Re: [FFmpeg-devel] [PATCH v2 1/2] avfilter/vf_blackdetect: add AVX2 SIMD version

[Zhao Zhili <quinkblack@foxmail.com>]

> On Jul 17, 2025, at 18:45, Niklas Haas <ffmpeg@haasn.xyz> wrote:
> 
> From: Niklas Haas <git@haasn.dev>
> 
> Requested by a user. Even with autovectorization enabled, the compiler
> performs a quite poor job of optimizing this function, due to not being
> able to take advantage of the pmaxub + pcmpeqb trick for counting the number
> of pixels less than or equal-to a threshold.
> 
> blackdetect8_c:                                       4625.0 ( 1.00x)
> blackdetect8_avx2:                                     155.1 (29.83x)
> blackdetect16_c:                                      2529.4 ( 1.00x)
> blackdetect16_avx2:                                    163.6 (15.46x)
> ---
> libavfilter/vf_blackdetect.c          | 33 +++---------
> libavfilter/vf_blackdetect.h          | 71 ++++++++++++++++++++++++++
> libavfilter/x86/Makefile              |  2 +
> libavfilter/x86/vf_blackdetect.asm    | 73 +++++++++++++++++++++++++++
> libavfilter/x86/vf_blackdetect_init.c | 34 +++++++++++++
> 5 files changed, 188 insertions(+), 25 deletions(-)
> create mode 100644 libavfilter/vf_blackdetect.h
> create mode 100644 libavfilter/x86/vf_blackdetect.asm
> create mode 100644 libavfilter/x86/vf_blackdetect_init.c
> 
> diff --git a/libavfilter/vf_blackdetect.c b/libavfilter/vf_blackdetect.c
> index 8be33a814d..b233bdfd60 100644
> --- a/libavfilter/vf_blackdetect.c
> +++ b/libavfilter/vf_blackdetect.c
> @@ -33,6 +33,7 @@
> #include "filters.h"
> #include "formats.h"
> #include "video.h"
> +#include "vf_blackdetect.h"
> 
> typedef struct BlackDetectContext {
>     const AVClass *class;
> @@ -53,6 +54,8 @@ typedef struct BlackDetectContext {
>     int          depth;
>     int          nb_threads;
>     unsigned int *counter;
> +
> +    ff_blackdetect_fn func;
> } BlackDetectContext;
> 
> #define OFFSET(x) offsetof(BlackDetectContext, x)
> @@ -133,6 +136,7 @@ static int config_input(AVFilterLink *inlink)
>     s->time_base = inlink->time_base;
>     s->black_min_duration = s->black_min_duration_time / av_q2d(s->time_base);
>     s->counter = av_calloc(s->nb_threads, sizeof(*s->counter));
> +    s->func = ff_blackdetect_get_fn(depth);
>     if (!s->counter)
>         return AVERROR(ENOMEM);
> 
> @@ -160,37 +164,16 @@ static int black_counter(AVFilterContext *ctx, void *arg,
>                          int jobnr, int nb_jobs)
> {
>     BlackDetectContext *s = ctx->priv;
> -    const unsigned int threshold = s->pixel_black_th_i;
> -    unsigned int *counterp = &s->counter[jobnr];
> -    AVFrame *in = arg;
> +    const AVFrame *in = arg;
>     const int plane = s->alpha ? 3 : 0;
>     const int linesize = in->linesize[plane];
> -    const int w = in->width;
>     const int h = in->height;
>     const int start = (h * jobnr) / nb_jobs;
>     const int end = (h * (jobnr+1)) / nb_jobs;
> -    const int size = end - start;
> -    unsigned int counter = 0;
> -
> -    if (s->depth == 8) {
> -        const uint8_t *p = in->data[plane] + start * linesize;
> -
> -        for (int i = 0; i < size; i++) {
> -            for (int x = 0; x < w; x++)
> -                counter += p[x] <= threshold;
> -            p += linesize;
> -        }
> -    } else {
> -        const uint16_t *p = (const uint16_t *)(in->data[plane] + start * linesize);
> -
> -        for (int i = 0; i < size; i++) {
> -            for (int x = 0; x < w; x++)
> -                counter += p[x] <= threshold;
> -            p += linesize / 2;
> -        }
> -    }
> 
> -    *counterp = counter;
> +    s->counter[jobnr] = s->func(in->data[plane] + start * linesize,
> +                                linesize, in->width, end - start,
> +                                s->pixel_black_th_i);
> 
>     return 0;
> }
> diff --git a/libavfilter/vf_blackdetect.h b/libavfilter/vf_blackdetect.h
> new file mode 100644
> index 0000000000..361da2c5bc
> --- /dev/null
> +++ b/libavfilter/vf_blackdetect.h
> @@ -0,0 +1,71 @@
> +/*
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +#ifndef AVFILTER_VF_BLACKDETECT_H
> +#define AVFILTER_VF_BLACKDETECT_H

