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diff --git a/Source/JavaScriptCore/dfg/DFGClobbersExitState.h b/Source/JavaScriptCore/dfg/DFGClobbersExitState.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. 
+ */
+
+#pragma once
+
+#if ENABLE(DFG_JIT)
+
+namespace JSC { namespace DFG {
+
+class Graph;
+struct Node;
+
+// A conservative approximation of whether the node will perform the kind of effect that would prevent
+// subsequent nodes from exiting to this node's exit origin. Exiting after an effect to that effect's
+// exit origin would cause the effect to execute a second time. Two kinds of such effects can exist:
+//
+// Observable heap or stack effect: If we perform such an effect and then exit to the same origin, that
+// effect will be executed a second time, which is incorrect.
+//
+// OSR exit state update: This doesn't do any observable side-effect, but it tells OSR exit that it
+// should recover some value as if an effect had happened. For example, a MovHint will tell OSR exit
+// that some bytecode variable now has a new value. If we exit to the exit origin of a MovHint after we
+// "execute" the MovHint, then the bytecode state will look as if we had already executed that bytecode
+// instruction. This could cause issues for example for bytecode like:
+//
+//     op_add r1, r1, r2
+//
+// which will get lowered to something like:
+//
+//     a: ArithAdd(...)
+//     b: MovHint(@a, r1)
+//
+// If we exit to the op_add after executing the MovHint, then r1 will already contain the result of the
+// add. Then after exit we'll do the add again, and r1 will have the wrong value. Because of object
+// allocation elimination and PutStack sinking, we can also have other OSR exit updates, like
+// KillStack, PutHint, among others. They don't do anything so long as we stay in optimized code, but
+// they tell OSR exit how to reconstitute state.
+
+bool clobbersExitState(Graph&, Node*);
+
+} } // namespace JSC::DFG
+
+#endif // ENABLE(DFG_JIT)