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diff --git a/Source/JavaScriptCore/runtime/AuxiliaryBarrier.h b/Source/JavaScriptCore/runtime/AuxiliaryBarrier.h
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+/*
+ * Copyright (C) 2016 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+namespace JSC {
+
+class JSCell;
+class VM;
+
+// An Auxiliary barrier is a barrier that does not try to reason about the value being stored into
+// it, other than interpreting a falsy value as not needing a barrier. It's OK to use this for either
+// JSCells or any other kind of data, so long as it responds to operator!().
+template<typename T>
+class AuxiliaryBarrier {
+public:
+    AuxiliaryBarrier(): m_value() { }
+    
+    template<typename U>
+    AuxiliaryBarrier(VM&, JSCell*, U&&);
+    
+    void clear() { m_value = T(); }
+    
+    template<typename U>
+    void set(VM&, JSCell*, U&&);
+    
+    const T& get() const { return m_value; }
+    
+    T* slot() { return &m_value; }
+    
+    explicit operator bool() const { return !!m_value; }
+    
+    template<typename U>
+    void setWithoutBarrier(U&& value) { m_value = std::forward<U>(value); }
+    
+private:
+    T m_value;
+};
+
+} // namespace JSC