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

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
@@ skipping 2193 matching lines @@
     CHECK(success);
     marking_deque_memory_committed_ = false;
   }
 }
 
 
 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();
 
   // Remove object groups after marking phase.
   heap()->isolate()->global_handles()->RemoveObjectGroups();
   heap()->isolate()->global_handles()->RemoveImplicitRefGroups();
 }
 
@@ skipping 2272 matching lines @@
   SlotsBuffer* buffer = *buffer_address;
   while (buffer != NULL) {
     SlotsBuffer* next_buffer = buffer->next();
     DeallocateBuffer(buffer);
     buffer = next_buffer;
   }
   *buffer_address = NULL;
 }
 }
 }  // namespace v8::internal