Keep maps when resetting elements.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/335553004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21838 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/elements.cc

@@ -1436,9 +1436,7 @@ class DictionaryElementsAccessor
     }
 
     if (new_length == 0) {
-      // If the length of a slow array is reset to zero, we clear
-      // the array and flush backing storage. This has the added
-      // benefit that the array returns to fast mode.
+      // Flush the backing store.
       JSObject::ResetElements(array);
     } else {
       DisallowHeapAllocation no_gc;

src/objects-inl.h

@@ -2738,6 +2738,9 @@ FixedArrayBase* Map::GetInitialElements() {
       GetHeap()->EmptyFixedTypedArrayForMap(this);
     ASSERT(!GetHeap()->InNewSpace(empty_array));
     return empty_array;
+  } else if (has_dictionary_elements()) {
+    ASSERT(!GetHeap()->InNewSpace(GetHeap()->empty_slow_element_dictionary()));
+    return GetHeap()->empty_slow_element_dictionary();
   } else {
     UNREACHABLE();
   }

src/objects.cc

@@ -4793,28 +4793,9 @@ void JSObject::TransformToFastProperties(Handle<JSObject> object,
 
 
 void JSObject::ResetElements(Handle<JSObject> object) {
-  if (object->map()->is_observed()) {
-    // Maintain invariant that observed elements are always in dictionary mode.
-    Isolate* isolate = object->GetIsolate();
-    Factory* factory = isolate->factory();
-    Handle<SeededNumberDictionary> dictionary =
-        SeededNumberDictionary::New(isolate, 0);
-    if (object->map() == *factory->sloppy_arguments_elements_map()) {
-      FixedArray::cast(object->elements())->set(1, *dictionary);
-    } else {
-      object->set_elements(*dictionary);
-    }
-    return;
-  }
-
-  ElementsKind elements_kind = GetInitialFastElementsKind();
-  if (!FLAG_smi_only_arrays) {
-    elements_kind = FastSmiToObjectElementsKind(elements_kind);
-  }
-  Handle<Map> map = JSObject::GetElementsTransitionMap(object, elements_kind);
-  DisallowHeapAllocation no_gc;
-  Handle<FixedArrayBase> elements(map->GetInitialElements());
-  JSObject::SetMapAndElements(object, map, elements);
+  Heap* heap = object->GetIsolate()->heap();
+  CHECK(object->map() != heap->sloppy_arguments_elements_map());
+  object->set_elements(object->map()->GetInitialElements());
 }