Speed up png decoding by eliminating branches from the core loop that fills ImageFrame with pixels

Source/core/platform/image-decoders/png/PNGImageDecoder.cpp

Index: Source/core/platform/image-decoders/png/PNGImageDecoder.cpp
diff --git a/Source/core/platform/image-decoders/png/PNGImageDecoder.cpp b/Source/core/platform/image-decoders/png/PNGImageDecoder.cpp
index 828328b956c2bb0c8d8d8de534228c2abee60223..e2bd53dc124a194128f8673275019c11138bba9b 100644
--- a/Source/core/platform/image-decoders/png/PNGImageDecoder.cpp
+++ b/Source/core/platform/image-decoders/png/PNGImageDecoder.cpp
@@ -489,18 +489,30 @@ void PNGImageDecoder::rowAvailable(unsigned char* rowBuffer, unsigned rowIndex,
 
     // Write the decoded row pixels to the frame buffer.
     ImageFrame::PixelData* address = buffer.getAddr(0, y);
-    bool nonTrivialAlpha = false;
+    unsigned alphaMask = 255;
     int width = size().width();
 
+    // Do not merge the loops below. They are hand rolled for each case for maximal performance.
     png_bytep pixel = row;
-    for (int x = 0; x < width; ++x, pixel += colorChannels) {
-        unsigned alpha = hasAlpha ? pixel[3] : 255;
-        buffer.setRGBA(address++, pixel[0], pixel[1], pixel[2], alpha);
-        nonTrivialAlpha |= alpha < 255;
+    if (hasAlpha) {
+        if (buffer.premultiplyAlpha()) {
+            for (int x = 0; x < width; ++x, pixel += 4) {
+                buffer.setRGBAPremultiply(address + x, pixel[0], pixel[1], pixel[2], pixel[3]);
+                alphaMask &= pixel[3];
+            }
+        } else {
+            for (int x = 0; x < width; ++x, pixel += 4) {
+                buffer.setRGBARaw(address + x, pixel[0], pixel[1], pixel[2], pixel[3]);
+                alphaMask &= pixel[3];
+            }
+        }
+    } else {
+        for (int x = 0; x < width; ++x, pixel += 3)
+            buffer.setRGB(address + x, pixel[0], pixel[1], pixel[2]);
     }
 
-    if (nonTrivialAlpha && !buffer.hasAlpha())
-        buffer.setHasAlpha(nonTrivialAlpha);
+    bool hasNonTrivialAlpha = alphaMask != 255;
+    buffer.setHasAlpha(hasNonTrivialAlpha);

[kbalazs_, 2013/06/25 13:20:18]

Oops, this war wrong, we cannot make !hasAlpha() just because there was no alpha
in the current line.

 }
 
 void PNGImageDecoder::pngComplete()