Use map transitions when defining accessor properties.

src/objects.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 686 matching lines @@
   inline bool name();                  \
   inline void set_##name(bool value);  \
 
 
 #define DECL_ACCESSORS(name, type)                                      \
   inline type* name();                                                  \
   inline void set_##name(type* value,                                   \
                          WriteBarrierMode mode = UPDATE_WRITE_BARRIER); \
 
 
+class AccessorPair;
 class DictionaryElementsAccessor;
 class ElementsAccessor;
+class Failure;
 class FixedArrayBase;
 class ObjectVisitor;
 class StringStream;
-class Failure;
 
 struct ValueInfo : public Malloced {
   ValueInfo() : type(FIRST_TYPE), ptr(NULL), str(NULL), number(0) { }
   InstanceType type;
   Object* ptr;
   const char* str;
   double number;
 };
 
 
@@ skipping 912 matching lines @@
 
   static void DefineAccessor(Handle<JSObject> object,
                              Handle<String> name,
                              Handle<Object> getter,
                              Handle<Object> setter,
                              PropertyAttributes attributes);
   MUST_USE_RESULT MaybeObject* DefineAccessor(String* name,
                                               Object* getter,
                                               Object* setter,
                                               PropertyAttributes attributes);
+  // Try to define a single accessor paying attention to map transitions.
+  // Returns a JavaScript null if this was not possible and we have to use the
+  // slow case. Note that we can fail due to allocations, too.
+  MUST_USE_RESULT MaybeObject* DefineFastAccessor(
+      String* name,
+      AccessorComponent component,
+      Object* accessor,
+      PropertyAttributes attributes);
   Object* LookupAccessor(String* name, AccessorComponent component);
 
   MUST_USE_RESULT MaybeObject* DefineAccessor(AccessorInfo* info);
 
   // Used from Object::GetProperty().
   MUST_USE_RESULT MaybeObject* GetPropertyWithFailedAccessCheck(
       Object* receiver,
       LookupResult* result,
       String* name,
       PropertyAttributes* attributes);
@@ skipping 6439 matching lines @@
 //   * a pointer to a map: a transition used to ensure map sharing
 class AccessorPair: public Struct {
  public:
   DECL_ACCESSORS(getter, Object)
   DECL_ACCESSORS(setter, Object)
 
   static inline AccessorPair* cast(Object* obj);
 
   MUST_USE_RESULT MaybeObject* CopyWithoutTransitions();
 
+  Object* get(AccessorComponent component) {
+    return component == ACCESSOR_GETTER ? getter() : setter();
+  }
+
+  void set(AccessorComponent component, Object* value) {
+    if (component == ACCESSOR_GETTER) {
+      set_getter(value);
+    } else {
+      set_setter(value);
+    }
+  }
+
   // Note: Returns undefined instead in case of a hole.
   Object* GetComponent(AccessorComponent component);
 
   // Set both components, skipping arguments which are a JavaScript null.
   void SetComponents(Object* getter, Object* setter) {
     if (!getter->IsNull()) set_getter(getter);
     if (!setter->IsNull()) set_setter(setter);
   }
 
   bool ContainsAccessor() {
@@ skipping 515 matching lines @@
     } else {
       value &= ~(1 << bit_position);
     }
     return value;
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_H_