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diff --git a/Source/JavaScriptCore/dfg/DFGMayExit.h b/Source/JavaScriptCore/dfg/DFGMayExit.h
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+/*
+ * Copyright (C) 2014-2016 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)
+
+namespace JSC { namespace DFG {
+
+class AtTailAbstractState;
+class Graph;
+struct Node;
+
+// A *very* conservative approximation of whether or not a node could possibly exit. Usually
+// returns true except in cases where we obviously don't expect an exit.
+
+enum ExitMode {
+    // The node does not exit at all. This means that it's legal to eliminate the first store in a
+    // program like:
+    //
+    // global = 1 // first store
+    // DoesNotExit(); // let's assume that this also doesn't read "global"
+    // global = 2 // second store
+    //
+    // It's legal to eliminate the first one since nobody will see it; the second store is always
+    // executed right after.
+    DoesNotExit,
+
+    // The node will exit, but only by properly throwing exceptions. A proper exception throw will
+    // divert execution to the matching op_catch and will not reexecute the exit origin. This means
+    // that the store elimination optimization above would be illegal, but the following would be OK:
+    //
+    // SideEffect(..., exit: bc#42)
+    // ExitsForExceptions(..., exit: #bc42, ExitInvalid)
+    //
+    // In particular, it's OK for a node that reports ExitsForExceptions to be executed in a context
+    // where !Node::origin.exitOK. That's because this node will not exit in a manner that could lead
+    // to the reexecution of SideEffect().
+    ExitsForExceptions,
+
+    // The node will exit to the exit origin. This means that we cannot do store elimination like for
+    // DoesNotExit and also we cannot place this node in an ExitInvalid context, since this will exit
+    // in a manner that will cause the reexecution of all previous operations within this exit origin.
+    Exits
+};
+
+// FIXME: This currently consumes the Check: flag produced by AI, and will claim that something doesn't
+// exit if the Check: flag was cleared. This makes it hard to use mayExit() for things like hoisting
+// (for example in LICM), since that wants to know if the node would exit if it was moved somewhere
+// else.
+// https://bugs.webkit.org/show_bug.cgi?id=148545
+
+ExitMode mayExit(Graph&, Node*);
+
+// Like mayExit(), but instead of using the Check: flag to determine if something exits, it
+// evaluates whether it will exit based on the tail state. This is useful for LICM. This *may* also
+// use the AtTailAbstractState to return more precise answers for other nodes.
+ExitMode mayExit(Graph&, Node*, AtTailAbstractState&);
+
+} } // namespace JSC::DFG
+
+namespace WTF {
+
+class PrintStream;
+
+void printInternal(PrintStream&, JSC::DFG::ExitMode);
+
+} // namespace WTF
+
+#endif // ENABLE(DFG_JIT)