Optimize clearing of map transitions.

src/snapshot/serialize.cc

Index: src/snapshot/serialize.cc
diff --git a/src/snapshot/serialize.cc b/src/snapshot/serialize.cc
index 4affefe7729065426b4f55fc6ecdd396be14d3bd..d4182c239eac17162d0f9e1d88522d5991d910b3 100644
--- a/src/snapshot/serialize.cc
+++ b/src/snapshot/serialize.cc
@@ -1974,7 +1974,7 @@ class UnlinkWeakCellScope {
     if (object->IsWeakCell()) {
       weak_cell_ = WeakCell::cast(object);
       next_ = weak_cell_->next();
-      weak_cell_->clear_next(object->GetHeap());
+      weak_cell_->clear_next(object->GetHeap()->the_hole_value());

[Yang, 2015/12/01 11:21:40]

Why does the caller need to know with what sentinel value we clear the weak
cell?

[ulan, 2015/12/01 12:49:52]

For efficiency, this function is used in a hot loop in mark-compact where we can
cache the value.

I think it is OK, since clear_next shouldn't be used by anyone except GC (and
serializer).

     }
   }