[Ffmpeg-devel] x86/SSE fix to ensure 16-byte alignment on local variables

[Rich Felker]

On Fri, Nov 10, 2006 at 10:30:20AM +0100, Guillaume Poirier wrote:
> Hi,
> 
> Christophe Mutricy wrote:
> >>Attached the patch against today's svn from the files Thorsten sent.
> > 
> > 
> > Arghh. I manage to send the worng one.
> > Here's the good one.
> 
> I think I can safely say that this patch is rejected in its corrent
> form. It leads to unreadable code (at least, to my eyes) to support a
> corner case.
> 
> I'm not even sure if a patch that would do smth like:
> 
> #ifndef BROKEN_STACK
>     DECLARE_ALIGNED_16(DCTELEM, d1[64]);
> #elif
>     // TJ: force alignment to 16.
>     //DCTELEM is short
>     //DECLARE_ALIGNED_16(DCTELEM, d1[64]);
>     DCTELEM d1_[64+8];
>     DCTELEM* d1 = (DCTELEM*)((((unsigned long)d1_)+0xf)&~0xf);
>     // end force
> #end
> 
> would be accepted. The maintainer would have to tell.

With gcc, BROKEN_STACK is _always_ the case. gcc simply does not align
the stack. However the code above is wrong in any case. It should
read:

DCTELEM d1_buf[64+8];
DCTELEM *d1 = d1_buf + (-(unsigned)d1_buf & 15)/sizeof(DCTELEM);

or similar. This is the ONLY reliable way to get aligned data on the
stack since gcc developers refuse to fix their broken crap and make
gcc actually align the stack...

Rich