Move LayoutObject::m_previousPosition... into a global map in ObjectPaintInvalidator

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

Index: third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp
diff --git a/third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp b/third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp
index 5dac2b6796a97570578b3be9562eb3e3e32904dc..9af86fea838e11e5b70ec8c6eed7bb275bbc0100 100644
--- a/third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp
+++ b/third_party/WebKit/Source/core/paint/ObjectPaintInvalidator.cpp
@@ -33,8 +33,15 @@ static void setPreviousSelectionVisualRect(const LayoutObject& object,
     selectionVisualRectMap().set(&object, rect);
 }
 
+typedef HashMap<const LayoutObject*, LayoutPoint> LocationInBackingMap;
+static LocationInBackingMap& locationInBackingMap() {
+  DEFINE_STATIC_LOCAL(LocationInBackingMap, map, ());
+  return map;
+}
+
 void ObjectPaintInvalidator::objectWillBeDestroyed(const LayoutObject& object) {
   selectionVisualRectMap().remove(&object);
+  locationInBackingMap().remove(&object);
 }
 
 // TODO(trchen): Use std::function<void, LayoutObject&> when available.
@@ -370,6 +377,25 @@ void ObjectPaintInvalidator::slowSetPaintingLayerNeedsRepaint() {
     paintingLayer->setNeedsRepaint();
 }
 
+LayoutPoint ObjectPaintInvalidator::previousLocationInBacking() const {
+  // TODO(wangxianzhu): Use a bit in LayoutObject to indicate whether the object
+  // has previous location in the map to avoid unnecessary map lookup.
+  auto it = locationInBackingMap().find(&m_object);

[chrishtr, 2016/11/04 18:20:07]

This lookup will be slow. How slow?

I think you should address the TODO in this patch to avoid making things slower.

[Xianzhu, 2016/11/04 19:11:03]

Done.

+  if (it == locationInBackingMap().end())
+    return m_object.previousVisualRect().location();
+  return it->value;
+}
+
+void ObjectPaintInvalidator::setPreviousLocationInBacking(
+    const LayoutPoint& location) {
+  // TODO(wangxianzhu): Use a bit in LayoutObject to indicate whether the object
+  // has previous location in the map to avoid unnecessary map lookup.
+  if (location == m_object.previousVisualRect().location())
+    locationInBackingMap().remove(&m_object);
+  else
+    locationInBackingMap().set(&m_object, location);
+}
+
 void ObjectPaintInvalidatorWithContext::fullyInvalidatePaint(
     PaintInvalidationReason reason,
     const LayoutRect& oldVisualRect,