Cache the absolute bounding box rect on RenderLayer.

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 281 matching lines @@
         DontConstrainForMask = 1 << 4,
         IncludeCompositedDescendants = 1 << 5,
         UseFragmentBoxes = 1 << 6,
         PretendLayerHasOwnBacking = 1 << 7,
         DefaultCalculateLayerBoundsFlags =  IncludeSelfTransform | UseLocalClipRectIfPossible | IncludeLayerFilterOutsets | UseFragmentBoxes
     };
     typedef unsigned CalculateLayerBoundsFlags;
 
     // Bounding box relative to some ancestor layer. Pass offsetFromRoot if known.
     LayoutRect boundingBox(const RenderLayer* rootLayer, CalculateLayerBoundsFlags = 0, const LayoutPoint* offsetFromRoot = 0) const;
-    // Bounding box in the coordinates of this layer.
-    LayoutRect localBoundingBox(CalculateLayerBoundsFlags = 0) const;
-    // Pixel snapped bounding box relative to the root.
-    IntRect absoluteBoundingBox() const;
 
     // Bounds used for layer overlap testing in RenderLayerCompositor.
     LayoutRect overlapBounds() const { return overlapBoundsIncludeChildren() ? calculateLayerBounds(this) : localBoundingBox(); }
 
     // If true, this layer's children are included in its bounds for overlap testing.
     // We can't rely on the children's positions if this layer has a filter that could have moved the children's pixels around.
     bool overlapBoundsIncludeChildren() const { return hasFilter() && renderer()->style()->filter().hasFilterThatMovesPixels(); }
 
     // Can pass offsetFromRoot if known.
     LayoutRect calculateLayerBounds(const RenderLayer* ancestorLayer, const LayoutPoint* offsetFromRoot = 0, CalculateLayerBoundsFlags = DefaultCalculateLayerBoundsFlags) const;
@@ skipping 156 matching lines @@
 
     RenderLayerBlendInfo& blendInfo() { return m_blendInfo; }
 
     void setOffsetFromSquashingLayerOrigin(IntSize offset) { m_compositingProperties.offsetFromSquashingLayerOrigin = offset; }
     IntSize offsetFromSquashingLayerOrigin() const { return m_compositingProperties.offsetFromSquashingLayerOrigin; }
 
     bool scrollsOverflow() const;
 
     bool hasDirectReasonsForCompositing() const { return compositingReasons() & CompositingReasonComboAllDirectReasons; }
 
+    const IntRect& cachedAbsoluteBoundingBox() { ASSERT(!m_needsToRecomputeBounds); return m_cachedAbsoluteBoundingBox; }
+    void setAbsoluteBoundingBoxForOverlap(const IntRect&);
+    void clearNeedsToRecomputeBounds();
+    bool needsToRecomputeBounds() { return m_needsToRecomputeBounds; }
+
 private:
+    // Bounding box in the coordinates of this layer.
+    LayoutRect localBoundingBox(CalculateLayerBoundsFlags = 0) const;
+
     bool hasOverflowControls() const;
 
     void setIsUnclippedDescendant(bool isUnclippedDescendant) { m_isUnclippedDescendant = isUnclippedDescendant; }
 
     void setAncestorChainHasSelfPaintingLayerDescendant();
     void dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
 
     void setAncestorChainHasOutOfFlowPositionedDescendant();
     void dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
 
@@ skipping 182 matching lines @@
     unsigned m_has3DTransformedDescendant : 1;
 
     unsigned m_containsDirtyOverlayScrollbars : 1;
 
     // This is an optimization added for <table>.
     // Currently cells do not need to update their repaint rectangles when scrolling. This also
     // saves a lot of time when scrolling on a table.
     const unsigned m_canSkipRepaintRectsUpdateOnScroll : 1;
 
     unsigned m_hasFilterInfo : 1;
+    unsigned m_needsToRecomputeBounds : 1;
 
     RenderLayerModelObject* m_renderer;
 
     RenderLayer* m_parent;
     RenderLayer* m_previous;
     RenderLayer* m_next;
     RenderLayer* m_first;
     RenderLayer* m_last;
 
     // Our current relative position offset.
     LayoutSize m_offsetForInFlowPosition;
 
     // Our (x,y) coordinates are in our parent layer's coordinate space.
     LayoutPoint m_topLeft;
 
     // The layer's width/height
     IntSize m_layerSize;
 
     // Cached normal flow values for absolute positioned elements with static left/top values.
     LayoutUnit m_staticInlinePosition;
     LayoutUnit m_staticBlockPosition;
 
+    IntRect m_cachedAbsoluteBoundingBox;

[esprehn, 2014/03/21 23:28:01]

m_absoluteBoundingBox ? cached seems like a detail.

[ojan, 2014/03/22 00:25:43]

I guess. I wanted to distinguish from boundingBox.

+
     OwnPtr<TransformationMatrix> m_transform;
 
     // Pointer to the enclosing RenderLayer that caused us to be paginated. It is 0 if we are not paginated.
     RenderLayer* m_enclosingPaginationLayer;
 
     // Pointer to the enclosing RenderLayer that establishes the 3d rendering context in which this layer participates.
     // If it 0, it does not participate in a 3d rendering context.
     RenderLayer* m_3dRenderingContextRoot;
 
     // Properties that are computed while updating compositing layers. These values may be dirty/invalid if
@@ skipping 50 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const WebCore::RenderLayer*);
 void showLayerTree(const WebCore::RenderObject*);
 #endif
 
 #endif // RenderLayer_h