avfilter/vf_blackdetect: add AVX2 SIMD version

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.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 */