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

src/heap/objects-visiting-inl.h

Index: src/heap/objects-visiting-inl.h
diff --git a/src/heap/objects-visiting-inl.h b/src/heap/objects-visiting-inl.h
index dcf8a9b02607508d75169d4740ab2321d27a309a..09b38237711886ea0773e2e0a995b6d9a79653d9 100644
--- a/src/heap/objects-visiting-inl.h
+++ b/src/heap/objects-visiting-inl.h
@@ -358,9 +358,9 @@ void StaticMarkingVisitor<StaticVisitor>::VisitWeakCell(Map* map,
   WeakCell* weak_cell = reinterpret_cast<WeakCell*>(object);
   Object* undefined = heap->undefined_value();
   // Enqueue weak cell in linked list of encountered weak collections.
-  // We can ignore weak cells with cleared values because they will always point
-  // to the undefined_value.
-  if (weak_cell->next() == undefined && weak_cell->value() != undefined) {
+  // We can ignore weak cells with cleared values because they will always
+  // contain smi zero.
+  if (weak_cell->next() == undefined && !weak_cell->cleared()) {

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

Perhaps this should just check for Sminess instead of clearedness, since there's
no point in queueing up a cell with a Smi in it.

[ulan, 2014/10/20 12:30:18]

Since the initialization function of the weak cell accepts only heap objects,
weak_cell->value()->IsSmi() is equivalent to weak_cell->value ==
Smi::FromInt(0).

     weak_cell->set_next(heap->encountered_weak_cells());
     heap->set_encountered_weak_cells(weak_cell);
   }