Moving prototype transitions into the header of the transition array.

src/objects.cc

 // 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 5117 matching lines @@
 
 // An iterator over all prototype transitions, reusing the map field of the
 // underlying array while it is running.
 class IntrusivePrototypeTransitionIterator {
  public:
   explicit IntrusivePrototypeTransitionIterator(HeapObject* proto_trans)
       : proto_trans_(proto_trans) { }
 
   void Start() {
     ASSERT(!IsIterating());
-    if (HasTransitions()) *Header() = Smi::FromInt(0);
+    *Header() = Smi::FromInt(0);

[Michael Starzinger, 2012/07/06 13:53:18]

Oh yeah, I like that change!

   }
 
   bool IsIterating() {
-    return HasTransitions() && (*Header())->IsSmi();
+    return (*Header())->IsSmi();
   }
 
   Map* Next() {
     ASSERT(IsIterating());
     int transitionNumber = Smi::cast(*Header())->value();
     if (transitionNumber < NumberOfTransitions()) {
       *Header() = Smi::FromInt(transitionNumber + 1);
       return GetTransition(transitionNumber);
     }
     *Header() = proto_trans_->GetHeap()->fixed_array_map();
     return NULL;
   }
 
  private:
-  bool HasTransitions() {
-    return proto_trans_->map()->IsSmi() || proto_trans_->IsFixedArray();
-  }
-
   Object** Header() {
     return HeapObject::RawField(proto_trans_, FixedArray::kMapOffset);
   }
 
   int NumberOfTransitions() {
-    ASSERT(HasTransitions());
     FixedArray* proto_trans = reinterpret_cast<FixedArray*>(proto_trans_);
     Object* num = proto_trans->get(Map::kProtoTransitionNumberOfEntriesOffset);
     return Smi::cast(num)->value();
   }
 
   Map* GetTransition(int transitionNumber) {
-    ASSERT(HasTransitions());
     FixedArray* proto_trans = reinterpret_cast<FixedArray*>(proto_trans_);
     return Map::cast(proto_trans->get(IndexFor(transitionNumber)));
   }
 
   int IndexFor(int transitionNumber) {
     return Map::kProtoTransitionHeaderSize +
         Map::kProtoTransitionMapOffset +
         transitionNumber * Map::kProtoTransitionElementsPerEntry;
   }
 
@@ skipping 29 matching lines @@
   // this map's map!
   void SetParent(TraversableMap* parent) { set_map_no_write_barrier(parent); }
 
   // Reset the current map's map, returning the parent previously stored in it.
   TraversableMap* GetAndResetParent() {
     TraversableMap* old_parent = static_cast<TraversableMap*>(map());
     set_map_no_write_barrier(GetHeap()->meta_map());
     return old_parent;
   }
 
-  // Can either be Smi (no instance descriptors), or a descriptor array with the
-  // header overwritten as a Smi (thus iterating).
-  TransitionArray* MutatedTransitions() {
-    Object* object = *HeapObject::RawField(instance_descriptors(),
-                                           DescriptorArray::kTransitionsOffset);
-    TransitionArray* transition_array = static_cast<TransitionArray*>(object);
-    return transition_array;
-  }
-
   // Start iterating over this map's children, possibly destroying a FixedArray
   // map (see explanation above).
   void ChildIteratorStart() {
     if (HasTransitionArray()) {
+      if (HasPrototypeTransitions()) {
+        IntrusivePrototypeTransitionIterator(prototype_transitions()).Start();
+      }
+
       IntrusiveMapTransitionIterator(transitions()).Start();
     }
-    IntrusivePrototypeTransitionIterator(
-        unchecked_prototype_transitions()).Start();
   }
 
   // If we have an unvisited child map, return that one and advance. If we have
   // none, return NULL and reset any destroyed FixedArray maps.
   TraversableMap* ChildIteratorNext() {
-    IntrusivePrototypeTransitionIterator
-        proto_iterator(unchecked_prototype_transitions());
-    if (proto_iterator.IsIterating()) {
-      Map* next = proto_iterator.Next();
-      if (next != NULL) return static_cast<TraversableMap*>(next);
-    }
     if (HasTransitionArray()) {
-      IntrusiveMapTransitionIterator
-          transitions_iterator(MutatedTransitions());
-      if (transitions_iterator.IsIterating()) {
-        Map* next = transitions_iterator.Next();
+      TransitionArray* transition_array = unchecked_transition_array();
+
+      if (transition_array->HasPrototypeTransitions()) {
+        HeapObject* proto_transitions =
+            transition_array->unchecked_prototype_transitions();
+        IntrusivePrototypeTransitionIterator proto_iterator(proto_transitions);
+        if (proto_iterator.IsIterating()) {
+          Map* next = proto_iterator.Next();
+          if (next != NULL) return static_cast<TraversableMap*>(next);
+        }
+      }
+
+      IntrusiveMapTransitionIterator transition_iterator(transition_array);
+      if (transition_iterator.IsIterating()) {
+        Map* next = transition_iterator.Next();
         if (next != NULL) return static_cast<TraversableMap*>(next);
       }
     }
+
     return NULL;
   }
 };
 
 
 // Traverse the transition tree in postorder without using the C++ stack by
 // doing pointer reversal.
 void Map::TraverseTransitionTree(TraverseCallback callback, void* data) {
   TraversableMap* current = static_cast<TraversableMap*>(this);
   current->ChildIteratorStart();
@@ skipping 2128 matching lines @@
     t->ClearElementsTransition();
   } else {
     // If there are no transitions to be cleared, return.
     // TODO(verwaest) Should be an assert, otherwise back pointers are not
     // properly cleared.
     if (transition_index == t->number_of_transitions()) return;
   }
 
   // If the final transition array does not contain any live transitions, remove
   // the transition array from the map.
-  if (transition_index == 0 && !t->HasElementsTransition()) {
+  if (transition_index == 0 &&
+      !t->HasElementsTransition() &&
+      !t->HasPrototypeTransitions()) {
     return ClearTransitions();
   }
 
   int trim = t->number_of_transitions() - transition_index;
   if (trim > 0) {
     RightTrimFixedArray(heap, t, trim * TransitionArray::kTransitionSize);
   }
 }
 
 
@@ skipping 1442 matching lines @@
     // Grow array by factor 2 over and above what we need.
     { MaybeObject* maybe_cache =
           GetHeap()->AllocateFixedArray(transitions * 2 * step + header);
       if (!maybe_cache->To(&new_cache)) return maybe_cache;
     }
 
     for (int i = 0; i < capacity * step; i++) {
       new_cache->set(i + header, cache->get(i + header));
     }
     cache = new_cache;
-    set_prototype_transitions(cache);
+    MaybeObject* set_result = set_prototype_transitions(cache);
+    if (set_result->IsFailure()) return set_result;
   }
 
   int last = transitions - 1;
 
   cache->set(header + last * step + kProtoTransitionPrototypeOffset, prototype);
   cache->set(header + last * step + kProtoTransitionMapOffset, map);
   SetNumberOfProtoTransitions(transitions);
 
   return cache;
 }
@@ skipping 4382 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal