Fix issue 303 by not shortcutting cons-symbols.

src/heap.cc

 // Copyright 2009 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 795 matching lines @@
     *p = first_word.ToForwardingAddress();
     return;
   }
 
   // Call the slow part of scavenge object.
   return ScavengeObjectSlow(p, object);
 }
 
 
 static inline bool IsShortcutCandidate(HeapObject* object, Map* map) {
-  // A ConsString object with Heap::empty_string() as the right side
-  // is a candidate for being shortcut by the scavenger.
+  // A ConsString with an empty string as the right side is a
+  // candidate for being shortcut by the scavenger unless it is a
+  // symbol. It's not common to have non-flat symbols, so we do not
+  // shortcut them thereby avoiding turning symbols into strings.
+  ASSERT(kNotStringTag != 0 && kSymbolTag != 0);
+  static const uint32_t kShortcutTypeMask =
+      kIsNotStringMask |
+      kIsSymbolMask |
+      kStringRepresentationMask;
   ASSERT(object->map() == map);
-  if (map->instance_type() >= FIRST_NONSTRING_TYPE) return false;
-  return (StringShape(map).representation_tag() == kConsStringTag) &&
-         (ConsString::cast(object)->unchecked_second() == Heap::empty_string());
+  InstanceType type = map->instance_type();
+  if ((type & kShortcutTypeMask) != kConsStringTag) return false;
+  ASSERT(object->IsString() && !object->IsSymbol());
+  return ConsString::cast(object)->unchecked_second() == Heap::empty_string();
 }
 
 
 void Heap::ScavengeObjectSlow(HeapObject** p, HeapObject* object) {
   ASSERT(InFromSpace(object));
   MapWord first_word = object->map_word();
   ASSERT(!first_word.IsForwardingAddress());
 
   // Optimization: Bypass flattened ConsString objects.
   if (IsShortcutCandidate(object, first_word.ToMap())) {
@@ skipping 2523 matching lines @@
 #ifdef DEBUG
 bool Heap::GarbageCollectionGreedyCheck() {
   ASSERT(FLAG_gc_greedy);
   if (Bootstrapper::IsActive()) return true;
   if (disallow_allocation_failure()) return true;
   return CollectGarbage(0, NEW_SPACE);
 }
 #endif
 
 } }  // namespace v8::internal