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diff --git a/Source/JavaScriptCore/dfg/DFGControlEquivalenceAnalysis.h b/Source/JavaScriptCore/dfg/DFGControlEquivalenceAnalysis.h
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+/*
+ * Copyright (C) 2016 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 "DFGBackwardsDominators.h"
+#include "DFGDominators.h"
+
+namespace JSC { namespace DFG {
+
+class ControlEquivalenceAnalysis {
+    WTF_MAKE_NONCOPYABLE(ControlEquivalenceAnalysis);
+    WTF_MAKE_FAST_ALLOCATED;
+public:
+    ControlEquivalenceAnalysis(Graph& graph)
+        : m_dominators(graph.ensureDominators())
+        , m_backwardsDominators(graph.ensureBackwardsDominators())
+    {
+    }
+    
+    // This returns true iff:
+    //
+    // - If b executes then a must have executed before it (a dominates b).
+    // - If a executes then b will execute after it (b backwards-dominates a).
+    //
+    // Note that like Dominators and BackwardsDominators, this analysis ignores OSR:
+    //
+    // - This may return true even if we OSR enter in beteen a and b. OSR entry would mean that b
+    //   could execute even if a had not executed. This is impossible in DFG SSA but it's possible
+    //   in DFG CPS.
+    // - This may return true even if we OSR exit in between a and b. OSR exit would mean that a
+    //   could execute even though b will not execute. This is possible in all forms of DFG IR.
+    //
+    // In DFG SSA you only have to worry about the definition being weaked by exits. This is usually
+    // OK, since we use this analysis to determine the cost of moving exits from one block to
+    // another. If we move an exit from b to a and a equivalently dominates b then at worst we have
+    // made the exit happen sooner. If we move an exit from b to a and a dominates b but not
+    // equivalently then we've done something much worse: the program may now exit even if it would
+    // not have ever exited before.
+    bool dominatesEquivalently(BasicBlock* a, BasicBlock* b)
+    {
+        return m_dominators.dominates(a, b)
+            && m_backwardsDominators.dominates(b, a);
+    }
+    
+    // This returns true iff the execution of a implies that b also executes and vice-versa.
+    bool areEquivalent(BasicBlock* a, BasicBlock* b)
+    {
+        return dominatesEquivalently(a, b)
+            || dominatesEquivalently(b, a);
+    }
+
+private:
+    Dominators& m_dominators;
+    BackwardsDominators& m_backwardsDominators;
+};
+
+} } // namespace JSC::DFG
+
+#endif // ENABLE(DFG_JIT)