Re: [FFmpeg-devel] [PATCH] swscale/x86/rgb2_rgb: Empty MMX state in ff_shuffle_bytes_2103_mmxext

[Michael Niedermayer <michael@niedermayer.cc>]

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On Tue, Aug 23, 2022 at 08:09:09PM +0200, Andreas Rheinhardt wrote:
> Michael Niedermayer:
> > On Tue, Aug 23, 2022 at 07:28:19PM +0200, Andreas Rheinhardt wrote:
> >> Michael Niedermayer:
> >>> On Mon, Aug 22, 2022 at 11:59:17PM +0200, Andreas Rheinhardt wrote:
> >>>> Andreas Rheinhardt:
> >>>>> Fixes FATE-failures with the the filter-2xbr filter-3xbr filter-4xbr
> >>>>> filter-ep2x filter-ep3x filter-hq2x filter-hq3x filter-hq4x
> >>>>> filter-paletteuse-bayer filter-paletteuse-bayer0
> >>>>> filter-paletteuse-nodither and filter-paletteuse-sierra2_4a tests
> >>>>> when using 32bit x86 with CPUFLAGS ranging from "mmx+mmxext" to
> >>>>> "mmx+mmxext+sse+sse2+sse3" (the relevant function is only overwritten
> >>>>> when using SSSE3).
> >>>>>
> >>>>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> >>>>> ---
> >>>>>  libswscale/x86/rgb_2_rgb.asm | 1 +
> >>>>>  1 file changed, 1 insertion(+)
> >>>>>
> >>>>> diff --git a/libswscale/x86/rgb_2_rgb.asm b/libswscale/x86/rgb_2_rgb.asm
> >>>>> index c695c61d5c..76ca1eec03 100644
> >>>>> --- a/libswscale/x86/rgb_2_rgb.asm
> >>>>> +++ b/libswscale/x86/rgb_2_rgb.asm
> >>>>> @@ -104,6 +104,7 @@ jge .end
> >>>>>      jl .loop_simd
> >>>>>  
> >>>>>  .end:
> >>>>> +    emms
> >>>>>      RET
> >>>>>  
> >>>>>  ;------------------------------------------------------------------------------
> >>>>
> >>>> I'd really love if someone with x86 assembly skills could look over this
> >>>> trivial patch and confirm whether it is indeed correct. All I currently
> >>>> know is that is works for me.
> >>>
> >>> emms needs to be called between MMX and float code, as far outside of loops
> >>> as possible
> >>> that would suggest outside the for() loops in rgbToRgbWrapper() and any
> >>> other code using it.
> >>
> >> But there is another aspect that the above is missing: Namely that if
> >> emms_c() is put outside of MMX functions, then it will be called even
> >> when it is unnecessary. In this case it is unnecessary for all modern
> >> CPUs, as this function is overridden when SSSE3 is available.
> > 
> > If you dont like that,
> > dont call it when its not needed or call it a few hundread times unnecessary
> > like your patch does.
> > or write only code that doesnt need emms 
> > maybe there are more options ...
> > 
> 
> If emms_c() is used as now outside of MMX functions, then a "dont call
> it when its not needed" would involve a check and would therefore still
> incur cost for users who don't use this. Also it is unclear how such a
> check would even look like given that one can use av_force_cpu_flags().
> See also 55fc2c5a892c50feb1b9a8f55b74ec6594755ddb.
> This patch also only calls it a few hundred times unnecessarily if one
> runs this without SSSE3. CPUs without SSSE3 are ancient today. For the
> non-ancient CPUs, using emms_c() adds an EMMS.

do whatever you prefer.
The best solution depends on assumptions.
The impact is biggest on old CPUs where EMMS is also a slow instruction
But as you say these are ancient today. very small impact on many vs
small to moderate impact on a today rare setup
the worst is if the bug is left open and time is wasted on bikesheding

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Avoid a single point of failure, be that a person or equipment.

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