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

* [FFmpeg-devel] [PATCH v2 1/2] avfilter/vf_blackdetect: add AVX2 SIMD version
@ 2025-07-17 10:45 Niklas Haas
  2025-07-17 10:45 ` [FFmpeg-devel] [PATCH v2 2/2] tests/checkasm: add test for vf_blackdetect Niklas Haas
  0 siblings, 1 reply; 2+ messages in thread
From: Niklas Haas @ 2025-07-17 10:45 UTC (permalink / raw)
  To: ffmpeg-devel; +Cc: Niklas Haas

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
+
+#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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