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diff --git a/Source/JavaScriptCore/runtime/InferredValue.h b/Source/JavaScriptCore/runtime/InferredValue.h
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+/*
+ * Copyright (C) 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. 
+ */
+
+#ifndef InferredValue_h
+#define InferredValue_h
+
+#include "JSCell.h"
+#include "Watchpoint.h"
+#include "WriteBarrier.h"
+
+namespace JSC {
+
+// Allocate one of these if you'd like to infer a constant value. Writes to the value should use
+// notifyWrite(). So long as exactly one value had ever been written and invalidate() has never been
+// called, and you register a watchpoint, you can rely on the inferredValue() being the one true
+// value.
+//
+// Commonly used for inferring singletons - in that case each allocation does notifyWrite(). But you
+// can use it for other things as well.
+
+class InferredValue : public JSCell {
+public:
+    typedef JSCell Base;
+    
+    static InferredValue* create(VM&);
+    
+    static const bool needsDestruction = true;
+    static void destroy(JSCell*);
+    
+    static Structure* createStructure(VM&, JSGlobalObject*, JSValue prototype);
+    
+    static void visitChildren(JSCell*, SlotVisitor&);
+    
+    DECLARE_INFO;
+    
+    // For the purpose of deciding whether or not to watch this variable, you only need
+    // to inspect inferredValue(). If this returns something other than the empty
+    // value, then it means that at all future safepoints, this watchpoint set will be
+    // in one of these states:
+    //
+    //    IsWatched: in this case, the variable's value must still be the
+    //        inferredValue.
+    //
+    //    IsInvalidated: in this case the variable's value may be anything but you'll
+    //        either notice that it's invalidated and not install the watchpoint, or
+    //        you will have been notified that the watchpoint was fired.
+    JSValue inferredValue() { return m_value.get(); }
+
+    // Forwards some WatchpointSet methods.
+    WatchpointState state() const { return m_set.state(); }
+    bool isStillValid() const { return m_set.isStillValid(); }
+    bool hasBeenInvalidated() const { return m_set.hasBeenInvalidated(); }
+    void add(Watchpoint* watchpoint) { m_set.add(watchpoint); }
+    
+    void notifyWrite(VM& vm, JSValue value, const FireDetail& detail)
+    {
+        if (LIKELY(m_set.stateOnJSThread() == IsInvalidated))
+            return;
+        notifyWriteSlow(vm, value, detail);
+    }
+    
+    void notifyWrite(VM& vm, JSValue value, const char* reason)
+    {
+        if (LIKELY(m_set.stateOnJSThread() == IsInvalidated))
+            return;
+        notifyWriteSlow(vm, value, reason);
+    }
+    
+    void invalidate(const FireDetail& detail)
+    {
+        m_value.clear();
+        m_set.invalidate(detail);
+    }
+    
+    static const unsigned StructureFlags = StructureIsImmortal | Base::StructureFlags;
+    
+    // We could have used Weak<>. But we want arbitrary JSValues, not just cells. It's also somewhat
+    // convenient to have eager notification of death.
+    //
+    // Also note that this should be a private class, but it isn't because Windows.
+    class ValueCleanup : public UnconditionalFinalizer {
+        WTF_MAKE_FAST_ALLOCATED;
+        
+    public:
+        ValueCleanup(InferredValue*);
+        virtual ~ValueCleanup();
+        
+    protected:
+        void finalizeUnconditionally() override;
+        
+    private:
+        InferredValue* m_owner;
+    };
+    
+private:
+    InferredValue(VM&);
+    ~InferredValue();
+    
+    JS_EXPORT_PRIVATE void notifyWriteSlow(VM&, JSValue, const FireDetail&);
+    JS_EXPORT_PRIVATE void notifyWriteSlow(VM&, JSValue, const char* reason);
+    
+    friend class ValueCleanup;
+    
+    InlineWatchpointSet m_set;
+    WriteBarrier<Unknown> m_value;
+    std::unique_ptr<ValueCleanup> m_cleanup;
+};
+
+// FIXME: We could have an InlineInferredValue, which only allocates the InferredValue object when
+// a notifyWrite() transitions us towards watching, and then clears the reference (allowing the object
+// to die) when we get invalidated.
+
+} // namespace JSC
+
+#endif // InferredValue_h
+