[heap] Old-to-new pointer updates need atomic accessors.

src/base/atomic-utils.h

Index: src/base/atomic-utils.h
diff --git a/src/base/atomic-utils.h b/src/base/atomic-utils.h
index 31db603bf94345bb84ce9ef7b135c513f8f9da3a..7931c7376ab4b5ba4bf27268bbeb124fec6acef8 100644
--- a/src/base/atomic-utils.h
+++ b/src/base/atomic-utils.h
@@ -51,6 +51,53 @@ class AtomicNumber {
   base::AtomicWord value_;
 };
 
+// This type uses no barrier accessors to change atomic word. Be careful with
+// data races.
+template <typename T>
+class NoBarrierAtomicValue {
+ public:
+  NoBarrierAtomicValue() : value_(0) {}
+
+  explicit NoBarrierAtomicValue(T initial)
+      : value_(cast_helper<T>::to_storage_type(initial)) {}
+
+  static NoBarrierAtomicValue* FromAddress(void* address) {
+    return reinterpret_cast<base::NoBarrierAtomicValue<T>*>(address);
+  }
+
+  V8_INLINE T Value() const {
+    return cast_helper<T>::to_return_type(base::NoBarrier_Load(&value_));
+  }
+
+  V8_INLINE void SetValue(T new_value) {
+    base::NoBarrier_Store(&value_, cast_helper<T>::to_storage_type(new_value));
+  }
+
+ private:
+  STATIC_ASSERT(sizeof(T) <= sizeof(base::AtomicWord));
+
+  template <typename S>
+  struct cast_helper {
+    static base::AtomicWord to_storage_type(S value) {
+      return static_cast<base::AtomicWord>(value);
+    }
+    static S to_return_type(base::AtomicWord value) {
+      return static_cast<S>(value);
+    }
+  };
+
+  template <typename S>
+  struct cast_helper<S*> {
+    static base::AtomicWord to_storage_type(S* value) {
+      return reinterpret_cast<base::AtomicWord>(value);
+    }
+    static S* to_return_type(base::AtomicWord value) {
+      return reinterpret_cast<S*>(value);
+    }
+  };
+
+  base::AtomicWord value_;
+};
 
 // Flag using T atomically. Also accepts void* as T.
 template <typename T>