+Object* Map::GetBackPointer() {
+ Object* object = READ_FIELD(this, kPrototypeTransitionsOrBackPointerOffset);
+ if (object->IsFixedArray()) {
+ return FixedArray::cast(object)->get(kProtoTransitionBackPointerOffset);
+ } else {
+ return object;
+ }
+}
+
+
+void Map::SetBackPointer(Object* value, WriteBarrierMode mode) {
+ Heap* heap = GetHeap();
+ ASSERT(instance_type() >= FIRST_JS_RECEIVER_TYPE);
+ ASSERT((value->IsUndefined() && GetBackPointer()->IsMap()) ||
+ (value->IsMap() && GetBackPointer()->IsUndefined()));
+ 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, GetBackPointer());
+#ifdef DEBUG
+ if (value != prototype_transitions()) {
+ ZapPrototypeTransitions();
+ }
+#endif
+ 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);