[FFmpeg-devel] [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due to Intel Gather Data Sampling mitigation performance loss (PR #20446)

[FFmpeg-devel] [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due to Intel Gather Data Sampling mitigation performance loss (PR #20446)

[legrosbuffle via ffmpeg-devel]

PR #20446 opened by legrosbuffle
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20446
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/20446.patch

    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.

    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)


>From 225382486832df97b74f84666aa5f895df503539 Mon Sep 17 00:00:00 2001
From: Alan Kelly <alankelly@google.com>
Date: Fri, 5 Sep 2025 15:17:25 +0000
Subject: [PATCH] swscale: Disable avx2 hscale 8to15 on IceLake and below due
 to Intel Gather Data Sampling mitigation performance loss

    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.

    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)
---
 libavutil/x86/cpu.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libavutil/x86/cpu.c b/libavutil/x86/cpu.c
index d6cd4fab9c..1a592f3bf4 100644
--- a/libavutil/x86/cpu.c
+++ b/libavutil/x86/cpu.c
@@ -244,8 +244,9 @@ 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;
     }
 
-- 
2.49.1

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