Implement explicit back pointers in transition tree.

src/objects-inl.h

 // 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 3390 matching lines @@
   if (object->IsSmi()) {
     WRITE_FIELD(this,
                 kInstanceDescriptorsOrBitField3Offset,
                 Smi::FromInt(value));
   } else {
     DescriptorArray::cast(object)->set_bit_field3_storage(value);
   }
 }
 
 
-FixedArray* Map::unchecked_prototype_transitions() {
-  return reinterpret_cast<FixedArray*>(
-      READ_FIELD(this, kPrototypeTransitionsOffset));
+Object* Map::back_pointer() {
+  Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
+  if (object->IsFixedArray()) {
+    return FixedArray::cast(object)->get(kProtoTransitionBackPointerOffset);
+  } else {
+    return object;
+  }
+}
+
+
+void Map::set_back_pointer(Object* value, WriteBarrierMode mode) {
+  ASSERT(value->IsUndefined() ||
+         (value->IsMap() && back_pointer()->IsUndefined() &&
+          instance_type() >= FIRST_JS_RECEIVER_TYPE));
+  Heap* heap = GetHeap();
+  Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
+  if (object->IsFixedArray()) {
+    FixedArray::cast(object)->set(
+        kProtoTransitionBackPointerOffset, value, mode);
+  } else {
+    WRITE_FIELD(this, kPrototypeTransitionsOrBackPointerOffset, value);
+    CONDITIONAL_WRITE_BARRIER(
+        heap, this, kPrototypeTransitionsOrBackPointerOffset, value, mode);
+  }
+}
+
+
+FixedArray* Map::prototype_transitions() {
+  Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
+  if (object->IsFixedArray()) {
+    return FixedArray::cast(object);
+  } else {
+    return GetHeap()->empty_fixed_array();
+  }
+}
+
+
+void Map::set_prototype_transitions(FixedArray* value, WriteBarrierMode mode) {
+  Heap* heap = GetHeap();
+  ASSERT(value != heap->empty_fixed_array());
+  value->set(kProtoTransitionBackPointerOffset, back_pointer());
+  WRITE_FIELD(this, kPrototypeTransitionsOrBackPointerOffset, value);
+  CONDITIONAL_WRITE_BARRIER(
+      heap, this, kPrototypeTransitionsOrBackPointerOffset, value, mode);
+
+}
+
+
+void Map::init_prototype_transitions(Object* undefined) {
+  ASSERT(undefined->IsUndefined());
+  WRITE_FIELD(this, kPrototypeTransitionsOrBackPointerOffset, undefined);
+}
+
+
+HeapObject* Map::unchecked_prototype_transitions() {
+  Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
+  return reinterpret_cast<HeapObject*>(object);
 }
 
 
 ACCESSORS(Map, code_cache, Object, kCodeCacheOffset)
-ACCESSORS(Map, prototype_transitions, FixedArray, kPrototypeTransitionsOffset)
 ACCESSORS(Map, constructor, Object, kConstructorOffset)
 
 ACCESSORS(JSFunction, shared, SharedFunctionInfo, kSharedFunctionInfoOffset)
 ACCESSORS(JSFunction, literals_or_bindings, FixedArray, kLiteralsOffset)
 ACCESSORS(JSFunction,
           next_function_link,
           Object,
           kNextFunctionLinkOffset)
 
 ACCESSORS(GlobalObject, builtins, JSBuiltinsObject, kBuiltinsOffset)
@@ skipping 1528 matching lines @@
 #undef WRITE_UINT32_FIELD
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_