[FFmpeg-devel] [PATCH] x86: Remove inline MMX assembly that clobbers the FPU state

[Martin Storsjö]

On Fri, 26 Jan 2024, Martin Storsjö wrote:

> On Fri, 26 Jan 2024, Martin Storsjö wrote:
>
>> These inline implementations of AV_COPY64, AV_SWAP64 and AV_ZERO64
>> are known to clobber the FPU state - which has to be restored
>> with the 'emms' instruction afterwards.
>> 
>> This was known and signaled with the FF_COPY_SWAP_ZERO_USES_MMX
>> define, which calling code seems to have been supposed to check,
>> in order to call emms_c() after using them. See
>> 0b1972d4096df5879038f0af776f87f41e90ebd4,
>> 29c4c0886d143790fcbeddbe40a23dfc6f56345c and
>> df215e575850e41b19aeb1fd99e53372a6b3d537 for history on earlier
>> fixes in the same area.
>> 
>> However, new code can use these AV_*64() macros without knowing
>> about the need to call emms_c().
>> 
>> Just get rid of these dangerous inline assembly snippets; this
>> doesn't make any difference for 64 bit architectures anyway.
>> 
>> Signed-off-by: Martin Storsjö <martin at martin.st>
>> ---
>> libavcodec/dca_core.c        | 16 ----------------
>> libavutil/x86/intreadwrite.h | 36 ------------------------------------
>> 2 files changed, 52 deletions(-)
>
> I forgot to add some more context here; the VVC tests fail on i386 in some 
> cases. 
> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20240125170518.61211-1-post@frankplowman.com/ 
> fixes this, by using av_log2() instead of the float log2() in the VVC 
> decoder. This patch fixes the same issue as well, by eliminating the FPU 
> state clobbering (so that float math functions anywhere in decoders work as 
> expected).

If there are no better suggestions here, I would like to go ahead and push 
this.

// Martin