Saving an extra backing store if the contents fill the entire border rect.

Source/core/rendering/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1591 matching lines @@
 bool CompositedLayerMapping::hasVisibleNonCompositingDescendantLayers() const
 {
     return hasVisibleNonCompositingDescendant(m_owningLayer);
 }
 
 bool CompositedLayerMapping::containsPaintedContent(bool isSimpleContainer) const
 {
     if (isSimpleContainer || paintsIntoCompositedAncestor() || m_artificiallyInflatedBounds || m_owningLayer->isReflection())
         return false;
 
+    if (renderer()->isCanvas() || renderer()->isVideo() || renderer()->isImage()) {
+        if (isAcceleratedCanvas(renderer())) {
+            if (contentsBox() == toRenderBox(renderer())->borderBoxRect())
+                return true;
+        }
+    }
+

[vivekg, 2014/02/18 12:30:39]

How about reordering like here? 

if (renderer()->isVideo() || 
    renderer()->isImage() || 
    (renderer()->isCanvas() && isAcceleratedCanvas(renderer()))) {
    if (contentsBox() == toRenderBox(renderer())->borderBoxRect())
        return true;
}

     if (isDirectlyCompositedImage())
         return false;
 
-    // FIXME: we could optimize cases where the image, video or canvas is known to fill the border box entirely,
-    // and set background color on the layer in that case, instead of allocating backing store and painting.
     if (renderer()->isVideo() && toRenderVideo(renderer())->shouldDisplayVideo())
         return m_owningLayer->hasBoxDecorationsOrBackground();
 
     return true;
 }
 
 // An image can be directly compositing if it's the sole content of the layer, and has no box decorations
 // that require painting. Direct compositing saves backing store.
 bool CompositedLayerMapping::isDirectlyCompositedImage() const
 {
@@ skipping 510 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore