bytereader: Use pointer instead of index access

Currently the scan uses Boyer-moore method and its performance is good.
but, it can be optimized from an implementation of view.

The original scan code is implemented by byte array and index-based access.
In _scan_for_start_code(), the index is increasing from start to end and the
base address of the byte array is referred to as return value.

In the case, index-based access can be replaced by pointer access, which
improve the performance by removing index-related operations.

Its performace is enhanced by approximately 8% on arm-based embedded devices.
Although it seems trivial, it can affect the overall performance because the
_scan_for_start_code() function is very often called when H.264/H.265 video is
played.

In addition, the technique can apply for all architectures and it is good in
view of readability and maintainability.

https://bugzilla.gnome.org/show_bug.cgi?id=731442

diff --git a/libs/gst/base/gstbytereader.c b/libs/gst/base/gstbytereader.c
index a037eb6..f9e4c7a 100644 (file)
--- a/libs/gst/base/gstbytereader.c
+++ b/libs/gst/base/gstbytereader.c
@@ -775,23 +775,21 @@ gst_byte_reader_dup_data (GstByteReader * reader, guint size, guint8 ** val)
 static inline gint
 _scan_for_start_code (const guint8 * data, guint offset, guint size)
 {
-  guint i = 0;
-
-  while (i <= (size - 4)) {
-    if (data[i + 2] > 1) {
-      i += 3;
-    } else if (data[i + 1]) {
-      i += 2;
-    } else if (data[i] || data[i + 2] != 1) {
-      i++;
+  guint8 *pdata = (guint8 *) data;
+  guint8 *pend = (guint8 *) (data + size - 4);
+
+  while (pdata <= pend) {
+    if (pdata[2] > 1) {
+      pdata += 3;
+    } else if (pdata[1]) {
+      pdata += 2;
+    } else if (*pdata || pdata[2] != 1) {
+      pdata++;
     } else {
-      break;
+      return (pdata - data + offset);
     }
   }
 
-  if (i <= (size - 4))
-    return i + offset;
-
   /* nothing found */
   return -1;
 }