Keep track of which maps are associated with prototype objects so we can tune the fast-case vs. has…

src/objects.cc

Index: src/objects.cc
===================================================================
--- src/objects.cc	(revision 11720)
+++ src/objects.cc	(working copy)
@@ -1580,7 +1580,7 @@
   }
 
   if (map()->unused_property_fields() == 0) {
-    if (properties()->length() > MaxFastProperties()) {
+    if (TooManyFastProperties(properties()->length())) {
       Object* obj;
       { MaybeObject* maybe_obj =
             NormalizeProperties(CLEAR_INOBJECT_PROPERTIES, 0);
@@ -1830,7 +1830,7 @@
                                                 Object* new_value,
                                                 PropertyAttributes attributes) {
   if (map()->unused_property_fields() == 0 &&
-      properties()->length() > MaxFastProperties()) {
+      TooManyFastProperties(properties()->length())) {
     Object* obj;
     { MaybeObject* maybe_obj =
           NormalizeProperties(CLEAR_INOBJECT_PROPERTIES, 0);
@@ -5042,6 +5042,7 @@
   return new_map;
 }
 
+
 void Map::UpdateCodeCache(Handle<Map> map,
                           Handle<String> name,
                           Handle<Code> code) {
@@ -7612,9 +7613,57 @@
 }
 
 
+MaybeObject* JSObject::OptimizeAsPrototype() {
+  if (IsGlobalObject()) return this;
+
+  // Make sure prototypes are fast objects and their maps have the bit set
+  // so they remain fast.
+  Map* proto_map = map();
+  if (!proto_map->used_for_prototype()) {
+    if (!HasFastProperties()) {
+      MaybeObject* new_proto = TransformToFastProperties(0);
+      if (new_proto->IsFailure()) return new_proto;
+      ASSERT(new_proto == this);
+      proto_map = map();
+      if (!proto_map->is_shared()) {
+        proto_map->set_used_for_prototype(true);
+      }
+    } else {
+      Heap* heap = GetHeap();
+      // We use the hole value as a singleton key in the prototype transition
+      // map so that we don't multiply the number of maps unnecessarily.
+      Map* new_map =
+          proto_map->GetPrototypeTransition(heap->the_hole_value());
+      if (new_map == NULL) {
+        MaybeObject* maybe_new_map = proto_map->CopyDropTransitions();
+        if (!maybe_new_map->To<Map>(&new_map)) return maybe_new_map;
+        new_map->set_used_for_prototype(true);
+        MaybeObject* ok =
+            proto_map->PutPrototypeTransition(heap->the_hole_value(),
+                                              new_map);
+        if (ok->IsFailure()) return ok;
+      }
+      ASSERT(!proto_map->is_shared() && !new_map->is_shared());
+      set_map(new_map);
+    }
+  }
+  return this;
+}
+
+
 MaybeObject* JSFunction::SetInstancePrototype(Object* value) {
   ASSERT(value->IsJSReceiver());
   Heap* heap = GetHeap();
+
+  // First some logic for the map of the prototype to make sure the
+  // used_for_prototype flag is set.
+  if (value->IsJSObject()) {
+    MaybeObject* ok = JSObject::cast(value)->OptimizeAsPrototype();
+    if (ok->IsFailure()) return ok;
+  }
+
+  // Now some logic for the maps of the objects that are created by using this
+  // function as a constructor.
   if (has_initial_map()) {
     // If the function has allocated the initial map
     // replace it with a copy containing the new prototype.
@@ -8786,7 +8835,7 @@
 }
 
 
-Object* Map::GetPrototypeTransition(Object* prototype) {
+Map* Map::GetPrototypeTransition(Object* prototype) {
   FixedArray* cache = prototype_transitions();
   int number_of_transitions = NumberOfProtoTransitions();
   const int proto_offset =
@@ -8796,8 +8845,7 @@
   for (int i = 0; i < number_of_transitions; i++) {
     if (cache->get(proto_offset + i * step) == prototype) {
       Object* map = cache->get(map_offset + i * step);
-      ASSERT(map->IsMap());
-      return map;
+      return Map::cast(map);
     }
   }
   return NULL;
@@ -8905,21 +8953,26 @@
   // Nothing to do if prototype is already set.
   if (map->prototype() == value) return value;
 
-  Object* new_map = map->GetPrototypeTransition(value);
+  if (value->IsJSObject()) {
+    MaybeObject* ok = JSObject::cast(value)->OptimizeAsPrototype();
+    if (ok->IsFailure()) return ok;
+  }
+
+  Map* new_map = map->GetPrototypeTransition(value);
   if (new_map == NULL) {
     { MaybeObject* maybe_new_map = map->CopyDropTransitions();
-      if (!maybe_new_map->ToObject(&new_map)) return maybe_new_map;
+      if (!maybe_new_map->To<Map>(&new_map)) return maybe_new_map;
     }
 
     { MaybeObject* maybe_new_cache =
-          map->PutPrototypeTransition(value, Map::cast(new_map));
+          map->PutPrototypeTransition(value, new_map);
       if (maybe_new_cache->IsFailure()) return maybe_new_cache;
     }
 
-    Map::cast(new_map)->set_prototype(value);
+    new_map->set_prototype(value);
   }
-  ASSERT(Map::cast(new_map)->prototype() == value);
-  real_receiver->set_map(Map::cast(new_map));
+  ASSERT(new_map->prototype() == value);
+  real_receiver->set_map(new_map);
 
   heap->ClearInstanceofCache();
   ASSERT(size == Size());