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diff --git a/Source/JavaScriptCore/heap/ConstraintVolatility.h b/Source/JavaScriptCore/heap/ConstraintVolatility.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 <wtf/PrintStream.h>
+
+namespace JSC {
+
+enum class ConstraintVolatility : uint8_t {
+    // The constraint needs to be validated, but it is unlikely to ever produce information.
+    // It's best to run it at the bitter end.
+    SeldomGreyed,
+    
+    // FIXME: We could introduce a new kind of volatility called GreyedByResumption, which
+    // would mean running all of the times that GreyedByExecution runs except as a root in a
+    // full GC.
+    // https://bugs.webkit.org/show_bug.cgi?id=166830
+    
+    // The constraint needs to be reevaluated anytime the mutator runs: so at GC start and
+    // whenever the GC resuspends after a resumption. This is almost always something that
+    // you'd call a "root" in a traditional GC.
+    GreyedByExecution,
+    
+    // The constraint needs to be reevaluated any time any object is marked and anytime the
+    // mutator resumes.
+    GreyedByMarking
+};
+    
+} // namespace JSC
+
+namespace WTF {
+
+inline void printInternal(PrintStream& out, JSC::ConstraintVolatility volatility)
+{
+    switch (volatility) {
+    case JSC::ConstraintVolatility::SeldomGreyed:
+        out.print("SeldomGreyed");
+        return;
+    case JSC::ConstraintVolatility::GreyedByExecution:
+        out.print("GreyedByExecuction");
+        return;
+    case JSC::ConstraintVolatility::GreyedByMarking:
+        out.print("GreyedByMarking");
+        return;
+    }
+    RELEASE_ASSERT_NOT_REACHED();
+}
+
+} // namespace WTF
+