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

[Martin Storsjö]

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).

// Martin