Optimize ParseRbsp method in H264 bitstream parser.

webrtc/common_video/h264/h264_common.cc

Index: webrtc/common_video/h264/h264_common.cc
diff --git a/webrtc/common_video/h264/h264_common.cc b/webrtc/common_video/h264/h264_common.cc
index a9cc6a25e62882e8619ed627aeb9cefee67e248f..4af434988dd784c54d982685942c37c306bb0a49 100644
--- a/webrtc/common_video/h264/h264_common.cc
+++ b/webrtc/common_video/h264/h264_common.cc
@@ -61,25 +61,27 @@ NaluType ParseNaluType(uint8_t data) {
 }
 
 std::unique_ptr<rtc::Buffer> ParseRbsp(const uint8_t* data, size_t length) {
-  std::unique_ptr<rtc::Buffer> rbsp_buffer(new rtc::Buffer(0, length));
-  const char* sps_bytes = reinterpret_cast<const char*>(data);
-  for (size_t i = 0; i < length;) {
+  std::unique_ptr<uint8_t[]> out(new uint8_t[length]);

[magjed_webrtc, 2017/03/03 13:14:14]

I think it would be better with an std::vector<uint8_t> here, and use push_back
instead of |j|. Also, we can change the return type to std::vector and that
removes an extra copy.

+
+  size_t j, i;
+  for (i = j = 0; i < length;) {
+    uint8_t* dst = out.get();
     // Be careful about over/underflow here. byte_length_ - 3 can underflow, and
     // i + 3 can overflow, but byte_length_ - i can't, because i < byte_length_
     // above, and that expression will produce the number of bytes left in
     // the stream including the byte at i.
-    if (length - i >= 3 && data[i] == 0 && data[i + 1] == 0 &&
-        data[i + 2] == 3) {
-      // Two rbsp bytes + the emulation byte.
-      rbsp_buffer->AppendData(sps_bytes + i, 2);
-      i += 3;
+    if (length - i >= 3 && !data[i] && !data[i + 1] && data[i + 2] == 3) {
+      // Two rbsp bytes.
+      dst[j++] = data[i++];
+      dst[j++] = data[i++];
+      // Skip the emulation byte.
+      i++;
     } else {
       // Single rbsp byte.
-      rbsp_buffer->AppendData(sps_bytes[i]);
-      ++i;
+      dst[j++] = data[i++];
     }
   }
-  return rbsp_buffer;
+  return std::unique_ptr<rtc::Buffer>(new rtc::Buffer(out.get(), j));
 }
 
 void WriteRbsp(const uint8_t* bytes, size_t length, rtc::Buffer* destination) {