Don't perform marking barrier for weak cell values.

src/objects-inl.h

 // 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.
 //
 // Review notes:
 //
 // - The use of macros in these inline functions may seem superfluous
 // but it is absolutely needed to make sure gcc generates optimal
 // code. gcc is not happy when attempting to inline too deep.
 //
@@ skipping 1899 matching lines @@
 
 
 void WeakCell::clear() {
   DCHECK(GetHeap()->gc_state() == Heap::MARK_COMPACT);
   WRITE_FIELD(this, kValueOffset, Smi::FromInt(0));
 }
 
 
 void WeakCell::initialize(HeapObject* val) {
   WRITE_FIELD(this, kValueOffset, val);
-  WRITE_BARRIER(GetHeap(), this, kValueOffset, val);
+  Heap* heap = GetHeap();
+  // We just have to execute the generational barrier here because we never
+  // mark through a weak cell and collect evacuation candidates when we process
+  // all weak cells.
+  if (heap->InNewSpace(val)) {
+    heap->RecordWrite(address(), kValueOffset);
+  }
 }
 
 
 bool WeakCell::cleared() const { return value() == Smi::FromInt(0); }
 
 
 Object* WeakCell::next() const { return READ_FIELD(this, kNextOffset); }
 
 
 void WeakCell::set_next(Object* val, WriteBarrierMode mode) {
@@ skipping 5678 matching lines @@
 #undef READ_SHORT_FIELD
 #undef WRITE_SHORT_FIELD
 #undef READ_BYTE_FIELD
 #undef WRITE_BYTE_FIELD
 #undef NOBARRIER_READ_BYTE_FIELD
 #undef NOBARRIER_WRITE_BYTE_FIELD
 
 } }  // namespace v8::internal
 
 #endif  // V8_OBJECTS_INL_H_