Use smi zero instead of undefine_value to zap dead weak cells.

src/objects-inl.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Review notes:
 //
 // - The use of macros in these inline functions may seem superfluous
 // but it is absolutely needed to make sure gcc generates optimal
 // code. gcc is not happy when attempting to inline too deep.
 //
@@ skipping 1912 matching lines @@
 Object* PropertyCell::type_raw() const {
   return READ_FIELD(this, kTypeOffset);
 }
 
 
 void PropertyCell::set_type_raw(Object* val, WriteBarrierMode ignored) {
   WRITE_FIELD(this, kTypeOffset, val);
 }
 
 
-HeapObject* WeakCell::value() const {
+Object* WeakCell::value() const {
   return HeapObject::cast(READ_FIELD(this, kValueOffset));
 }
 
 
-void WeakCell::clear(HeapObject* undefined) {
-  WRITE_FIELD(this, kValueOffset, undefined);
+void WeakCell::clear() {
+  DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT);
+  WRITE_FIELD(this, kValueOffset, Smi::FromInt(0));
 }
 
 
 void WeakCell::initialize(HeapObject* val) {
   WRITE_FIELD(this, kValueOffset, val);
   WRITE_BARRIER(GetHeap(), this, kValueOffset, val);
 }
 
 
+bool WeakCell::cleared() const { return value() == Smi::FromInt(0); }
+
+
 Object* WeakCell::next() const { return READ_FIELD(this, kNextOffset); }
 
 
 void WeakCell::set_next(Object* val, WriteBarrierMode mode) {
   WRITE_FIELD(this, kNextOffset, val);
   if (mode == UPDATE_WRITE_BARRIER) {
     WRITE_BARRIER(GetHeap(), this, kNextOffset, val);
   }
 }
 
@@ skipping 5326 matching lines @@
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 #undef NOBARRIER_READ_BYTE_FIELD
 #undef NOBARRIER_WRITE_BYTE_FIELD
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_