Introduce a flag for tracing retaining path in GC.

src/heap/objects-visiting.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_OBJECTS_VISITING_H_
 #define V8_OBJECTS_VISITING_H_
 
 #include "src/allocation.h"
 #include "src/layout-descriptor.h"
 
@@ skipping 363 matching lines @@
 
   static VisitorDispatchTable<Callback> table_;
 };
 
 
 template <typename StaticVisitor>
 VisitorDispatchTable<typename StaticNewSpaceVisitor<StaticVisitor>::Callback>
     StaticNewSpaceVisitor<StaticVisitor>::table_;
 
 
+#ifdef TRACE_RETAINING_PATH
+void SetCurrentRetainer(HeapObject* obj);
+void ResetCurrentRetainer(HeapObject* obj);
+#endif
+
+
 // Base class for visitors used to transitively mark the entire heap.
 // IterateBody returns nothing.
 // Certain types of objects might not be handled by this base class and
 // no visitor function is registered by the generic initialization. A
 // specialized visitor function needs to be provided by the inheriting
 // class itself for those cases.
 //
 // This class is intended to be used in the following way:
 //
 //   class SomeVisitor : public StaticMarkingVisitor<SomeVisitor> {
 //     ...
 //   }
 //
 // This is an example of Curiously recurring template pattern.
 template <typename StaticVisitor>
 class StaticMarkingVisitor : public StaticVisitorBase {
  public:
   static void Initialize();
 
   INLINE(static void IterateBody(Map* map, HeapObject* obj)) {
+#ifdef TRACE_RETAINING_PATH
+    SetCurrentRetainer(obj);
+#endif
     table_.GetVisitor(map)(map, obj);
+#ifdef TRACE_RETAINING_PATH
+    ResetCurrentRetainer(obj);
+#endif
   }
 
   INLINE(static void VisitPropertyCell(Map* map, HeapObject* object));
   INLINE(static void VisitWeakCell(Map* map, HeapObject* object));
   INLINE(static void VisitCodeEntry(Heap* heap, Address entry_address));
   INLINE(static void VisitEmbeddedPointer(Heap* heap, RelocInfo* rinfo));
   INLINE(static void VisitCell(Heap* heap, RelocInfo* rinfo));
   INLINE(static void VisitDebugTarget(Heap* heap, RelocInfo* rinfo));
   INLINE(static void VisitCodeTarget(Heap* heap, RelocInfo* rinfo));
   INLINE(static void VisitCodeAgeSequence(Heap* heap, RelocInfo* rinfo));
@@ skipping 72 matching lines @@
 // the next element. Given the head of the list, this function removes dead
 // elements from the list and if requested records slots for next-element
 // pointers. The template parameter T is a WeakListVisitor that defines how to
 // access the next-element pointers.
 template <class T>
 Object* VisitWeakList(Heap* heap, Object* list, WeakObjectRetainer* retainer);
 }
 }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_VISITING_H_