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diff --git a/Source/JavaScriptCore/dfg/DFGValueStrength.h b/Source/JavaScriptCore/dfg/DFGValueStrength.h
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+++ b/Source/JavaScriptCore/dfg/DFGValueStrength.h
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+/*
+ * Copyright (C) 2014, 2015 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. ``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
+ * 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
+
+#if ENABLE(DFG_JIT)
+
+#include <wtf/PrintStream.h>
+
+namespace JSC { namespace DFG {
+
+enum ValueStrength {
+    // The value has been used for optimization and it arose through inference. We don't want the
+    // fact that we optimized the code to result in the GC keeping this value alive unnecessarily,
+    // so we'd rather kill the code and recompile than keep the object alive longer.
+    WeakValue,
+    
+    // The code will keep this value alive. This is true of constants that were present in the
+    // source. String constants tend to be strong.
+    StrongValue
+};
+
+inline ValueStrength merge(ValueStrength a, ValueStrength b)
+{
+    switch (a) {
+    case WeakValue:
+        return b;
+    case StrongValue:
+        return StrongValue;
+    }
+    RELEASE_ASSERT_NOT_REACHED();
+
+    return WeakValue;
+}
+
+} } // namespace JSC::DFG
+
+namespace WTF {
+
+void printInternal(PrintStream&, JSC::DFG::ValueStrength);
+
+} // namespace WTF
+
+#endif // ENABLE(DFG_JIT)