In-object double fields unboxing (for 64-bit only).

src/runtime/runtime-object.cc

Index: src/runtime/runtime-object.cc
diff --git a/src/runtime/runtime-object.cc b/src/runtime/runtime-object.cc
index 74cb8cb29344b273fb5a9a4aba88b5b3778d18c5..800659d61df91a31b088ae111fb4a7fefd74bb16 100644
--- a/src/runtime/runtime-object.cc
+++ b/src/runtime/runtime-object.cc
@@ -1398,9 +1398,8 @@ RUNTIME_FUNCTION(Runtime_LoadMutableDouble) {
     RUNTIME_ASSERT(field_index.outobject_array_index() <
                    object->properties()->length());
   }
-  Handle<Object> raw_value(object->RawFastPropertyAt(field_index), isolate);
-  RUNTIME_ASSERT(raw_value->IsMutableHeapNumber());
-  return *Object::WrapForRead(isolate, raw_value, Representation::Double());
+  return *JSObject::FastPropertyAt(object, Representation::Double(),
+                                   field_index);
 }