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

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 91bd0092244536b8293999e80640e56f9235a762..057331c27c7a4f56a7aa65393c376073e94f388c 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1930,14 +1930,12 @@ void PropertyCell::set_type_raw(Object* val, WriteBarrierMode ignored) {
 }
 
 
-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() { WRITE_FIELD(this, kValueOffset, Smi::FromInt(0)); }

[Erik Corry Chromium.org, 2014/10/20 08:00:18]

If this should only be called by the GC, can we assert that we are in a GC here?

[ulan, 2014/10/20 12:30:19]

Done.

 
 
 void WeakCell::initialize(HeapObject* val) {
@@ -1946,6 +1944,9 @@ void WeakCell::initialize(HeapObject* val) {
 }
 
 
+bool WeakCell::cleared() const { return value() == Smi::FromInt(0); }
+
+
 Object* WeakCell::next() const { return READ_FIELD(this, kNextOffset); }