Read object pointer atomically while updating slots

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 2906 matching lines @@
     DCHECK((RelocInfo::IsJSReturn(rinfo->rmode()) &&
             rinfo->IsPatchedReturnSequence()) ||
            (RelocInfo::IsDebugBreakSlot(rinfo->rmode()) &&
             rinfo->IsPatchedDebugBreakSlotSequence()));
     Object* target = Code::GetCodeFromTargetAddress(rinfo->call_address());
     VisitPointer(&target);
     rinfo->set_call_address(Code::cast(target)->instruction_start());
   }
 
   static inline void UpdateSlot(Heap* heap, Object** slot) {
-    Object* obj = *slot;
+    Object* obj = reinterpret_cast<Object*>(
+        base::NoBarrier_Load(reinterpret_cast<base::AtomicWord*>(slot)));
 
     if (!obj->IsHeapObject()) return;
 
     HeapObject* heap_obj = HeapObject::cast(obj);
 
     MapWord map_word = heap_obj->map_word();
     if (map_word.IsForwardingAddress()) {
       DCHECK(heap->InFromSpace(heap_obj) ||
              MarkCompactCollector::IsOnEvacuationCandidate(heap_obj));
       HeapObject* target = map_word.ToForwardingAddress();
-      *slot = target;
+      base::NoBarrier_CompareAndSwap(
+          reinterpret_cast<base::AtomicWord*>(slot),
+          reinterpret_cast<base::AtomicWord>(obj),
+          reinterpret_cast<base::AtomicWord>(target));
       DCHECK(!heap->InFromSpace(target) &&
              !MarkCompactCollector::IsOnEvacuationCandidate(target));
     }
   }
 
  private:
   inline void UpdatePointer(Object** p) { UpdateSlot(heap_, p); }
 
   Heap* heap_;
 };
@@ skipping 1496 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