WIP: Prune the prepaint tree walk

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PrePaintTreeWalk.h"
 
 #include "core/dom/DocumentLifecycle.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
 #include "core/layout/LayoutMultiColumnSpannerPlaceholder.h"
@@ skipping 42 matching lines @@
       localContext.needToUpdatePaintPropertySubtree = true;
     }
   }
   // 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
   // the ability to do local updates that don't change the subtree.
   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);
 
 #if DCHECK_IS_ON()
   frameView.layoutView()->assertSubtreeClearedPaintInvalidationFlags();
 #endif
 
   frameView.clearNeedsPaintPropertyUpdate();
 }
 
@@ skipping 13 matching lines @@
       // mayNeedpaintInvalidation will be true when locations change which will
       // affect paint properties (e.g., PaintOffset).
       localContext.needToUpdatePaintPropertySubtree = true;
     } else if (object.shouldDoFullPaintInvalidation()) {
       // shouldDoFullPaintInvalidation will be true when locations or overflow
       // changes which will affect paint properties (e.g., PaintOffset, scroll).
       localContext.needToUpdatePaintPropertySubtree = true;
     }
   }
 
+  // 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
   // the ability to do local updates that don't change the subtree.
   if (localContext.needToUpdatePaintPropertySubtree)
     object.getMutableForPainting().setNeedsPaintPropertyUpdate();
 
   // TODO(pdr): Ensure multi column works with incremental property tree
   // construction.
   if (object.isLayoutMultiColumnSpannerPlaceholder()) {
     // Walk multi-column spanner as if it replaces the placeholder.
     // Set the flag so that the tree builder can specially handle out-of-flow
     // 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()) {
     // Column spanners are walked through their placeholders. See above.
     if (child->isColumnSpanAll())
       continue;
     walk(*child, localContext);
   }
 
   if (object.isLayoutPart()) {
     const LayoutPart& layoutPart = toLayoutPart(object);
     Widget* widget = layoutPart.widget();
     if (widget && widget->isFrameView()) {
       localContext.treeBuilderContext.current.paintOffset +=
           layoutPart.replacedContentRect().location() -
           widget->frameRect().location();
       localContext.treeBuilderContext.current.paintOffset =
           roundedIntPoint(localContext.treeBuilderContext.current.paintOffset);
       walk(*toFrameView(widget), localContext);
     }
     // TODO(pdr): Investigate RemoteFrameView (crbug.com/579281).
   }
 
   object.getMutableForPainting().clearPaintInvalidationFlags();
   object.getMutableForPainting().clearNeedsPaintPropertyUpdate();
+  object.getMutableForPainting().clearDescendantNeedsPaintPropertyUpdate();
 }
 
 }  // namespace blink