Altivec + 16 byte alignment

[Franz Sirl]

At 18:25 11.02.2003, John David Anglin wrote:
> > I have a problem in that altivec requires all inputs to be 16 byte
> > aligned, so I am placing my vectors in a structure like this:
> >
> > struct vector {
> >         float x,y,z,w;
> > }__attribute__((aligned(16)));
>
>We had a similar problem with respect to the alignment of data for
>the ldcw instruction on the PA which needs 16-byte alignment on PA 1.X.
>We have found an alternative solution.  However, the patch below
>will allow the aligned attribute to be respected for stack locals
>up to PREFERRED_STACK_BOUNDARY.  Note however, that alignment of
>stack locals is only guaranteed up to STACK_BOUNDARY.  The default
>alignment for structs on the stack is BIGGEST_ALIGNMENT, which I
>presume is set 64 (8 bytes).
>
>It may well be with altvec you really want BIGGEST_ALIGNMENT set to
>256, but the change would affect the ABI and alignment of long doubles.
>The i386 code provides an example of what needs doing if there are
>types such as long doubles that don't require natural alignment.

BIGGEST_ALIGNMENT is 128, but STACK_BOUNDARY is 64 unless you give 
-mabi=altivec (in 3.2+). Now, if we would increase STACK_BOUNDARY to be 
always 128 on powerpc-linux-gnu, would this affect binary compatibility at 
all? I can't think of a situation where this would happen, but maybe I'm wrong?
If changing STACK_BOUNDARY doesn't affect binary compatibility, I will 
submit a patch to set it to 128 in rs6000/linux.h.

Franz.