This break fate-source.

https://ffmpeg.org/pipermail/ffmpeg-devel/2025-July/346792.html

> +
> +#include <stddef.h>
> +#include <stdint.h>
> +
> +typedef unsigned (*ff_blackdetect_fn)(const uint8_t *src, ptrdiff_t stride,
> +                                                   ptrdiff_t width, ptrdiff_t height,
> +                                                   unsigned threshold);
> +
> +ff_blackdetect_fn ff_blackdetect_get_fn_x86(int depth);
> +
> +static unsigned count_pixels8_c(const uint8_t *src, ptrdiff_t stride,
> +                                ptrdiff_t width, ptrdiff_t height,
> +                                unsigned threshold)
> +{
> +    unsigned int counter = 0;
> +    while (height--) {
> +        for (int x = 0; x < width; x++)
> +            counter += src[x] <= threshold;
> +        src += stride;
> +    }
> +    return counter;
> +}
> +
> +static unsigned count_pixels16_c(const uint8_t *src, ptrdiff_t stride,
> +                                 ptrdiff_t width, ptrdiff_t height,
> +                                 unsigned threshold)
> +{
> +    unsigned int counter = 0;
> +    while (height--) {
> +        const uint16_t *src16 = (const uint16_t *) src;
> +        for (int x = 0; x < width; x++)
> +            counter += src16[x] <= threshold;
> +        src += stride;
> +    }
> +    return counter;
> +}
> +
> +
> +static inline ff_blackdetect_fn ff_blackdetect_get_fn(int depth)
> +{
> +    ff_blackdetect_fn fn = NULL;
> +#if ARCH_X86
> +    fn = ff_blackdetect_get_fn_x86(depth);
> +#endif
> +
> +    if (!fn)
> +        fn = depth == 8 ? count_pixels8_c : count_pixels16_c;
> +    return fn;
> +}
> +
> +#endif /* AVFILTER_VF_BLACKDETECT_H */
> diff --git a/libavfilter/x86/Makefile b/libavfilter/x86/Makefile
> index 0efe3f8d2c..86f7119a7b 100644
> --- a/libavfilter/x86/Makefile
> +++ b/libavfilter/x86/Makefile
> @@ -3,6 +3,7 @@ OBJS-$(CONFIG_SCENE_SAD)                     += x86/scene_sad_init.o
> OBJS-$(CONFIG_AFIR_FILTER)                   += x86/af_afir_init.o
> OBJS-$(CONFIG_ANLMDN_FILTER)                 += x86/af_anlmdn_init.o
> OBJS-$(CONFIG_ATADENOISE_FILTER)             += x86/vf_atadenoise_init.o
> +OBJS-$(CONFIG_BLACKDETECT_FILTER)            += x86/vf_blackdetect_init.o
> OBJS-$(CONFIG_BLEND_FILTER)                  += x86/vf_blend_init.o
> OBJS-$(CONFIG_BWDIF_FILTER)                  += x86/vf_bwdif_init.o
> OBJS-$(CONFIG_COLORSPACE_FILTER)             += x86/colorspacedsp_init.o
> @@ -49,6 +50,7 @@ X86ASM-OBJS-$(CONFIG_SCENE_SAD)              += x86/scene_sad.o
> X86ASM-OBJS-$(CONFIG_AFIR_FILTER)            += x86/af_afir.o
> X86ASM-OBJS-$(CONFIG_ANLMDN_FILTER)          += x86/af_anlmdn.o
> X86ASM-OBJS-$(CONFIG_ATADENOISE_FILTER)      += x86/vf_atadenoise.o
> +X86ASM-OBJS-$(CONFIG_BLACKDETECT_FILTER)     += x86/vf_blackdetect.o
> X86ASM-OBJS-$(CONFIG_BLEND_FILTER)           += x86/vf_blend.o
> X86ASM-OBJS-$(CONFIG_BWDIF_FILTER)           += x86/vf_bwdif.o
> X86ASM-OBJS-$(CONFIG_COLORSPACE_FILTER)      += x86/colorspacedsp.o
> diff --git a/libavfilter/x86/vf_blackdetect.asm b/libavfilter/x86/vf_blackdetect.asm
> new file mode 100644
> index 0000000000..78c24c5adc
> --- /dev/null
> +++ b/libavfilter/x86/vf_blackdetect.asm
