[LookupIterator] Add fast-path for reloading accessor property information

src/lookup.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/lookup.h"
 
 #include "src/bootstrapper.h"
 #include "src/deoptimizer.h"
 #include "src/elements.h"
 #include "src/field-type.h"
@@ skipping 350 matching lines @@
   // Can only be called when the receiver is a JSObject. JSProxy has to be
   // handled via a trap. Adding properties to primitive values is not
   // observable.
   Handle<JSObject> receiver = GetStoreTarget();
 
   if (!IsElement() && !receiver->map()->is_dictionary_map()) {
     holder_ = receiver;
     Handle<Map> old_map(receiver->map(), isolate_);
     Handle<Map> new_map = Map::TransitionToAccessorProperty(
         old_map, name_, component, accessor, attributes);
+    bool simple_transition = new_map->GetBackPointer() == receiver->map();
     JSObject::MigrateToMap(receiver, new_map);
 
+    if (simple_transition) {
+      int number = new_map->LastAdded();

[jochen (gone - plz use gerrit), 2016/03/04 09:58:03]

why's LastAdded not return uint32_t?

+      number_ = static_cast<uint32_t>(number);
+      property_details_ = new_map->GetLastDescriptorDetails();
+      state_ = ACCESSOR;
+      return;
+    }
+
     ReloadPropertyInformation();
-
     if (!new_map->is_dictionary_map()) return;
   }
 
   Handle<AccessorPair> pair;
   if (state() == ACCESSOR && GetAccessors()->IsAccessorPair()) {
     pair = Handle<AccessorPair>::cast(GetAccessors());
     // If the component and attributes are identical, nothing has to be done.
     if (pair->get(component) == *accessor) {
       if (property_details().attributes() == attributes) return;
     } else {
@@ skipping 335 matching lines @@
     // Fall through.
     default:
       return NOT_FOUND;
   }
   UNREACHABLE();
   return state_;
 }
 
 }  // namespace internal
 }  // namespace v8