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

[Nuo Mi <nuomi2021@gmail.com>]

On Fri, Jan 26, 2024 at 9:45 PM Martin Storsjö <martin@martin.st> 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@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
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