WIP: Prune the prepaint tree walk

third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp

Index: third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
diff --git a/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp b/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
index bc59d57869f2f1cd2c9e5df2f5555e721242fad7..e617cb16368927db90ca409c5640c6e06dfe0653 100644
--- a/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
+++ b/third_party/WebKit/Source/core/paint/PrePaintTreeWalk.cpp
@@ -60,12 +60,15 @@ void PrePaintTreeWalk::walk(FrameView& frameView,
   if (localContext.needToUpdatePaintPropertySubtree)
     frameView.setNeedsPaintPropertyUpdate();
 
-  m_propertyTreeBuilder.updateProperties(frameView,
-                                         localContext.treeBuilderContext);
+  auto propertyTreeStructureChanged = m_propertyTreeBuilder.updateProperties(
+      frameView, localContext.treeBuilderContext);
 
   m_paintInvalidator.invalidatePaintIfNeeded(
       frameView, localContext.paintInvalidatorContext);
 
+  if (propertyTreeStructureChanged)
+    localContext.needToUpdatePaintPropertySubtree = true;
+
   if (LayoutView* layoutView = frameView.layoutView())
     walk(*layoutView, localContext);
 
@@ -99,6 +102,17 @@ void PrePaintTreeWalk::walk(const LayoutObject& object,
     }
   }
 
+  // Early out from the treewalk if possible.
+  if (!object.descendantNeedsPaintPropertyUpdate() &&
+      !localContext.needToUpdatePaintPropertySubtree &&
+      !context.paintInvalidatorContext.forcedSubtreeInvalidationFlags &&
+      !object.shouldCheckForPaintInvalidationRegardlessOfPaintInvalidationState()) {
+    object.getMutableForPainting().clearPaintInvalidationFlags();
+    object.getMutableForPainting().clearNeedsPaintPropertyUpdate();
+    object.getMutableForPainting().clearDescendantNeedsPaintPropertyUpdate();
+    return;
+  }
+
   // Paint properties can depend on their ancestor properties so ensure the
   // entire subtree is rebuilt on any changes.
   // TODO(pdr): Add additional granularity to the needs update approach such as
@@ -114,19 +128,25 @@ void PrePaintTreeWalk::walk(const LayoutObject& object,
     // positioned descendants if their containers are between the multi-column
     // container and the spanner. See PaintPropertyTreeBuilder for details.
     localContext.treeBuilderContext.isUnderMultiColumnSpanner = true;
-    walk(*toLayoutMultiColumnSpannerPlaceholder(object)
-              .layoutObjectInFlowThread(),
-         localContext);
+    const auto& placeholder = toLayoutMultiColumnSpannerPlaceholder(object);
+    walk(*placeholder.layoutObjectInFlowThread(), localContext);
     object.getMutableForPainting().clearPaintInvalidationFlags();
+    object.getMutableForPainting().clearNeedsPaintPropertyUpdate();
+    object.getMutableForPainting().clearDescendantNeedsPaintPropertyUpdate();
     return;
   }
 
-  m_propertyTreeBuilder.updatePropertiesForSelf(
+  bool propertyTreeStructureChanged = false;
+  propertyTreeStructureChanged |= m_propertyTreeBuilder.updatePropertiesForSelf(
       object, localContext.treeBuilderContext);
   m_paintInvalidator.invalidatePaintIfNeeded(
       object, localContext.paintInvalidatorContext);
-  m_propertyTreeBuilder.updatePropertiesForChildren(
-      object, localContext.treeBuilderContext);
+  propertyTreeStructureChanged |=
+      m_propertyTreeBuilder.updatePropertiesForChildren(
+          object, localContext.treeBuilderContext);
+
+  if (propertyTreeStructureChanged)
+    localContext.needToUpdatePaintPropertySubtree = true;
 
   for (const LayoutObject* child = object.slowFirstChild(); child;
        child = child->nextSibling()) {
@@ -152,6 +172,7 @@ void PrePaintTreeWalk::walk(const LayoutObject& object,
 
   object.getMutableForPainting().clearPaintInvalidationFlags();
   object.getMutableForPainting().clearNeedsPaintPropertyUpdate();
+  object.getMutableForPainting().clearDescendantNeedsPaintPropertyUpdate();
 }
 
 }  // namespace blink