Implement explicit back pointers in transition tree.

src/profile-generator.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2128 matching lines @@
                        "prototype", map->prototype(), Map::kPrototypeOffset);
   SetInternalReference(map, entry,
                        "constructor", map->constructor(),
                        Map::kConstructorOffset);
   if (!map->instance_descriptors()->IsEmpty()) {
     TagObject(map->instance_descriptors(), "(map descriptors)");
     SetInternalReference(map, entry,
                          "descriptors", map->instance_descriptors(),
                          Map::kInstanceDescriptorsOrBitField3Offset);
   }
-  TagObject(map->prototype_transitions(), "(prototype transitions)");
-  SetInternalReference(map, entry,
-                       "prototype_transitions", map->prototype_transitions(),
-                       Map::kPrototypeTransitionsOffset);
+  if (map->unchecked_prototype_transitions()->IsFixedArray()) {
+    TagObject(map->prototype_transitions(), "(prototype transitions)");
+    SetInternalReference(map, entry,
+                         "prototype_transitions", map->prototype_transitions(),
+                         Map::kPrototypeTransitionsOrBackPointerOffset);
+  } else {
+    SetInternalReference(map, entry,
+                         "back_pointer", map->GetBackPointer(),
+                         Map::kPrototypeTransitionsOrBackPointerOffset);
+  }
   SetInternalReference(map, entry,
                        "code_cache", map->code_cache(),
                        Map::kCodeCacheOffset);
 }
 
 
 void V8HeapExplorer::ExtractSharedFunctionInfoReferences(
     HeapEntry* entry, SharedFunctionInfo* shared) {
   HeapObject* obj = shared;
   SetInternalReference(obj, entry,
@@ skipping 1882 matching lines @@
 
 
 void HeapSnapshotJSONSerializer::SortHashMap(
     HashMap* map, List<HashMap::Entry*>* sorted_entries) {
   for (HashMap::Entry* p = map->Start(); p != NULL; p = map->Next(p))
     sorted_entries->Add(p);
   sorted_entries->Sort(SortUsingEntryValue);
 }
 
 } }  // namespace v8::internal