update3dRenderingContext is 13.5% of GraphicsLayerUpdater::rebuildTree

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 20f618d211c61b254a05595db9ba40aaa1bb68a3..5cc4c9a2f728b4f0f89e28be4d96134427987393 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -136,7 +136,6 @@ RenderLayer::RenderLayer(RenderLayerModelObject* renderer, LayerType type)
     , m_staticInlinePosition(0)
     , m_staticBlockPosition(0)
     , m_enclosingPaginationLayer(0)
-    , m_3dRenderingContextRoot(0)
     , m_groupedMapping(0)
     , m_repainter(renderer)
     , m_clipper(renderer)
@@ -562,20 +561,24 @@ void RenderLayer::updateTransform()
         dirty3DTransformedDescendantStatus();
 }
 
-// Note: this function assumes that all ancestors have an updated 3d rendering context root.
-void RenderLayer::update3dRenderingContext()
+static RenderLayer* enclosingLayerForContainingBlock(RenderLayer* layer)
 {
-    m_3dRenderingContextRoot = 0;
+    if (RenderObject* containingBlock = layer->renderer()->containingBlock())
+        return containingBlock->enclosingLayer();
+    return 0;
+}
 
-    if (!shouldFlattenTransform())
-        m_3dRenderingContextRoot = this;
+RenderLayer* RenderLayer::renderingContextRoot()
+{
+    RenderLayer* renderingContext = 0;
 
-    if (RenderObject* containingBlock = renderer()->containingBlock()) {
-        if (RenderLayer* ancestorLayer = containingBlock->enclosingLayer()) {
-            if (!ancestorLayer->shouldFlattenTransform())
-                m_3dRenderingContextRoot = ancestorLayer->renderingContextRoot();
-        }
-    }
+    if (shouldPreserve3D())
+        renderingContext = this;
+
+    for (RenderLayer* current = enclosingLayerForContainingBlock(this); current && current->shouldPreserve3D(); current = enclosingLayerForContainingBlock(current))
+        renderingContext = current;
+
+    return renderingContext;
 }
 
 TransformationMatrix RenderLayer::currentTransform(RenderStyle::ApplyTransformOrigin applyOrigin) const