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

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

 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
  * Copyright (C) Research In Motion Limited 2009-2010. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 157 matching lines @@
         // Use fix point multiplier instead of integer division or floating point math.
         // This multipler produces exactly the same result for all values in range 0 - 255.
         static const unsigned fixPointShift = 24;
         static const unsigned fixPointMult = static_cast<unsigned>(1.0 / 255.0 * (1 << fixPointShift)) + 1;
         // Multiplies unsigned value by fixpoint value and converts back to unsigned.
         static unsigned fixPointUnsignedMultiply(unsigned fixed, unsigned v)
         {
             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
         {
             return m_bitmap->bitmap().width();
         }
 
         int height() const
         {
             return m_bitmap->bitmap().height();
         }
@@ skipping 236 matching lines @@
 
         IntSize m_size;
         bool m_sizeAvailable;
         bool m_isAllDataReceived;
         bool m_failed;
     };
 
 } // namespace WebCore
 
 #endif