h264dsp: Factorize code into a new function, h264_find_start_code_candidate

This performs the start code search which was previously part of
h264_find_frame_end() - the most CPU intensive part of the function.

By itself, this results in a performance regression:
              Before          After
              Mean   StdDev   Mean   StdDev  Change
Overall time  2925.6 26.2     3068.5 31.7    -4.7%

but this can more than be made up for by platform-optimised
implementations of the function.

Signed-off-by: Martin Storsjö <martin@martin.st>

1 files changed, 29 insertions, 0 deletions

diff --git a/libavcodec/h264dsp.c b/libavcodec/h264dsp.c
index 3ca6abe..a901dbb 100644
--- a/libavcodec/h264dsp.c
+++ b/libavcodec/h264dsp.c
@@ -53,6 +53,34 @@
 #include "h264addpx_template.c"
 #undef BIT_DEPTH
 
+static int h264_find_start_code_candidate_c(const uint8_t *buf, int size)
+{
+    int i = 0;
+#if HAVE_FAST_UNALIGNED
+    /* we check i < size instead of i + 3 / 7 because it is
+     * simpler and there must be FF_INPUT_BUFFER_PADDING_SIZE
+     * bytes at the end.
+     */
+#if HAVE_FAST_64BIT
+    while (i < size &&
+            !((~*(const uint64_t *)(buf + i) &
+                    (*(const uint64_t *)(buf + i) - 0x0101010101010101ULL)) &
+                    0x8080808080808080ULL))
+        i += 8;
+#else
+    while (i < size &&
+            !((~*(const uint32_t *)(buf + i) &
+                    (*(const uint32_t *)(buf + i) - 0x01010101U)) &
+                    0x80808080U))
+        i += 4;
+#endif
+#endif
+    for (; i < size; i++)
+        if (!buf[i])
+            break;
+    return i;
+}
+
 av_cold void ff_h264dsp_init(H264DSPContext *c, const int bit_depth,
                              const int chroma_format_idc)
 {
@@ -133,6 +161,7 @@ av_cold void ff_h264dsp_init(H264DSPContext *c, const int bit_depth,
         H264_DSP(8);
         break;
     }
+    c->h264_find_start_code_candidate = h264_find_start_code_candidate_c;
 
     if (ARCH_ARM) ff_h264dsp_init_arm(c, bit_depth, chroma_format_idc);
     if (ARCH_PPC) ff_h264dsp_init_ppc(c, bit_depth, chroma_format_idc);