WIP: Prune the prepaint tree walk

third_party/WebKit/Source/core/layout/LayoutObject.h

Index: third_party/WebKit/Source/core/layout/LayoutObject.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutObject.h b/third_party/WebKit/Source/core/layout/LayoutObject.h
index 8478f86e332c7f6041d231b7552db5f38e8c24dc..061ef652c3601955383c94d41e771573959f3202 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.h
@@ -1707,6 +1707,12 @@ class CORE_EXPORT LayoutObject : public ImageResourceObserver,
     void clearNeedsPaintPropertyUpdate() {
       m_layoutObject.clearNeedsPaintPropertyUpdate();
     }
+    void setDescendantNeedsPaintPropertyUpdate() {
+      m_layoutObject.setDescendantNeedsPaintPropertyUpdate();
+    }
+    void clearDescendantNeedsPaintPropertyUpdate() {
+      m_layoutObject.clearDescendantNeedsPaintPropertyUpdate();
+    }
 
    protected:
     friend class PaintPropertyTreeBuilder;
@@ -1734,16 +1740,36 @@ class CORE_EXPORT LayoutObject : public ImageResourceObserver,
   // state (location, transform, etc) as well as properties from ancestors.
   // When these inputs change, setNeedsPaintPropertyUpdate will cause a property
   // tree update during the next document lifecycle update.
-  // TODO(pdr): Add additional granularity such as the ability to signal that
-  // only a local paint property update is needed.
+  //
+  // In addition to tracking if an object needs its own paint properties
+  // updated, |descendantNeedsPaintPropertyUpdate| is used to track if any
+  // descendant needs an update too. This bit is up the tree, crossing frames.
+  //
+  // TODO(pdr): We may want to add more granular paint property update types
+  // such as the ability to only update one property type.
   void setNeedsPaintPropertyUpdate() {
     m_bitfields.setNeedsPaintPropertyUpdate(true);
+    // TODO(pdr): paintInvalidationParent is not inlined. May want to optimize
+    // this to check m_parent first.
+    if (auto* parent = paintInvalidationParent())
+      parent->setDescendantNeedsPaintPropertyUpdate();
+  }
+  void setDescendantNeedsPaintPropertyUpdate() {
+    if (!m_bitfields.descendantNeedsPaintPropertyUpdate()) {
+      m_bitfields.setDescendantNeedsPaintPropertyUpdate(true);
+      // TODO(pdr): paintInvalidationParent is not inlined. May want to optimize
+      // this to check m_parent first.
+      if (auto* parent = paintInvalidationParent())
+        parent->setDescendantNeedsPaintPropertyUpdate();
+    } else {
+      auto* parent = paintInvalidationParent();
+      DCHECK(!parent || parent->descendantNeedsPaintPropertyUpdate());
+    }
   }
-  // TODO(pdr): This can be removed once we have more granular update flags.
   void setAllAncestorsNeedPaintPropertyUpdate() {
-    if (m_parent) {
-      m_parent->setNeedsPaintPropertyUpdate();
-      m_parent->setAllAncestorsNeedPaintPropertyUpdate();
+    if (auto* parent = paintInvalidationParent()) {
+      parent->setNeedsPaintPropertyUpdate();
+      parent->setAllAncestorsNeedPaintPropertyUpdate();
     }
   }
   void clearNeedsPaintPropertyUpdate() {
@@ -1753,6 +1779,13 @@ class CORE_EXPORT LayoutObject : public ImageResourceObserver,
   bool needsPaintPropertyUpdate() const {
     return m_bitfields.needsPaintPropertyUpdate();
   }
+  void clearDescendantNeedsPaintPropertyUpdate() {
+    DCHECK_EQ(document().lifecycle().state(), DocumentLifecycle::InPrePaint);
+    m_bitfields.setDescendantNeedsPaintPropertyUpdate(false);
+  }
+  bool descendantNeedsPaintPropertyUpdate() const {
+    return m_bitfields.descendantNeedsPaintPropertyUpdate();
+  }
 
   void setIsScrollAnchorObject() { m_bitfields.setIsScrollAnchorObject(true); }
   // Clears the IsScrollAnchorObject bit if and only if no ScrollAnchors still
@@ -2149,6 +2182,7 @@ class CORE_EXPORT LayoutObject : public ImageResourceObserver,
           m_hasPreviousSelectionVisualRect(false),
           m_hasPreviousBoxGeometries(false),
           m_needsPaintPropertyUpdate(true),
+          m_descendantNeedsPaintPropertyUpdate(true),
           m_backgroundChangedSinceLastPaintInvalidation(false),
           m_positionedState(IsStaticallyPositioned),
           m_selectionState(SelectionNone),
@@ -2321,13 +2355,17 @@ class CORE_EXPORT LayoutObject : public ImageResourceObserver,
     // Whether the paint properties need to be updated. For more details, see
     // LayoutObject::needsPaintPropertyUpdate().
     ADD_BOOLEAN_BITFIELD(needsPaintPropertyUpdate, NeedsPaintPropertyUpdate);
+    // Whether the paint properties of a descendant need to be updated. For more
+    // details, see LayoutObject::descendantNeedsPaintPropertyUpdate().
+    ADD_BOOLEAN_BITFIELD(descendantNeedsPaintPropertyUpdate,
+                         DescendantNeedsPaintPropertyUpdate);
 
     ADD_BOOLEAN_BITFIELD(backgroundChangedSinceLastPaintInvalidation,
                          BackgroundChangedSinceLastPaintInvalidation);
 
    protected:
     // Use protected to avoid warning about unused variable.
-    unsigned m_unusedBits : 8;
+    unsigned m_unusedBits : 7;
 
    private:
     // This is the cached 'position' value of this object