Add infrastructure to keep track of references to prototypes.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index ea84ebf335f99af66765fdd5a25b3a4738305bfd..b35c156451282c69b029b048b79e9948ea3ea1be 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -88,6 +88,7 @@
 //         - ScopeInfo
 //         - TransitionArray
 //         - ScriptContextTable
+//         - WeakFixedArray
 //       - FixedDoubleArray
 //       - ExternalArray
 //         - ExternalUint8ClampedArray
@@ -942,6 +943,7 @@ template <class C> inline bool Is(Object* obj);
   V(DependentCode)                 \
   V(FixedArray)                    \
   V(FixedDoubleArray)              \
+  V(WeakFixedArray)                \
   V(ConstantPoolArray)             \
   V(Context)                       \
   V(ScriptContextTable)            \
@@ -1815,6 +1817,10 @@ class JSObject: public JSReceiver {
   static void OptimizeAsPrototype(Handle<JSObject> object,
                                   PrototypeOptimizationMode mode);
   static void ReoptimizeIfPrototype(Handle<JSObject> object);
+  static void RegisterPrototypeUser(Handle<JSObject> prototype,
+                                    Handle<HeapObject> user);
+  static void UnregisterPrototypeUser(Handle<JSObject> prototype,
+                                      Handle<HeapObject> user);
 
   // Retrieve interceptors.
   InterceptorInfo* GetNamedInterceptor();
@@ -2588,6 +2594,46 @@ class FixedDoubleArray: public FixedArrayBase {
 };
 
 
+class WeakFixedArray : public FixedArray {
+ public:
+  inline Object* get(int index) const;
+
+  static bool StoreAnywhere(Handle<WeakFixedArray> array,

[Toon Verwaest, 2014/12/01 15:23:54]

What about combining StoreAnywhere with GrowAndStore into
Handle<WFA> Add(Handle<WFA>, ...)?

[Jakob Kummerow, 2014/12/02 12:32:15]

Done.

+                            Handle<HeapObject> value,
+                            bool may_be_duplicate = false);
+
+  static Handle<WeakFixedArray> GrowAndStore(Handle<WeakFixedArray> original,
+                                             Handle<HeapObject> value);
+  void Delete(Handle<HeapObject> value);

[Toon Verwaest, 2014/12/01 15:23:54]

And then rename to Remove to match Add.

[Jakob Kummerow, 2014/12/02 12:32:15]

Done.

+
+  DECLARE_CAST(WeakFixedArray)
+
+ private:
+  static const int kLastUsedIndexIndex = 0;
+  static const int kReservedFieldCount = 1;
+
+  static void Set(Handle<WeakFixedArray> array, int index,
+                  Handle<HeapObject> value);
+  inline void clear(int index);
+  inline int length() const;
+  inline bool IsEmptySlot(int index) const;
+
+  inline int last_used_index() const;
+  inline void set_last_used_index(int index);
+
+  static void CopyFrom(Handle<WeakFixedArray> target,
+                       Handle<WeakFixedArray> source);
+
+  friend class Factory;  // For kReservedFieldCount.

[Toon Verwaest, 2014/12/01 15:23:54]

Just move the method onto WeakFixedArray rather than having this indirection
over the factory. Just like DescriptorArray::Allocate. In that case, you don't
need to friend anything.

[Jakob Kummerow, 2014/12/02 12:32:15]

Done. I've merged CopyFrom into it, to avoid redundant initialization.

+
+  // Disallow inherited setters.
+  void set(int index, Smi* value);
+  void set(int index, Object* value);
+  void set(int index, Object* value, WriteBarrierMode mode);
+  DISALLOW_IMPLICIT_CONSTRUCTORS(WeakFixedArray);
+};
+
+
 // ConstantPoolArray describes a fixed-sized array containing constant pool
 // entries.
 //
@@ -5883,6 +5929,11 @@ class Map: public HeapObject {
 
   // [prototype]: implicit prototype object.
   DECL_ACCESSORS(prototype, Object)
+  // TODO(jkummerow): make set_prototype private.
+  void SetPrototype(Handle<Object> prototype,
+                    PrototypeOptimizationMode proto_mode = FAST_PROTOTYPE);
+  bool ShouldRegisterAsPrototypeUser(Handle<JSObject> prototype);
+  bool CanUseOptimizationsBasedOnPrototypeRegistry();
 
   // [constructor]: points back to the function responsible for this map.
   DECL_ACCESSORS(constructor, Object)
@@ -6212,10 +6263,11 @@ class Map: public HeapObject {
   // the original map.  That way we can transition to the same map if the same
   // prototype is set, rather than creating a new map every time.  The
   // transitions are in the form of a map where the keys are prototype objects
-  // and the values are the maps the are transitioned to.
+  // and the values are the maps they transition to.
   static const int kMaxCachedPrototypeTransitions = 256;
   static Handle<Map> TransitionToPrototype(Handle<Map> map,
-                                           Handle<Object> prototype);
+                                           Handle<Object> prototype,
+                                           PrototypeOptimizationMode mode);
 
   static const int kMaxPreAllocatedPropertyFields = 255;