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

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 2726 matching lines @@
   }
   heap()->set_encountered_weak_collections(Smi::FromInt(0));
 }
 
 
 void MarkCompactCollector::ProcessAndClearWeakCells() {
   HeapObject* undefined = heap()->undefined_value();
   Object* weak_cell_obj = heap()->encountered_weak_cells();
   while (weak_cell_obj != Smi::FromInt(0)) {
     WeakCell* weak_cell = reinterpret_cast<WeakCell*>(weak_cell_obj);
-    HeapObject* value = weak_cell->value();
+    Object* value = weak_cell->value();
     if (!MarkCompactCollector::IsMarked(value)) {

[Erik Corry Chromium.org, 2014/10/20 08:00:17]

IsMarked will assert that the value is a heap object, so you should handle the
non-heap-object case here or make value a HeapObject pointer and use
HeapObject::cast() if you don't think it can happen.

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

Done.

-      weak_cell->clear(undefined);
+      weak_cell->clear();
     } else {
       Object** slot = HeapObject::RawField(weak_cell, WeakCell::kValueOffset);
       heap()->mark_compact_collector()->RecordSlot(slot, slot, value);
     }
     weak_cell_obj = weak_cell->next();
     weak_cell->set_next(undefined, SKIP_WRITE_BARRIER);
   }
   heap()->set_encountered_weak_cells(Smi::FromInt(0));
 }
 
@@ skipping 1685 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