Simple BlinkGC heap compaction.

third_party/WebKit/Source/platform/heap/Visitor.h

Index: third_party/WebKit/Source/platform/heap/Visitor.h
diff --git a/third_party/WebKit/Source/platform/heap/Visitor.h b/third_party/WebKit/Source/platform/heap/Visitor.h
index f93d1e4d52fd9ce3516a7bc9399758f7c60a2343..92a7e6f131ad304585eaa1f41da51ea4af78deb2 100644
--- a/third_party/WebKit/Source/platform/heap/Visitor.h
+++ b/third_party/WebKit/Source/platform/heap/Visitor.h
@@ -243,10 +243,26 @@ class VisitorHelper {
   void registerWeakMembers(const T* obj) {
     registerWeakMembers(obj, &TraceMethodDelegate<T, method>::trampoline);
   }
+
   void registerWeakMembers(const void* object, WeakCallback callback) {
     Derived::fromHelper(this)->registerWeakMembers(object, object, callback);
   }
 
+  template <typename T>
+  void registerBackingStoreReference(T** slot) {
+    Derived::fromHelper(this)->registerMovingObjectReference(
+        reinterpret_cast<MovableReference*>(slot));
+  }
+
+  template <typename T>
+  void registerBackingStoreCallback(T* backingStore,
+                                    MovingObjectCallback callback,
+                                    void* callbackData) {
+    Derived::fromHelper(this)->registerMovingObjectCallback(
+        reinterpret_cast<MovableReference>(backingStore), callback,
+        callbackData);
+  }
+
   inline ThreadState* state() const { return m_state; }
   inline ThreadHeap& heap() const { return state()->heap(); }
 
@@ -254,7 +270,7 @@ class VisitorHelper {
   template <typename T>
   static void handleWeakCell(Visitor* self, void* object);
 
-  ThreadState* m_state;
+  ThreadState* const m_state;
 };
 
 // Visitor is used to traverse the Blink object graph. Used for the
@@ -284,6 +300,13 @@ class PLATFORM_EXPORT Visitor : public VisitorHelper<Visitor> {
     // This visitor is used to trace objects during weak processing.
     // This visitor is allowed to trace only already marked objects.
     WeakProcessing,
+    // Perform global marking along with preparing for additional sweep
+    // compaction of heap arenas afterwards. Compared to the GlobalMarking
+    // visitor, this visitor will also register references to objects
+    // that might be moved during arena compaction -- the compaction
+    // pass will then fix up those references when the object move goes
+    // ahead.
+    GlobalMarkCompacting,
   };
 
   static std::unique_ptr<Visitor> create(ThreadState*, BlinkGC::GCType);
@@ -310,7 +333,10 @@ class PLATFORM_EXPORT Visitor : public VisitorHelper<Visitor> {
   // object. If collection backings are reachable from other
   // locations we strongify them to avoid issues with iterators and
   // weak processing.
-  virtual void registerDelayedMarkNoTracing(const void*) = 0;
+  // A pointer to the location holding the backing store reference
+  // is passed in. This is done so as to potentially allow that
+  // location to be updated by the garbage collector.
+  virtual void registerDelayedMarkNoTracing(void**) = 0;
 
   // If the object calls this during the regular trace callback, then the
   // WeakCallback argument may be called later, when the strong roots
@@ -344,10 +370,21 @@ class PLATFORM_EXPORT Visitor : public VisitorHelper<Visitor> {
 
   virtual bool ensureMarked(const void*) = 0;
 
+  virtual void registerMovingObjectReference(MovableReference*) = 0;
+
+  virtual void registerMovingObjectCallback(MovableReference,
+                                            MovingObjectCallback,
+                                            void*) = 0;
+
   virtual void registerWeakCellWithCallback(void**, WeakCallback) = 0;
 
   inline MarkingMode getMarkingMode() const { return m_markingMode; }
 
+  inline bool isGlobalMarking() const {
+    return m_markingMode == GlobalMarking ||
+           m_markingMode == GlobalMarkCompacting;
+  }
+
  protected:
   Visitor(ThreadState*, MarkingMode);
 
@@ -356,7 +393,6 @@ class PLATFORM_EXPORT Visitor : public VisitorHelper<Visitor> {
     return static_cast<Visitor*>(helper);
   }
 
-  ThreadState* m_state;
   const MarkingMode m_markingMode;
 };