Also allow the empty object map to keep transitions.

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 1583 matching lines @@
 
   FixedArray* values = NULL;
 
   if (map()->unused_property_fields() == 0) {
     // Make room for the new value
     MaybeObject* maybe_values =
         properties()->CopySize(properties()->length() + kFieldsAdded);
     if (!maybe_values->To(&values)) return maybe_values;
   }
 
-  // Only allow map transition if the object isn't the global object.
-  TransitionFlag flag = isolate->empty_object_map() != map()
-      ? INSERT_TRANSITION
-      : OMIT_TRANSITION;
+  TransitionFlag flag = INSERT_TRANSITION;
 
   Map* new_map;
   MaybeObject* maybe_new_map = map()->CopyAddDescriptor(&new_field, flag);
   if (!maybe_new_map->To(&new_map)) return maybe_new_map;
 
   if (map()->unused_property_fields() == 0) {
     ASSERT(values != NULL);
     set_properties(values);
     new_map->set_unused_property_fields(kFieldsAdded - 1);
   } else {
     new_map->set_unused_property_fields(map()->unused_property_fields() - 1);
   }
 
   set_map(new_map);
   return FastPropertyAtPut(index, value);
 }
 
 
 MaybeObject* JSObject::AddConstantFunctionProperty(
     String* name,
     JSFunction* function,
     PropertyAttributes attributes) {
   // Allocate new instance descriptors with (name, function) added
   ConstantFunctionDescriptor d(name, function, attributes, 0);
 
-  Heap* heap = GetHeap();
   TransitionFlag flag =
-      // Do not add transitions to the empty object map (map of "new Object()"),
-      // nor to global objects.
-      (map() == heap->isolate()->empty_object_map() || IsGlobalObject() ||
+      // Do not add transitions to  global objects.
+      (IsGlobalObject() ||
       // Don't add transitions to special properties with non-trivial
       // attributes.
-      // TODO(verwaest): Once we support attribute changes, these transitions
-      // should be kept as well.
        attributes != NONE)
       ? OMIT_TRANSITION
       : INSERT_TRANSITION;
 
   Map* new_map;
   MaybeObject* maybe_new_map = map()->CopyAddDescriptor(&d, flag);
   if (!maybe_new_map->To(&new_map)) return maybe_new_map;
 
   set_map(new_map);
   return function;
@@ skipping 183 matching lines @@
   Map* old_target = old_map->GetTransition(transition_index);
   Object* result;
 
   MaybeObject* maybe_result =
       ConvertDescriptorToField(name, new_value, attributes);
   if (!maybe_result->To(&result)) return maybe_result;
 
   if (!HasFastProperties()) return result;
 
   // This method should only be used to convert existing transitions. Objects
   // with the map of "new Object()" cannot have transitions in the first place.

[Michael Starzinger, 2013/01/30 10:37:16]

This comment is also outdated. Drop the second sentence.

   Map* new_map = map();
-  ASSERT(new_map != GetIsolate()->empty_object_map());
 
   // TODO(verwaest): From here on we lose existing map transitions, causing
   // invalid back pointers. This will change once we can store multiple
   // transitions with the same key.
 
   bool owned_descriptors = old_map->owns_descriptors();
   if (owned_descriptors ||
       old_target->instance_descriptors() == old_map->instance_descriptors()) {
     // Since the conversion above generated a new fast map with an additional
     // property which can be shared as well, install this descriptor pointer
@@ skipping 550 matching lines @@
 
 MaybeObject* JSObject::GetElementsTransitionMapSlow(ElementsKind to_kind) {
   Map* start_map = map();
   ElementsKind from_kind = start_map->elements_kind();
 
   if (from_kind == to_kind) {
     return start_map;
   }
 
   bool allow_store_transition =
-      // Only remember the map transition if the object's map is NOT equal to
-      // the global object_function's map and there is not an already existing
+      // Only remember the map transition if there is not an already existing
       // non-matching element transition.
-      (GetIsolate()->empty_object_map() != map()) &&
       !start_map->IsUndefined() && !start_map->is_shared() &&
       IsFastElementsKind(from_kind);
 
   // Only store fast element maps in ascending generality.
   if (IsFastElementsKind(to_kind)) {
     allow_store_transition &=
         IsTransitionableFastElementsKind(from_kind) &&
         IsMoreGeneralElementsKindTransition(from_kind, to_kind);
   }
 
@@ skipping 11423 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