Merged: [heap] Use CAS to update old to new slots in Scavenger.

src/heap/scavenger.cc

 // Copyright 2015 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/heap/scavenger.h"
 
 #include "src/contexts.h"
 #include "src/heap/heap.h"
 #include "src/heap/objects-visiting-inl.h"
 #include "src/heap/scavenger-inl.h"
@@ skipping 167 matching lines @@
                                    HeapObject* object, int object_size) {
     Heap* heap = map->GetHeap();
 
     AllocationResult allocation =
         heap->old_space()->AllocateRaw(object_size, alignment);
 
     HeapObject* target = NULL;  // Initialization to please compiler.
     if (allocation.To(&target)) {
       MigrateObject(heap, object, target, object_size);
 
-      // Update slot to new target.
-      *slot = target;
+      // Update slot to new target using CAS. A concurrent sweeper thread my
+      // filter the slot concurrently.
+      HeapObject* old = *slot;
+      base::Release_CompareAndSwap(reinterpret_cast<base::AtomicWord*>(slot),
+                                   reinterpret_cast<base::AtomicWord>(old),
+                                   reinterpret_cast<base::AtomicWord>(target));
 
       if (object_contents == POINTER_OBJECT) {
         heap->promotion_queue()->insert(
             target, object_size,
             Marking::IsBlack(ObjectMarking::MarkBitFrom(object)));
       }
       heap->IncrementPromotedObjectsSize(object_size);
       return true;
     }
     return false;
@@ skipping 239 matching lines @@
 void ScavengeVisitor::ScavengePointer(Object** p) {
   Object* object = *p;
   if (!heap_->InNewSpace(object)) return;
 
   Scavenger::ScavengeObject(reinterpret_cast<HeapObject**>(p),
                             reinterpret_cast<HeapObject*>(object));
 }
 
 }  // namespace internal
 }  // namespace v8