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swscale: Disable avx2 hscale 8to15 on IceLake and below due to Intel Gather Data Sampling mitigation performance loss

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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)
This commit is contained in:
Alan Kelly
2025-09-05 15:17:25 +00:00
committed by kierank
parent 1dcb0c6c3e
commit f4b044bbe3
1 changed files with 3 additions and 2 deletions
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libavutil/x86/cpu.c
+3 -2
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@@ -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;
}