Remove elements transitions from the transition array.

src/transitions.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 23 matching lines @@
 #include "objects.h"
 #include "v8checks.h"
 
 namespace v8 {
 namespace internal {
 
 
 // TransitionArrays are fixed arrays used to hold map transitions for property,
 // constant, and element changes. They can either be simple transition arrays
 // that store a single property transition, or a full transition array that has
-// space for elements transitions, prototype transitions and multiple property
-// transitons. The details related to property transitions are accessed in the
-// descriptor array of the target map. In the case of a simple transition, the
-// key is also read from the descriptor array of the target map.
+// prototype transitions and multiple property transitons. The details related
+// to property transitions are accessed in the descriptor array of the target
+// map. In the case of a simple transition, the key is also read from the
+// descriptor array of the target map.
 //
 // The simple format of the these objects is:
 // [0] Undefined or back pointer map
 // [1] Single transition
 //
 // The full format is:
 // [0] Undefined or back pointer map
-// [1] Smi(0) or elements transition map
-// [2] Smi(0) or fixed array of prototype transitions
-// [3] First transition
+// [1] Smi(0) or fixed array of prototype transitions
+// [2] First transition
 // [length() - kTransitionSize] Last transition
 class TransitionArray: public FixedArray {
  public:
   // Accessors for fetching instance transition at transition number.
   inline Name* GetKey(int transition_number);
   inline void SetKey(int transition_number, Name* value);
   inline Object** GetKeySlot(int transition_number);
   int GetSortedKeyIndex(int transition_number) { return transition_number; }
 
   Name* GetSortedKey(int transition_number) {
     return GetKey(transition_number);
   }
 
   inline Map* GetTarget(int transition_number);
   inline void SetTarget(int transition_number, Map* target);
 
   inline PropertyDetails GetTargetDetails(int transition_number);
 
-  inline Map* elements_transition();
-  inline void set_elements_transition(
-      Map* target,
-      WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
   inline bool HasElementsTransition();
-  inline void ClearElementsTransition();
 
   inline Object* back_pointer_storage();
   inline void set_back_pointer_storage(
       Object* back_pointer,
       WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
 
   inline FixedArray* GetPrototypeTransitions();
   inline void SetPrototypeTransitions(
       FixedArray* prototype_transitions,
       WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
@@ skipping 28 matching lines @@
   inline void NoIncrementalWriteBarrierCopyFrom(TransitionArray* origin,
                                                 int origin_transition,
                                                 int target_transition);
 
   // Search a transition for a given property name.
   inline int Search(Name* name);
 
   // Allocates a TransitionArray.
   MUST_USE_RESULT static MaybeObject* Allocate(int number_of_transitions);
 
-  bool IsSimpleTransition() { return length() == kSimpleTransitionSize; }
-  bool IsFullTransitionArray() { return length() >= kFirstIndex; }
+  bool IsSimpleTransition() {
+    return length() == kSimpleTransitionSize &&
+        get(kSimpleTransitionTarget)->IsHeapObject() &&
+        // The IntrusivePrototypeTransitionIterator may have set the map of the
+        // prototype transitions array to a smi. In that case, there are
+        // prototype transitions, hence this transition array is a full
+        // transition array.
+        (*HeapObject::RawField(HeapObject::cast(get(kSimpleTransitionTarget)),

[titzer, 2013/07/31 13:55:46]

Any reason to use a RawField as opposed to the map() accessor through the
HeapObject?

[Toon Verwaest, 2013/07/31 15:05:23]

Done.

+                               HeapObject::kMapOffset))->IsMap() &&
+        get(kSimpleTransitionTarget)->IsMap();
+  }
+
+  bool IsFullTransitionArray() {
+    return length() > kFirstIndex ||
+        (length() == kFirstIndex && !IsSimpleTransition());
+  }
 
   // Casting.
   static inline TransitionArray* cast(Object* obj);
 
   // Constant for denoting key was not found.
   static const int kNotFound = -1;
 
   static const int kBackPointerStorageIndex = 0;
 
   // Layout for full transition arrays.
-  static const int kElementsTransitionIndex = 1;
-  static const int kPrototypeTransitionsIndex = 2;
-  static const int kFirstIndex = 3;
+  static const int kPrototypeTransitionsIndex = 1;
+  static const int kFirstIndex = 2;
 
   // Layout for simple transition arrays.
   static const int kSimpleTransitionTarget = 1;
   static const int kSimpleTransitionSize = 2;
   static const int kSimpleTransitionIndex = 0;
   STATIC_ASSERT(kSimpleTransitionIndex != kNotFound);
 
   static const int kBackPointerStorageOffset = FixedArray::kHeaderSize;
 
   // Layout for the full transition array header.
-  static const int kElementsTransitionOffset = kBackPointerStorageOffset +
-                                               kPointerSize;
-  static const int kPrototypeTransitionsOffset = kElementsTransitionOffset +
+  static const int kPrototypeTransitionsOffset = kBackPointerStorageOffset +
                                                  kPointerSize;
 
   // Layout of map transition entries in full transition arrays.
   static const int kTransitionKey = 0;
   static const int kTransitionTarget = 1;
   static const int kTransitionSize = 2;
 
 #ifdef OBJECT_PRINT
   // Print all the transitions.
   inline void PrintTransitions() {
@@ skipping 30 matching lines @@
                                            Name* key,
                                            Map* target);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(TransitionArray);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TRANSITIONS_H_