Avoid tree walks when computing RenderLayer::scrollParent

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 ef40889fb15879f48508ec0638e98382c9c74ccb..c564aa75a1f60d85ddc62382416319ea3a383fb9 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -1133,23 +1133,49 @@ RenderLayer* RenderLayer::enclosingCompositingLayerForRepaint(IncludeSelfOrNot i
     return 0;
 }
 
-RenderLayer* RenderLayer::ancestorCompositedScrollingLayer() const
+void RenderLayer::clearAncestorDependentPropertyCache()
 {
-    ASSERT(isAllowedToQueryCompositingState());
+    ASSERT(isInCompositingUpdate());
+    m_ancestorDependentPropertyCache.clear();
+}
+
+void RenderLayer::ensureAncestorDependentPropertyCache() const
+{
+    ASSERT(isInCompositingUpdate());
+    if (m_ancestorDependentPropertyCache)
+        return;
+    m_ancestorDependentPropertyCache = adoptPtr(new AncestorDependentPropertyCache());
+}
 
+RenderLayer* RenderLayer::ancestorCompositedScrollingLayer() const
+{
     if (!renderer()->acceleratedCompositingForOverflowScrollEnabled())
         return 0;
 
+    ASSERT(isInCompositingUpdate() || !m_ancestorDependentPropertyCache);
+
+    if (m_ancestorDependentPropertyCache && !m_ancestorDependentPropertyCache->ancestorCompositedScrollingLayerDirty())
+        return m_ancestorDependentPropertyCache->ancestorCompositedScrollingLayer();
+
     RenderObject* containingBlock = renderer()->containingBlock();
     if (!containingBlock)
         return 0;
 
+    if (isInCompositingUpdate())
+        ensureAncestorDependentPropertyCache();
+
+    RenderLayer* ancestorCompositedScrollingLayer = 0;
     for (RenderLayer* ancestorLayer = containingBlock->enclosingLayer(); ancestorLayer; ancestorLayer = ancestorLayer->parent()) {
-        if (ancestorLayer->needsCompositedScrolling())
-            return ancestorLayer;
+        if (ancestorLayer->needsCompositedScrolling()) {
+            ancestorCompositedScrollingLayer = ancestorLayer;
+            break;
+        }
     }
 
-    return 0;
+    if (m_ancestorDependentPropertyCache)
+        m_ancestorDependentPropertyCache->setAncestorCompositedScrollingLayer(ancestorCompositedScrollingLayer);
+
+    return ancestorCompositedScrollingLayer;
 }
 
 RenderLayer* RenderLayer::ancestorScrollingLayer() const
@@ -1677,6 +1703,10 @@ RenderLayer* RenderLayer::scrollParent() const
     if (stackingNode()->isNormalFlowOnly())
         return 0;
 
+    // We should never have an ancestor dependent property cache outside of the
+    // compositing update phase.
+    ASSERT(isInCompositingUpdate() || !m_ancestorDependentPropertyCache);
+
     // A layer scrolls with its containing block. So to find the overflow scrolling layer
     // that we scroll with respect to, we must ascend the layer tree until we reach the
     // first overflow scrolling div at or above our containing block. I will refer to this
@@ -1689,8 +1719,10 @@ RenderLayer* RenderLayer::scrollParent() const
     // our scrolling ancestor, and we will therefore not scroll with it. In this case, we must
     // be a composited layer since the compositor will need to take special measures to ensure
     // that we scroll with our scrolling ancestor and it cannot do this if we do not promote.
-    RenderLayer* scrollParent = ancestorCompositedScrollingLayer();
+    if (m_ancestorDependentPropertyCache && !m_ancestorDependentPropertyCache->scrollParentDirty())
+        return m_ancestorDependentPropertyCache->scrollParent();
 
+    RenderLayer* scrollParent = ancestorCompositedScrollingLayer();
     if (!scrollParent || scrollParent->stackingNode()->isStackingContainer())
         return 0;
 
@@ -1699,8 +1731,18 @@ RenderLayer* RenderLayer::scrollParent() const
     for (RenderLayer* ancestor = parent(); ancestor && ancestor != scrollParent; ancestor = ancestor->parent()) {
         if (ancestor->stackingNode()->isStackingContainer())
             return 0;
+        if (!isInCompositingUpdate())
+            continue;
+        if (AncestorDependentPropertyCache* ancestorCache = ancestor->m_ancestorDependentPropertyCache.get()) {
+            if (!ancestorCache->ancestorCompositedScrollingLayerDirty() && ancestorCache->ancestorCompositedScrollingLayer() == scrollParent) {
+                scrollParent = ancestorCache->scrollParent();
+                break;
+            }
+        }
     }
 
+    if (m_ancestorDependentPropertyCache)
+        m_ancestorDependentPropertyCache->setScrollParent(scrollParent);
     return scrollParent;
 }
 
@@ -3520,6 +3562,11 @@ bool RenderLayer::isAllowedToQueryCompositingState() const
     return renderer()->document().lifecycle().state() >= DocumentLifecycle::InCompositingUpdate;
 }
 
+bool RenderLayer::isInCompositingUpdate() const
+{
+    return renderer()->document().lifecycle().state() == DocumentLifecycle::InCompositingUpdate;
+}
+
 CompositedLayerMappingPtr RenderLayer::compositedLayerMapping() const
 {
     ASSERT(isAllowedToQueryCompositingState());
@@ -4059,6 +4106,36 @@ DisableCompositingQueryAsserts::DisableCompositingQueryAsserts()
 
 COMPILE_ASSERT(1 << RenderLayer::ViewportConstrainedNotCompositedReasonBits >= RenderLayer::NumNotCompositedReasons, too_many_viewport_constrained_not_compositing_reasons);
 
+RenderLayer::AncestorDependentPropertyCache::AncestorDependentPropertyCache()
+    : m_ancestorCompositedScrollingLayer(0)
+    , m_scrollParent(0)
+    , m_ancestorCompositedScrollingLayerDirty(true)
+    , m_scrollParentDirty(true) { }
+
+RenderLayer* RenderLayer::AncestorDependentPropertyCache::scrollParent() const
+{
+    ASSERT(!m_scrollParentDirty);
+    return m_scrollParent;
+}
+
+void RenderLayer::AncestorDependentPropertyCache::setScrollParent(RenderLayer* scrollParent)
+{
+    m_scrollParent = scrollParent;
+    m_scrollParentDirty = false;
+}
+
+RenderLayer* RenderLayer::AncestorDependentPropertyCache::ancestorCompositedScrollingLayer() const
+{
+    ASSERT(!m_ancestorCompositedScrollingLayerDirty);
+    return m_ancestorCompositedScrollingLayer;
+}
+
+void RenderLayer::AncestorDependentPropertyCache::setAncestorCompositedScrollingLayer(RenderLayer* layer)
+{
+    m_ancestorCompositedScrollingLayer = layer;
+    m_ancestorCompositedScrollingLayerDirty = false;
+}
+
 } // namespace WebCore
 
 #ifndef NDEBUG