Before overapproximating the weak closure, make sure the marking deque is empty

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 44262e42b162b58974cf7b2092ea141047856d2a..aacae732e06d0ce127dc9d77539aa13e9ce032ad 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -2211,11 +2211,18 @@ void MarkCompactCollector::OverApproximateWeakClosure() {
   GCTracer::Scope gc_scope(heap()->tracer(),
                            GCTracer::Scope::MC_INCREMENTAL_WEAKCLOSURE);
 
+  // Make sure that the marking deque is empty, even if it has overflown.
+  // Otherwise, we might miss some objects in an object group that would
+  // otherwise made the entire group alive.
+  ProcessMarkingDeque();
+
   RootMarkingVisitor root_visitor(heap());
   isolate()->global_handles()->IterateObjectGroups(
       &root_visitor, &IsUnmarkedHeapObjectWithHeap);
   MarkImplicitRefGroups();
 
+  DCHECK(marking_deque_.overflowed());

[Hannes Payer (out of office), 2015/03/10 13:09:28]

Why do we check here for overflowed?

+
   // Remove object groups after marking phase.
   heap()->isolate()->global_handles()->RemoveObjectGroups();
   heap()->isolate()->global_handles()->RemoveImplicitRefGroups();