[FFmpeg-devel] [PATCH] swscale: Break loop-carried dependency enabling parallel out of order execution of the gathers.

[Hendrik Leppkes]

On Mon, Aug 4, 2025 at 7:19 PM Jacob Lifshay <programmerjake at gmail.com> wrote:
>
>
>
> On August 4, 2025 6:49:20 AM PDT, Alan Kelly via ffmpeg-devel <ffmpeg-devel at ffmpeg.org> wrote:
> > The gather is unmasked but the instruction does a merge into ymm4, which
> > depends on the value of ymm4 from the previous loop iteration. The
> > out-of-order scheduler does not know statically that the instruction is
> > fully unmasked, preventing parallel out-of-order execution of the
> > gathers.
> > ---
> >  libswscale/x86/scale_avx2.asm | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/libswscale/x86/scale_avx2.asm b/libswscale/x86/scale_avx2.asm
> > index b4b852d60b..90ee8b0a0e 100644
> > --- a/libswscale/x86/scale_avx2.asm
> > +++ b/libswscale/x86/scale_avx2.asm
> > @@ -68,8 +68,10 @@ cglobal hscale8to15_%1, 7, 9, 16, pos0, dst, w, srcmem, filter, fltpos, fltsize,
> >  .innerloop:
> >  %endif
> >      vpcmpeqd  m13, m13
> > +    pxor m3, m3  ; break loop-carried dependency
>
> this is in AVX2 code, so you should use vpxor since pxor will just clear the lower 128 bits and leave the upper 128 bits unmodified. actually, on some older intel cpus it will cause a huge stall due to not being v-prefixed:
> https://stackoverflow.com/questions/41303780/why-is-this-sse-code-6-times-slower-without-vzeroupper-on-skylake/41349852#41349852
>

The v is actually automatically added by the pre-processor through
x86inc.asm if the function is marked as avx - its a bit confusing
because all other instructions are explicitly using it however, so it
might still be a good idea to be explicit about it.

As for the patch itself, any numbers?

- Hendrik