> @@ -0,0 +1,73 @@
> +;*****************************************************************************
> +;* x86-optimized functions for blackdetect filter
> +;*
> +;* Copyright (C) 2025 Niklas Haas
> +;*
> +;* This file is part of FFmpeg.
> +;*
> +;* FFmpeg is free software; you can redistribute it and/or
> +;* modify it under the terms of the GNU Lesser General Public
> +;* License as published by the Free Software Foundation; either
> +;* version 2.1 of the License, or (at your option) any later version.
> +;*
> +;* FFmpeg is distributed in the hope that it will be useful,
> +;* but WITHOUT ANY WARRANTY; without even the implied warranty of
> +;* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +;* Lesser General Public License for more details.
> +;*
> +;* You should have received a copy of the GNU Lesser General Public
> +;* License along with FFmpeg; if not, write to the Free Software
> +;* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> +;*****************************************************************************
> +
> +%include "libavutil/x86/x86util.asm"
> +
> +SECTION .text
> +
> +%macro count_pixels_fn 1 ; depth
> +cglobal blackdetect_%1, 5, 7, 2, src, stride, width, height, threshold
> +        movd xm1, thresholdd
> +    %if %1 == 8
> +        vpbroadcastb m1, xm1
> +    %else
> +        vpbroadcastw m1, xm1
> +        shl widthq, 1
> +    %endif
> +        add srcq, widthq
> +        neg widthq
> +        xor r4, r4
> +        mov r5, widthq
> +        jmp .start
> +.loop:
> +        popcnt r6d, r6d
> +        add r4, r6
> +.start:
> +        movu m0, [srcq + r5]
> +    %if %1 == 8
> +        pmaxub m0, m1
> +        pcmpeqb m0, m1
> +    %else
> +        pmaxuw m0, m1
> +        pcmpeqw m0, m1
> +    %endif
> +        pmovmskb r6d, m0
> +        add r5, mmsize
> +        jl .loop
> +        ; handle tail by shifting away unused high elements
> +        shlx r6d, r6d, r5d
> +        popcnt r6d, r6d
> +        add r4, r6
> +        add srcq, strideq
> +        mov r5, widthq
> +        dec heightq
> +        jg .start
> +    %if %1 > 8
> +        shr r4, 1
> +    %endif
> +        mov rax, r4
> +        RET
> +%endmacro
> +
> +INIT_YMM avx2
> +count_pixels_fn 8
> +count_pixels_fn 16
> diff --git a/libavfilter/x86/vf_blackdetect_init.c b/libavfilter/x86/vf_blackdetect_init.c
> new file mode 100644
> index 0000000000..3780072589
> --- /dev/null
> +++ b/libavfilter/x86/vf_blackdetect_init.c
> @@ -0,0 +1,34 @@
> +/*
> + * Copyright (c) 2025 Niklas Haas
> + *
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +#include "libavutil/attributes.h"
> +#include "libavutil/x86/cpu.h"
> +#include "libavfilter/vf_blackdetect.h"
> +
> +unsigned ff_blackdetect_8_avx2(const uint8_t *, ptrdiff_t, ptrdiff_t, ptrdiff_t, unsigned);
> +unsigned ff_blackdetect_16_avx2(const uint8_t *, ptrdiff_t, ptrdiff_t, ptrdiff_t, unsigned);
> +
> +av_cold ff_blackdetect_fn ff_blackdetect_get_fn_x86(int depth)
> +{
> +    int cpu_flags = av_get_cpu_flags();
> +    if (EXTERNAL_AVX2(cpu_flags))
> +        return depth == 8 ? ff_blackdetect_8_avx2 : ff_blackdetect_16_avx2;
> +    return NULL;
> +}
> -- 
> 2.50.1
> 
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