[Ffmpeg-devel] x86/SSE fix to ensure 16-byte alignment on local variables
Sat Nov 11 04:10:50 CET 2006
On Fri, Nov 10, 2006 at 10:30:20AM +0100, Guillaume Poirier wrote:
> 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);
> // TJ: force alignment to 16.
> //DCTELEM is short
> //DECLARE_ALIGNED_16(DCTELEM, d1);
> DCTELEM d1_[64+8];
> DCTELEM* d1 = (DCTELEM*)((((unsigned long)d1_)+0xf)&~0xf);
> // end force
> 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
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...
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