Re: [FFmpeg-devel] [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due to Intel Gather Data Sampling mitigation performance loss - Alan Kelly via ffmpeg-devel

From: Alan Kelly via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>
To: ffmpeg-devel@ffmpeg.org
Cc: Alan Kelly <alankelly@google.com>
Subject: Re: [FFmpeg-devel] [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due to Intel Gather Data Sampling mitigation performance loss
Date: Fri, 8 Aug 2025 14:23:58 +0200
Message-ID: <CABaBdZoABsGnNgB4i+xSnK1c1k6mT5b=1_8nSbfp6GDHrsPENQ@mail.gmail.com> (raw)
In-Reply-To: <20250808122142.2957853-1-alankelly@google.com>

On Fri, Aug 8, 2025 at 2:21 PM Alan Kelly <alankelly@google.com> wrote:

> Intel provided a microcode update to mitigate this security
> vulnerability which has a huge negative performance impact on gather
> instructions. This means that hscale 8to15 avx2, which uses gather
> extensively, is no longer faster than SSSE3 on impacted CPUs.
> ---
>  libavutil/x86/cpu.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/libavutil/x86/cpu.c b/libavutil/x86/cpu.c
> index d6cd4fab9c..923c63e0c4 100644
> --- a/libavutil/x86/cpu.c
> +++ b/libavutil/x86/cpu.c
> @@ -244,9 +244,11 @@ int ff_get_cpu_flags_x86(void)
>              family == 6 && model < 23)
>              rval |= AV_CPU_FLAG_SSSE3SLOW;
>
> -        /* Haswell has slow gather */
> -        if ((rval & AV_CPU_FLAG_AVX2) && family == 6 && model < 70)
> +        /* Ice Lake and below have slow gather due to Gather Data Sampling
> +         * mitigation. */
> +        if ((rval & AV_CPU_FLAG_AVX2) && family == 6 && model < 143) {
>              rval |= AV_CPU_FLAG_SLOW_GATHER;
> +        }
>      }
>
>  #endif /* cpuid */
> --
> 2.50.1.703.g449372360f-goog
>

https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/gather-data-sampling.html

Broadwell:
hscale_8_to_15__fs_4_dstW_512_c:                      3379.5 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2:                    615.7 ( 5.49x)
hscale_8_to_15__fs_4_dstW_512_ssse3:                   613.4 ( 5.51x)
hscale_8_to_15__fs_4_dstW_512_avx2:                    495.7 ( 6.82x)

Skylake:
hscale_8_to_15__fs_4_dstW_512_c:                      3411.4 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2:                    591.0 ( 5.77x)
hscale_8_to_15__fs_4_dstW_512_ssse3:                   591.5 ( 5.77x)
hscale_8_to_15__fs_4_dstW_512_avx2:                   1386.2 ( 2.46x)

Cascade Lake:
hscale_8_to_15__fs_4_dstW_512_c:                      3231.3 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2:                    517.9 ( 6.24x)
hscale_8_to_15__fs_4_dstW_512_ssse3:                   521.6 ( 6.19x)
hscale_8_to_15__fs_4_dstW_512_avx2:                   1775.0 ( 1.82x)

Sapphire Rapids:
hscale_8_to_15__fs_4_dstW_512_c:                      1840.0 ( 1.00x)
hscale_8_to_15__fs_4_dstW_512_sse2:                    287.9 ( 6.39x)
hscale_8_to_15__fs_4_dstW_512_ssse3:                   293.8 ( 6.26x)
hscale_8_to_15__fs_4_dstW_512_avx2:                    219.2 ( 8.40x)
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