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diff --git a/Source/JavaScriptCore/heap/MarkingConstraint.h b/Source/JavaScriptCore/heap/MarkingConstraint.h
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+/*
+ * Copyright (C) 2017 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
+
+#include "ConstraintVolatility.h"
+#include "VisitingTimeout.h"
+#include <wtf/FastMalloc.h>
+#include <wtf/Function.h>
+#include <wtf/MonotonicTime.h>
+#include <wtf/Noncopyable.h>
+#include <wtf/text/CString.h>
+
+namespace JSC {
+
+class MarkingConstraintSet;
+class SlotVisitor;
+
+class MarkingConstraint {
+    WTF_MAKE_NONCOPYABLE(MarkingConstraint);
+    WTF_MAKE_FAST_ALLOCATED;
+public:
+    JS_EXPORT_PRIVATE MarkingConstraint(
+        CString abbreviatedName, CString name,
+        ::Function<void(SlotVisitor&, const VisitingTimeout&)>,
+        ConstraintVolatility);
+    
+    JS_EXPORT_PRIVATE MarkingConstraint(
+        CString abbreviatedName, CString name,
+        ::Function<void(SlotVisitor&, const VisitingTimeout&)>,
+        ::Function<double(SlotVisitor&)>,
+        ConstraintVolatility);
+    
+    JS_EXPORT_PRIVATE ~MarkingConstraint();
+    
+    unsigned index() const { return m_index; }
+    
+    const char* abbreviatedName() const { return m_abbreviatedName.data(); }
+    const char* name() const { return m_name.data(); }
+    
+    void resetStats();
+    
+    size_t lastVisitCount() const { return m_lastVisitCount; }
+    
+    void execute(SlotVisitor&, bool& didVisitSomething, MonotonicTime timeout);
+    
+    double quickWorkEstimate(SlotVisitor& visitor)
+    {
+        if (!m_quickWorkEstimateFunction)
+            return 0;
+        return m_quickWorkEstimateFunction(visitor);
+    }
+    
+    double workEstimate(SlotVisitor& visitor)
+    {
+        return lastVisitCount() + quickWorkEstimate(visitor);
+    }
+    
+    ConstraintVolatility volatility() const { return m_volatility; }
+    
+private:
+    friend class MarkingConstraintSet; // So it can set m_index.
+    
+    unsigned m_index { UINT_MAX };
+    CString m_abbreviatedName;
+    CString m_name;
+    ::Function<void(SlotVisitor&, const VisitingTimeout& timeout)> m_executeFunction;
+    ::Function<double(SlotVisitor&)> m_quickWorkEstimateFunction;
+    ConstraintVolatility m_volatility;
+    size_t m_lastVisitCount { 0 };
+};
+
+} // namespace JSC
+