Reduce space usage of simple transitions and descriptors holders.

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 73 matching lines @@
   inline FixedArray* GetPrototypeTransitions();
   inline void SetPrototypeTransitions(
       FixedArray* prototype_transitions,
       WriteBarrierMode mode = UPDATE_WRITE_BARRIER);
   inline Object** GetPrototypeTransitionsSlot();
   inline bool HasPrototypeTransitions();
   inline HeapObject* UncheckedPrototypeTransitions();
 
   // Returns the number of transitions in the array.
   int number_of_transitions() {
-    ASSERT(length() >= kFirstIndex);
+    if (IsSimpleTransition()) return 1;
     int len = length();
     return len <= kFirstIndex ? 0 : (len - kFirstIndex) / kTransitionSize;
   }
 
   inline int number_of_entries() { return number_of_transitions(); }
 
   // Allocate a new transition array with a single entry.
   static MUST_USE_RESULT MaybeObject* NewWith(
-      String* name,
+      SimpleTransitionFlag flag,
+      String* key,
       Map* target,
       JSGlobalPropertyCell* descriptor_pointer,
       Object* back_pointer);
 
+  static MUST_USE_RESULT MaybeObject* AllocateDescriptorsHolder(
+      JSGlobalPropertyCell* descriptor_pointer);
+
+  MUST_USE_RESULT MaybeObject* ExtendToFullTransitionArray();
+
   // Copy the transition array, inserting a new transition.
   // TODO(verwaest): This should not cause an existing transition to be
   // overwritten.
   MUST_USE_RESULT MaybeObject* CopyInsert(String* name, Map* target);
 
   // Copy a single transition from the origin array.
   inline void NoIncrementalWriteBarrierCopyFrom(TransitionArray* origin,
                                                 int origin_transition,
                                                 int target_transition);
 
   // Search a transition for a given property name.
   inline int Search(String* name);
 
   // Allocates a TransitionArray.
   MUST_USE_RESULT static MaybeObject* Allocate(
       int number_of_transitions,
       JSGlobalPropertyCell* descriptors_cell);
 
+  bool IsDescriptorsHolder() { return length() == kDescriptorsHolderSize; }
+  bool IsSimpleTransition() { return length() == kSimpleTransitionSize; }
+  bool IsFullTransitionArray() { return length() >= kFirstIndex; }
+
   // Casting.
   static inline TransitionArray* cast(Object* obj);
 
   // Constant for denoting key was not found.
   static const int kNotFound = -1;
 
   static const int kDescriptorsPointerIndex = 0;
   static const int kBackPointerStorageIndex = 1;
+  static const int kDescriptorsHolderSize = 2;
+
+  // Layout for full transition arrays.
   static const int kElementsTransitionIndex = 2;
   static const int kPrototypeTransitionsIndex = 3;
   static const int kFirstIndex = 4;
 
-  // Layout transition array header.
+  // Layout for simple transition arrays.
+  static const int kSimpleTransitionTarget = 2;
+  static const int kSimpleTransitionSize = 3;
+  static const int kSimpleTransitionIndex = 0;
+  STATIC_ASSERT(kSimpleTransitionIndex != kNotFound);
+
   static const int kDescriptorsPointerOffset = FixedArray::kHeaderSize;
   static const int kBackPointerStorageOffset = kDescriptorsPointerOffset +
                                                kPointerSize;
+
+  // Layout for the full transition array header.
   static const int kElementsTransitionOffset = kBackPointerStorageOffset +
                                                kPointerSize;
   static const int kPrototypeTransitionsOffset = kElementsTransitionOffset +
                                                  kPointerSize;
 
-  // Layout of map transition.
+  // 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() {
     PrintTransitions(stdout);
   }
   void PrintTransitions(FILE* out);
@@ skipping 27 matching lines @@
                                            String* key,
                                            Map* target);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(TransitionArray);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TRANSITIONS_H_