Re: [FFmpeg-devel] [PATCH 1/2] libavutil/cpu: Add AV_CPU_FLAG_SLOW_GATHER.

[Lynne <dev@lynne.ee>]

20 Dec 2021, 15:59 by alankelly-at-google.com@ffmpeg.org:

> This flag is set on Haswell and earlier and all AMD cpus.
> ---
>  Sets this flag on Zen 3 and earlier.
>  libavutil/cpu.h     |  1 +
>  libavutil/x86/cpu.c | 14 +++++++++++++-
>  2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/libavutil/cpu.h b/libavutil/cpu.h
> index ae443eccad..ce9bf14bf7 100644
> --- a/libavutil/cpu.h
> +++ b/libavutil/cpu.h
> @@ -54,6 +54,7 @@
>  #define AV_CPU_FLAG_BMI1        0x20000 ///< Bit Manipulation Instruction Set 1
>  #define AV_CPU_FLAG_BMI2        0x40000 ///< Bit Manipulation Instruction Set 2
>  #define AV_CPU_FLAG_AVX512     0x100000 ///< AVX-512 functions: requires OS support even if YMM/ZMM registers aren't used
> +#define AV_CPU_FLAG_SLOW_GATHER  0x2000000 ///< CPU has slow gathers.
>  
>  #define AV_CPU_FLAG_ALTIVEC      0x0001 ///< standard
>  #define AV_CPU_FLAG_VSX          0x0002 ///< ISA 2.06
> diff --git a/libavutil/x86/cpu.c b/libavutil/x86/cpu.c
> index bcd41a50a2..11467ba99d 100644
> --- a/libavutil/x86/cpu.c
> +++ b/libavutil/x86/cpu.c
> @@ -146,8 +146,16 @@ int ff_get_cpu_flags_x86(void)
>  if (max_std_level >= 7) {
>  cpuid(7, eax, ebx, ecx, edx);
>  #if HAVE_AVX2
> -        if ((rval & AV_CPU_FLAG_AVX) && (ebx & 0x00000020))
> +        if ((rval & AV_CPU_FLAG_AVX) && (ebx & 0x00000020)) {
>  rval |= AV_CPU_FLAG_AVX2;
> +            cpuid(1, eax, ebx, ecx, std_caps);
> +            family = ((eax >> 8) & 0xf) + ((eax >> 20) & 0xff);
> +            model  = ((eax >> 4) & 0xf) + ((eax >> 12) & 0xf0);
> +            /* Haswell has slow gather */
> +            if(family == 6 && model < 70)
>

Shouldn't this check for the vendor as well?
Nit: space after the if.


> +                rval |= AV_CPU_FLAG_SLOW_GATHER;
> +        }
> +
>  #if HAVE_AVX512 /* F, CD, BW, DQ, VL */
>  if ((xcr0_lo & 0xe0) == 0xe0) { /* OPMASK/ZMM state */
>  if ((rval & AV_CPU_FLAG_AVX2) && (ebx & 0xd0030000) == 0xd0030000)
> @@ -196,6 +204,10 @@ int ff_get_cpu_flags_x86(void)
>  used unless explicitly disabled by checking AV_CPU_FLAG_AVXSLOW. */
>  if ((family == 0x15 || family == 0x16) && (rval & AV_CPU_FLAG_AVX))
>  rval |= AV_CPU_FLAG_AVXSLOW;
> +
> +        /* Zen 3 and earlier have slow gather */
>

Nit: put a space after the start of a comment and indent
to the same level as the line below.


> +            if((rval & AV_CPU_FLAG_AVX2) & family <= 25)
>

Mistake: you're ANDing the conditions, you want `&& family <= 0x19`
(hex because the other checks are all hex too).
Nit: space after the if.


> +                rval |= AV_CPU_FLAG_SLOW_GATHER;
>  }
>  
>  /* XOP and FMA4 use the AVX instruction coding scheme, so they can't be
>
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