYPE_ALIGN to 128 bits (size of v16qi)
> and in 4.0 and earlier finalize_type_size used to decrease the size
> to GET_MODE_ALIGNMENT (TImode), which is 64 on s390{,x}.
>
> Was this change intentional?  If yes, I think it should be documented in 4.1
> release notes, but I still hope it wasn't intentional.
>  
No, it wasn't.  The change was supposed to affect structures only.
As I understand the documentation ,the expected behaviour would be to 
limit the alignment
to BIGGEST_ALIGNMENT, unless the user has specified a larger alignment 
with the
aligned attribute.

One possible solution appears to be along the lines of
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/stor-layout.c.diff?cvsroot=gcc&r1=1.239&r2=1.240 
,
but with the comment changed to explain what we want to preserve now.

However, I think it is probably better to start out with the right 
alignment, by fix this code
in layout_type to take BIGGEST_ALIGNMENT into account:
   case VECTOR_TYPE:
...
       /* Always naturally align vectors.  This prevents ABI changes
          depending on whether or not native vector modes are 
supported.  */
       TYPE_ALIGN (type) = tree_low_cst (TYPE_SIZE (type), 0);