Move LayoutObject to satellite NodeLayoutData that hangs from a Node.

third_party/WebKit/Source/core/dom/Node.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004-2011, 2014 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 39 matching lines @@
 class Event;
 class ExceptionState;
 class GetRootNodeOptions;
 class HTMLQualifiedName;
 class HTMLSlotElement;
 class IntRect;
 class EventDispatchHandlingState;
 class NodeList;
 class NodeListsNodeData;
 class NodeOrString;
+class NodeLayoutData;
 class NodeRareData;
 class QualifiedName;
 class RegisteredEventListener;
 class LayoutBox;
 class LayoutBoxModelObject;
 class LayoutObject;
 class ComputedStyle;
 class SVGQualifiedName;
 class ShadowRoot;
 template <typename NodeType>
@@ skipping 21 matching lines @@
   kFailed = 3 << kNodeCustomElementShift,
 
   kNotDefinedFlag = 2 << kNodeCustomElementShift,
 };
 
 enum class SlotChangeType {
   kInitial,
   kChained,
 };
 
+class NodeLayoutData {
+ public:
+  explicit NodeLayoutData(LayoutObject* layout_object)
+      : layout_object_(layout_object) {}
+  LayoutObject* GetLayoutObject() const { return layout_object_; }
+  void SetLayoutObject(LayoutObject* layout_object) {
+    layout_object_ = layout_object;

[esprehn, 2017/04/13 00:30:47]

DCHECK_NE(&SharedEmptyData(), this);

[nainar, 2017/04/13 15:02:02]

Done. 

+  }
+  static NodeLayoutData& SharedEmptyData() {
+    static NodeLayoutData shared_empty_data_(nullptr);
+    return shared_empty_data_;
+  }
+
+ private:
+  LayoutObject* layout_object_;
+};
+
 class NodeRareDataBase {
  public:
-  LayoutObject* GetLayoutObject() const { return layout_object_; }
-  void SetLayoutObject(LayoutObject* layout_object) {
-    layout_object_ = layout_object;
+  NodeLayoutData* GetNodeLayoutData() const {
+    return node_layout_data_ ? node_layout_data_

[esprehn, 2017/04/13 00:30:47]

The point of the SharedEmptyData() is that we don't need this null check, we
should always either have node_layout_data_ be SharedEmptyData() or a unique
one, it's not nullable.

[nainar, 2017/04/13 15:02:02]

Changed to 
return node_layout_data_;

+                             : &NodeLayoutData::SharedEmptyData();
+  }
+  void SetNodeLayoutData(NodeLayoutData* node_layout_data) {

[esprehn, 2017/04/13 00:30:47]

DCHECK(node_layout_data)

[nainar, 2017/04/13 15:02:02]

Done.

+    node_layout_data_ = node_layout_data;
   }
 
  protected:
-  NodeRareDataBase(LayoutObject* layout_object)
-      : layout_object_(layout_object) {}
+  NodeRareDataBase(NodeLayoutData* node_layout_data)
+      : node_layout_data_(node_layout_data) {}
 
  protected:
-  // LayoutObjects are fully owned by their DOM node. See LayoutObject's
-  // LIFETIME documentation section.
-  LayoutObject* layout_object_;
+  NodeLayoutData* node_layout_data_;

[esprehn, 2017/04/13 00:30:47]

you need a destructor for NodeRareDataBase and inside ~Node, that does:

if (node_layout_data_ && node_layout_data_ != SharedEmptyData())
  delete node_layout_data_;

or you need to make NodeLayoutData GarbageCollected and then this should all
work automatically since the static Persistent in SharedEmptyData would keep it
alive. Making it garbage collected is less code which is nice, you'll need to
modify the tracing function here:
https://cs.chromium.org/webrtc/src/third_party/WebKit/Source/core/dom/Node.cp...

Right now you're leaking this object though.

[nainar, 2017/04/13 15:02:02]

Done. 

