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

Source/core/platform/image-decoders/ImageDecoder.h

Index: Source/core/platform/image-decoders/ImageDecoder.h
diff --git a/Source/core/platform/image-decoders/ImageDecoder.h b/Source/core/platform/image-decoders/ImageDecoder.h
index e58d53fffa1973458b4a6ba836295b48061f1308..24555cd522e6c34b7ecfcc9f9deef57f112f2393 100644
--- a/Source/core/platform/image-decoders/ImageDecoder.h
+++ b/Source/core/platform/image-decoders/ImageDecoder.h
@@ -175,24 +175,46 @@ namespace WebCore {
             return  (fixed * v) >> fixPointShift;
         }
 
+        inline void setRGB(PixelData* dest, unsigned r, unsigned g, unsigned b)
+        {
+            *dest = SkPackARGB32NoCheck(255, r, g, b);
+        }
+
         inline void setRGBA(PixelData* dest, unsigned r, unsigned g, unsigned b, unsigned a)
         {
-            if (m_premultiplyAlpha && a < 255) {
-                if (!a) {
-                    *dest = 0;
-                    return;
-                }
+            if (m_premultiplyAlpha)
+                setRGBAPremultiply(dest, r, g, b, a);
+            else
+                setRGBARaw(dest, r, g, b, a);
+        }
+
+        inline void setRGBAPremultiply(PixelData* dest, unsigned r, unsigned g, unsigned b, unsigned a)
+        {
+            ASSERT(m_premultiplyAlpha);
 
+            if (!a) {

[Noel Gordon, 2013/06/26 05:15:26]

Why did you change the shape of this routine?  The test of 0 was done after the
a< 255 test before and for good reason.   Why do we need the ASSERT at all (the
routine does what it says)?

+                *dest = 0;
+                return;
+            }
+            if (a < 255) {
                 unsigned alphaMult = a * fixPointMult;
                 r = fixPointUnsignedMultiply(r, alphaMult);
                 g = fixPointUnsignedMultiply(g, alphaMult);
                 b = fixPointUnsignedMultiply(b, alphaMult);
             }
+
             // Call the "NoCheck" version since we may deliberately pass non-premultiplied
             // values, and we don't want an assert.
             *dest = SkPackARGB32NoCheck(a, r, g, b);
         }
 
+        inline void setRGBARaw(PixelData* dest, unsigned r, unsigned g, unsigned b, unsigned a)
+        {
+            ASSERT(!m_premultiplyAlpha);

[Noel Gordon, 2013/06/26 05:15:26]

Ditto re the ASSERT, why do we need it all when the routine does what it says?

+
+            *dest = SkPackARGB32NoCheck(a, r, g, b);
+        }
+
     private:
         int width() const
         {