Replace ad-hoc weakness in prototype transitions with WeakCell.

src/transitions.cc

Index: src/transitions.cc
diff --git a/src/transitions.cc b/src/transitions.cc
index 63bb7fa68f515db5bb90e64fc4629e1807c6b898..c39534bbb566a2930934d2a56482517a2d18e1c6 100644
--- a/src/transitions.cc
+++ b/src/transitions.cc
@@ -233,17 +233,19 @@ bool TransitionArray::CanHaveMoreTransitions(Handle<Map> map) {
 
 
 // static
-Handle<Map> TransitionArray::PutPrototypeTransition(Handle<Map> map,
-                                                    Handle<Object> prototype,
-                                                    Handle<Map> target_map) {
+void TransitionArray::PutPrototypeTransition(Handle<Map> map,
+                                             Handle<Object> prototype,
+                                             Handle<Map> target_map) {
   DCHECK(HeapObject::cast(*prototype)->map()->IsMap());
   // Don't cache prototype transition if this map is either shared, or a map of
   // a prototype.
-  if (map->is_prototype_map()) return map;
-  if (map->is_dictionary_map() || !FLAG_cache_prototype_transitions) return map;
+  if (map->is_prototype_map()) return;
+  if (map->is_dictionary_map() || !FLAG_cache_prototype_transitions) return;
 
   const int header = kProtoTransitionHeaderSize;
 
+  Handle<WeakCell> target_cell = Map::WeakCellForMap(target_map);
+
   Handle<FixedArray> cache(GetPrototypeTransitions(*map));
   int capacity = cache->length() - header;
   int transitions = NumberOfPrototypeTransitions(*cache) + 1;
@@ -251,7 +253,7 @@ Handle<Map> TransitionArray::PutPrototypeTransition(Handle<Map> map,
   if (transitions > capacity) {
     // Grow array by factor 2 up to MaxCachedPrototypeTransitions.
     int new_capacity = Min(kMaxCachedPrototypeTransitions, transitions * 2);
-    if (new_capacity == capacity) return map;
+    if (new_capacity == capacity) return;
 
     cache = FixedArray::CopySize(cache, header + new_capacity);
     if (capacity < 0) {
@@ -267,10 +269,8 @@ Handle<Map> TransitionArray::PutPrototypeTransition(Handle<Map> map,
   int last = NumberOfPrototypeTransitions(*cache);
   int entry = header + last;
 
-  cache->set(entry, *target_map);
+  cache->set(entry, *target_cell);
   SetNumberOfPrototypeTransitions(*cache, last + 1);
-
-  return map;
 }
 
 
@@ -281,8 +281,12 @@ Handle<Map> TransitionArray::GetPrototypeTransition(Handle<Map> map,
   FixedArray* cache = GetPrototypeTransitions(*map);
   int number_of_transitions = NumberOfPrototypeTransitions(cache);
   for (int i = 0; i < number_of_transitions; i++) {
-    Map* target = Map::cast(cache->get(kProtoTransitionHeaderSize + i));
-    if (target->prototype() == *prototype) return handle(target);
+    WeakCell* target_cell =
+        WeakCell::cast(cache->get(kProtoTransitionHeaderSize + i));
+    if (!target_cell->cleared() &&
+        Map::cast(target_cell->value())->prototype() == *prototype) {
+      return handle(Map::cast(target_cell->value()));
+    }
   }
   return Handle<Map>();
 }
@@ -436,8 +440,9 @@ void TransitionArray::TraverseTransitionTreeInternal(Map* map,
       FixedArray* proto_trans = transitions->GetPrototypeTransitions();
       for (int i = 0; i < NumberOfPrototypeTransitions(proto_trans); ++i) {
         int index = TransitionArray::kProtoTransitionHeaderSize + i;
-        TraverseTransitionTreeInternal(Map::cast(proto_trans->get(index)),
-                                       callback, data);
+        WeakCell* cell = WeakCell::cast(proto_trans->get(index));
+        TraverseTransitionTreeInternal(Map::cast(cell->value()), callback,
+                                       data);
       }
     }
     for (int i = 0; i < transitions->number_of_transitions(); ++i) {