 };
 
 class Node;
 WILL_NOT_BE_EAGERLY_TRACED_CLASS(Node);
 
 // This class represents a DOM node in the DOM tree.
 // https://dom.spec.whatwg.org/#interface-node
 class CORE_EXPORT Node : public EventTarget {
   DEFINE_WRAPPERTYPEINFO();
   friend class TreeScope;
@@ skipping 435 matching lines @@
   virtual bool CanStartSelection() const;
 
   // -----------------------------------------------------------------------------
   // Integration with layout tree
 
   // As layoutObject() includes a branch you should avoid calling it repeatedly
   // in hot code paths.
   // Note that if a Node has a layoutObject, it's parentNode is guaranteed to
   // have one as well.
   LayoutObject* GetLayoutObject() const {
-    return HasRareData() ? data_.rare_data_->GetLayoutObject()
-                         : data_.layout_object_;
+    return HasRareData()
+               ? data_.rare_data_->GetNodeLayoutData()->GetLayoutObject()
+               : data_.node_layout_data_->GetLayoutObject();
   }
   void SetLayoutObject(LayoutObject* layout_object) {

[esprehn, 2017/04/13 00:30:47]

This function has become so big, I'd suggest moving out of line into the .cpp
file. The one function call is unlikely to be a perf issue since we're already
doing something so expensive when we call setLayoutObject.

[nainar, 2017/04/13 15:02:02]

Done. 

-    if (HasRareData())
-      data_.rare_data_->SetLayoutObject(layout_object);
-    else
-      data_.layout_object_ = layout_object;
+    if (HasRareData()) {

[nainar, 2017/04/13 00:10:22]

Can make this less complicated by adding a EnsureLayoutObject method on
NodeLayoutData, 

NodeLayoutData* NodeLayoutData::EnsureLayoutObject(layout_object) {
   if this == &NodeLayoutData::SharedEmptyData()
       return new NodeLayoutData(layout_object)
   else 
       SetLayoutObject(layout_object);
}

+      data_.rare_data_->GetNodeLayoutData() ==

[esprehn, 2017/04/13 00:30:47]

we should put this in a method on NodeLayoutData like bool IsShared()

[nainar, 2017/04/13 15:02:02]

Done.

+              &NodeLayoutData::SharedEmptyData()
+          ? data_.rare_data_->SetNodeLayoutData(
+                new NodeLayoutData(layout_object))
+          : data_.rare_data_->GetNodeLayoutData()->SetLayoutObject(
+                layout_object);
+    } else {
+      if (data_.node_layout_data_ == &NodeLayoutData::SharedEmptyData())

[esprehn, 2017/04/13 00:30:47]

if (data_.node_layout_data_->IsShared())

[nainar, 2017/04/13 15:02:02]

Done.

+        data_.node_layout_data_ = new NodeLayoutData(layout_object);
+      else
+        data_.node_layout_data_->SetLayoutObject(layout_object);
+    }
   }
 
   // Use these two methods with caution.
   LayoutBox* GetLayoutBox() const;
   LayoutBoxModelObject* GetLayoutBoxModelObject() const;
 
   struct AttachContext {
     STACK_ALLOCATED();
     ComputedStyle* resolved_style = nullptr;
     bool performing_reattach = false;
@@ skipping 334 matching lines @@
   const HeapHashSet<TraceWrapperMember<MutationObserverRegistration>>*
   TransientMutationObserverRegistry();
 
   uint32_t node_flags_;
   Member<ContainerNode> parent_or_shadow_host_node_;
   Member<TreeScope> tree_scope_;
   Member<Node> previous_;
   Member<Node> next_;
   // When a node has rare data we move the layoutObject into the rare data.
   union DataUnion {
-    DataUnion() : layout_object_(nullptr) {}
+    DataUnion() : node_layout_data_(&NodeLayoutData::SharedEmptyData()) {}
     // LayoutObjects are fully owned by their DOM node. See LayoutObject's
     // LIFETIME documentation section.
-    LayoutObject* layout_object_;
+    NodeLayoutData* node_layout_data_;
     NodeRareDataBase* rare_data_;
   } data_;
 };
 
 inline void Node::SetParentOrShadowHostNode(ContainerNode* parent) {
   DCHECK(IsMainThread());
   parent_or_shadow_host_node_ = parent;
   ScriptWrappableVisitor::WriteBarrier(
       this, reinterpret_cast<Node*>(parent_or_shadow_host_node_.Get()));
 }
@@ skipping 52 matching lines @@
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showNode(const blink::Node*);
 void showTree(const blink::Node*);
 void showNodePath(const blink::Node*);
 #endif
 
 #endif  // Node_h