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

[Jacob Lifshay]

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

Jacob