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

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

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.

- Andreas
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