Re: [FFmpeg-devel] [PATCH 1/4] avutil/x86/pixelutils: Empty MMX state in ff_pixelutils_sad_8x8_mmxext

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Andreas Rheinhardt:
> We currently mostly do not empty the MMX state in our MMX
> DSP functions; instead we only do so before code that might
> be using x87 code. This is a violation of the System V i386 ABI
> (and maybe of other ABIs, too):
> "The CPU shall be in x87 mode upon entry to a function. Therefore,
> every function that uses the MMX registers is required to issue an
> emms or femms instruction after using MMX registers, before returning
> or calling another function." (See 2.2.1 in [1])
> This patch does not intend to change all these functions to abide
> by the ABI; it only does so for ff_pixelutils_sad_8x8_mmxext, as this
> function can by called by external users, because it is exported
> via the pixelutils API. Without this, the following fragment will
> assert (on x86/x64):
>     uint8_t src1[8 * 8], src2[8 * 8];
>     av_pixelutils_sad_fn fn = av_pixelutils_get_sad_fn(3, 3, 0, NULL);
>     fn(src1, 8, src2, 8);
>     av_assert0_fpu();
> 
> [1]: https://raw.githubusercontent.com/wiki/hjl-tools/x86-psABI/intel386-psABI-1.1.pdf
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
>  libavutil/x86/pixelutils.asm | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/libavutil/x86/pixelutils.asm b/libavutil/x86/pixelutils.asm
> index fbe9b45971..0bcccb51f5 100644
> --- a/libavutil/x86/pixelutils.asm
> +++ b/libavutil/x86/pixelutils.asm
> @@ -43,6 +43,7 @@ cglobal pixelutils_sad_8x8, 4,4,0, src1, stride1, src2, stride2
>      lea         src2q, [src2q + 2*stride2q]
>  %endrep
>      movd        eax, m2
> +    emms
>      RET
>  
>  ;-------------------------------------------------------------------------------

Will apply this patchset tomorrow unless there are objections.

- Andreas

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