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 d220118368e77d07b973ec92c701061ddf06b6a5..6f9ee8f55eb1099c106f26efa88d42c2a05aa8c0 100644
--- a/src/heap/objects-visiting-inl.h
+++ b/src/heap/objects-visiting-inl.h
@@ -191,6 +191,8 @@ void StaticMarkingVisitor<StaticVisitor>::Initialize() {
 
   table_.Register(kVisitPropertyCell, &VisitPropertyCell);
 
+  table_.Register(kVisitWeakCell, &VisitWeakCell);
+
   table_.template RegisterSpecializations<DataObjectVisitor, kVisitDataObject,
                                           kVisitDataObjectGeneric>();
 
@@ -350,6 +352,19 @@ void StaticMarkingVisitor<StaticVisitor>::VisitPropertyCell(
 
 
 template <typename StaticVisitor>
+void StaticMarkingVisitor<StaticVisitor>::VisitWeakCell(Map* map,
+                                                        HeapObject* object) {
+  Heap* heap = map->GetHeap();
+  WeakCell* weak_cell = reinterpret_cast<WeakCell*>(object);
+  // Enqueue weak cell in linked list of encountered weak collections.
+  if (weak_cell->next() == heap->undefined_value()) {

[Erik Corry, 2014/10/14 13:37:22]

I think this should set to smi zero.

Since there's no set_value function, there's no need to chain up the weak cell
if it only contains undefined.

Even if there was a set_value method (is there actually one? the design doc says
there isn't...) the write barrier should ensure that we come back here if it is
used.

[ulan, 2014/10/14 14:05:41]

Good idea about skipping cleared weak cells. Added a check.

About setting to smi zero: undefined and smi zero help us distinguish the
following cases:
- weak cell is not in the list (next() == undefined)
- weak cell is the last element of the list (next() == smi zero)
- weak cell is in the list and not the last element (next() == pointer to a weak
cell)

We could probably use smi zero for first two cases, but then it would complicate
debugging.

+    weak_cell->set_next(heap->encountered_weak_cells());
+    heap->set_encountered_weak_cells(weak_cell);
+  }
+}
+
+
+template <typename StaticVisitor>
 void StaticMarkingVisitor<StaticVisitor>::VisitAllocationSite(
     Map* map, HeapObject* object) {
   Heap* heap = map->GetHeap();