[Liboil] alignment problems with sse2

[David Schleef]

On Tue, Aug 29, 2006 at 12:40:02PM +0200, Matthias Drochner wrote:
> Hi -
> some of the sse2 optimized functions in liboil-0.3.9 cause
> GPFs for me occasionally, on a Pentium 4 running NetBSD-current
> (which uses a gcc 4.1.2 prerelease).
> 
> This appears to be caused by aligned moves of __m128i variables
> allocated on the stack from/to xmm registers. Depending on
> stack alignment the variables are aligned to a 16-byte boundary
> or not, which explains that the problem occurs only sometimes.
> (I also wouldn't expect this to happen on amd64 because stack
> alignment is defined by the ABI there.)
> 
> I believe this is gcc's fault: If I compile a small test function:
> #include <emmintrin.h>
> void
> mist(__m128i *p)
> {
>         __m128i a;
> 
>         a = *p;
> }
> 
> I'm getting code which doesn't enforce alignment:
> mist:
>         pushl   %ebp
>         movl    %esp, %ebp
>         subl    $24, %esp
>         movl    8(%ebp), %eax
>         movdqa  (%eax), %xmm0
>         movdqa  %xmm0, -24(%ebp)    <--HERE!
>         leave
>         ret
> 
> Am I right here? If yes, is this a known gcc problem or should
> it be reported to the gcc people?

This is a known problem, see 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=368991

The best solution at this point (not yet implemented) is to fix
all the liboil functions to not use local variables for SSE.



dave...

-- 
David Schleef
Big Kitten LLC (http://www.bigkitten.com/) -- data acquisition on Linux