optimizing layout performance when only transform3d matrix changed by

Source/core/rendering/compositing/RenderLayerCompositor.h

 /*
  * Copyright (C) 2009 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 31 matching lines @@
 class RenderPart;
 class RenderVideo;
 class ScrollingCoordinator;
 class StickyPositionViewportConstraints;
 
 enum CompositingUpdateType {
     CompositingUpdateAfterStyleChange,
     CompositingUpdateAfterLayout,
     CompositingUpdateOnScroll,
     CompositingUpdateOnCompositedScroll,
-    CompositingUpdateAfterCanvasContextChange
+    CompositingUpdateAfterCanvasContextChange,
+    CompositingMinUpdateAfterStyleChange,

[Ian Vollick, 2014/03/18 11:59:16]

"Min" and "fast" don't convey enough meaning.
How about,
CompositingUpdateAfterCompositedStyleChange,
..with a comment saying that if a property has changed in such a way that
compositing reasons cannot be impacted, then use this.

We should also use this for opacity (provided you're not transition to or from
1.0) and filters, where appropriate.

 };
 
 // RenderLayerCompositor manages the hierarchy of
 // composited RenderLayers. It determines which RenderLayers
 // become compositing, and creates and maintains a hierarchy of
 // GraphicsLayers based on the RenderLayer painting order.
 //
 // There is one RenderLayerCompositor per RenderView.
 
 class RenderLayerCompositor FINAL : public GraphicsLayerClient {
@@ skipping 303 matching lines @@
     // Enclosing layer for overflow controls and the clipping layer
     OwnPtr<GraphicsLayer> m_overflowControlsHostLayer;
 
     // Layers for overflow controls
     OwnPtr<GraphicsLayer> m_layerForHorizontalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForVerticalScrollbar;
     OwnPtr<GraphicsLayer> m_layerForScrollCorner;
 #if USE(RUBBER_BANDING)
     OwnPtr<GraphicsLayer> m_layerForOverhangShadow;
 #endif
+    bool m_layersChanged;
 };
 
 
 } // namespace WebCore
 
 #endif // RenderLayerCompositor_h