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diff --git a/src/jit/valuenumtype.h b/src/jit/valuenumtype.h
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+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+
+// Defines the type "ValueNum".
+
+// This file exists only to break an include file cycle -- had been in ValueNum.h.  But that
+// file wanted to include gentree.h to get GT_COUNT, and gentree.h wanted ton include ValueNum.h to
+// the ValueNum type.
+
+/*****************************************************************************/
+#ifndef _VALUENUMTYPE_H_
+#define _VALUENUMTYPE_H_
+/*****************************************************************************/
+
+// We will represent ValueNum's as unsigned integers.
+typedef UINT32 ValueNum;
+
+// There are two "kinds" of value numbers, which differ in their modeling of the actions of other threads.
+// "Liberal" value numbers assume that the other threads change contents of heap locations only at
+// synchronization points.  Liberal VNs are appropriate, for example, in identifying CSE opportunities.
+// "Conservative" value numbers assume that the contents of heap locations change arbitrarily between 
+// every two accesses.  Conservative VNs are appropriate, for example, in assertion prop, where an observation
+// of a property of the value in some storage location is used to perform an optimization downstream on
+// an operation involving the contents of that storage location.  If other threads may modify the storage
+// location between the two accesses, the observed property may no longer hold -- and conservative VNs make
+// it clear that the values need not be the same.
+//
+enum ValueNumKind { VNK_Liberal, VNK_Conservative };
+
+struct ValueNumPair
+{
+private:
+    ValueNum m_liberal;
+    ValueNum m_conservative;
+public:
+    ValueNum GetLiberal() const       { return m_liberal; }
+    void SetLiberal(ValueNum vn)      { m_liberal = vn; }
+    ValueNum GetConservative() const  { return m_conservative; }
+    void SetConservative(ValueNum vn) { m_conservative = vn; }
+
+    ValueNum* GetLiberalAddr()        { return &m_liberal; }
+    ValueNum* GetConservativeAddr()   { return &m_conservative; }
+
+    ValueNum Get(ValueNumKind vnk)    { return vnk == VNK_Liberal ? m_liberal : m_conservative; }
+
+    void SetBoth(ValueNum vn)
+    {
+        m_liberal = vn;
+        m_conservative = vn;
+    }
+
+    void operator=(const ValueNumPair& vn2)
+    {
+        m_liberal = vn2.m_liberal; 
+        m_conservative = vn2.m_conservative;
+    }
+
+    // Initializes both elements to "NoVN".  Defined in ValueNum.cpp.
+    ValueNumPair();
+
+    ValueNumPair(ValueNum lib, ValueNum cons) : m_liberal(lib), m_conservative(cons) {}
+
+    // True iff neither element is "NoVN".  Defined in ValueNum.cpp.
+    bool BothDefined() const;
+
+    bool BothEqual() const { return m_liberal == m_conservative; }
+};
+
+#endif // _VALUENUMTYPE_H_