Improve PNG decode performance: avoid branching in the row write loop

No change in behavior, no new tests.

BUG=258324

Review URL: https://codereview.chromium.org/18099004

git-svn-id: svn://svn.chromium.org/blink/trunk@175666 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/platform/image-decoders/ImageFrame.h

@@ -162,11 +162,19 @@ public:
         setRGBA(getAddr(x, y), r, g, b, a);
     }
 
+    inline void setRGBA(PixelData* dest, unsigned r, unsigned g, unsigned b, unsigned a)
+    {
+        if (m_premultiplyAlpha)
+            setRGBAPremultiply(dest, r, g, b, a);
+        else
+            *dest = SkPackARGB32NoCheck(a, r, g, b);
+    }
+
     static const unsigned div255 = static_cast<unsigned>(1.0 / 255 * (1 << 24)) + 1;
 
-    inline void setRGBA(PixelData* dest, unsigned r, unsigned g, unsigned b, unsigned a)
+    static inline void setRGBAPremultiply(PixelData* dest, unsigned r, unsigned g, unsigned b, unsigned a)
     {
-        if (m_premultiplyAlpha && a < 255) {
+        if (a < 255) {
             if (!a) {
                 *dest = 0;
                 return;
@@ -183,7 +191,7 @@ public:
         *dest = SkPackARGB32NoCheck(a, r, g, b);
     }
 
-    inline void setRGBARaw(PixelData* dest, unsigned r, unsigned g, unsigned b, unsigned a)
+    static inline void setRGBARaw(PixelData* dest, unsigned r, unsigned g, unsigned b, unsigned a)
     {
         *dest = SkPackARGB32NoCheck(a, r, g, b);
     }

third_party/WebKit/Source/platform/image-decoders/png/PNGImageDecoder.cpp

@@ -489,20 +489,33 @@ void PNGImageDecoder::rowAvailable(unsigned char* rowBuffer, unsigned rowIndex,
     }
 #endif
 
-    // Write the decoded row pixels to the frame buffer.
+    // Write the decoded row pixels to the frame buffer. The repetitive
+    // form of the row write loops is for speed.
     ImageFrame::PixelData* address = buffer.getAddr(0, y);
-    bool nonTrivialAlpha = false;
+    unsigned alphaMask = 255;
     int width = size().width();
 
     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++, pixel[0], pixel[1], pixel[2], pixel[3]);
+                alphaMask &= pixel[3];
+            }
+        } else {
+            for (int x = 0; x < width; ++x, pixel += 4) {
+                buffer.setRGBARaw(address++, pixel[0], pixel[1], pixel[2], pixel[3]);
+                alphaMask &= pixel[3];
+            }
+        }
+    } else {
+        for (int x = 0; x < width; ++x, pixel += 3) {
+            buffer.setRGBARaw(address++, pixel[0], pixel[1], pixel[2], 255);
+        }
     }
 
-    if (nonTrivialAlpha && !buffer.hasAlpha())
-        buffer.setHasAlpha(nonTrivialAlpha);
+    if (alphaMask != 255 && !buffer.hasAlpha())
+        buffer.setHasAlpha(true);
 
     buffer.setPixelsChanged(true);
 }