Refactor BlockDifference() to use MediaInitializer().

media/video/capture/screen/differ_block.cc

Index: media/video/capture/screen/differ_block.cc
diff --git a/media/video/capture/screen/differ_block.cc b/media/video/capture/screen/differ_block.cc
index d263ac4312ad5e2ddbd9b5cea9dcd85e180b6def..5c3c93a93d7dbf8745dac54d78733db4e4d5ead6 100644
--- a/media/video/capture/screen/differ_block.cc
+++ b/media/video/capture/screen/differ_block.cc
@@ -9,6 +9,8 @@
 #include "media/video/capture/screen/differ_block_sse2.h"
 
 namespace media {
+typedef int (*BlockDifferenceProc)(const uint8*, const uint8*, int);
+static BlockDifferenceProc g_block_difference_proc_ = NULL;
 
 int BlockDifference_C(const uint8* image1, const uint8* image2, int stride) {
   int width_bytes = kBlockSize * kBytesPerPixel;
@@ -22,28 +24,28 @@ int BlockDifference_C(const uint8* image1, const uint8* image2, int stride) {
   return 0;
 }
 
-int BlockDifference(const uint8* image1, const uint8* image2, int stride) {
-  static int (*diff_proc)(const uint8*, const uint8*, int) = NULL;
+void InitializeCPUSpecificBlockDifference() {
+  g_block_difference_proc_ = &BlockDifference_C;
 
-  if (!diff_proc) {
-#if defined(ARCH_CPU_ARM_FAMILY) || defined(ARCH_CPU_MIPS_FAMILY)
-    // For ARM and MIPS processors, always use C version.
-    // TODO(hclam): Implement a NEON version.
-    diff_proc = &BlockDifference_C;
+#if defined(ARCH_CPU_X86_FAMILY)
+#if defined(__SSE2__)
+  const bool kHasSSE2 = true;
 #else
-    base::CPU cpu;
-    // For x86 processors, check if SSE2 is supported.
-    if (cpu.has_sse2() && kBlockSize == 32) {
-      diff_proc = &BlockDifference_SSE2_W32;
-    } else if (cpu.has_sse2() && kBlockSize == 16) {
-      diff_proc = &BlockDifference_SSE2_W16;
-    } else {
-      diff_proc = &BlockDifference_C;
-    }
+  const bool kHasSSE2 = base::CPU().has_sse2();
 #endif
+
+  if (kHasSSE2) {
+    if (kBlockSize == 32) {
+      g_block_difference_proc_ = &BlockDifference_SSE2_W32;
+    } else if (kBlockSize == 16) {
+      g_block_difference_proc_ = &BlockDifference_SSE2_W16;
+    }
   }
+#endif
+}
 
-  return diff_proc(image1, image2, stride);
+int BlockDifference(const uint8* image1, const uint8* image2, int stride) {
+  return g_block_difference_proc_(image1, image2, stride);
 }
 
 }  // namespace media