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diff --git a/Source/JavaScriptCore/dfg/DFGMayExit.cpp b/Source/JavaScriptCore/dfg/DFGMayExit.cpp
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+++ b/Source/JavaScriptCore/dfg/DFGMayExit.cpp
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+/*
+ * Copyright (C) 2014, 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. 
+ */
+
+#include "config.h"
+#include "DFGMayExit.h"
+
+#if ENABLE(DFG_JIT)
+
+#include "DFGGraph.h"
+#include "DFGNode.h"
+#include "Operations.h"
+
+namespace JSC { namespace DFG {
+
+namespace {
+
+class EdgeMayExit {
+public:
+    EdgeMayExit()
+        : m_result(false)
+    {
+    }
+    
+    void operator()(Node*, Edge edge)
+    {
+        // FIXME: Maybe this should call mayHaveTypeCheck(edge.useKind()) instead.
+        // https://bugs.webkit.org/show_bug.cgi?id=148545
+        if (edge.willHaveCheck()) {
+            m_result = true;
+            return;
+        }
+
+        switch (edge.useKind()) {
+        // These are shady because nodes that have these use kinds will typically exit for
+        // unrelated reasons. For example CompareEq doesn't usually exit, but if it uses ObjectUse
+        // then it will.
+        case ObjectUse:
+        case ObjectOrOtherUse:
+            m_result = true;
+            break;
+            
+        // These are shady because they check the structure even if the type of the child node
+        // passes the StringObject type filter.
+        case StringObjectUse:
+        case StringOrStringObjectUse:
+            m_result = true;
+            break;
+            
+        default:
+            break;
+        }
+    }
+    
+    bool result() const { return m_result; }
+    
+private:
+    bool m_result;
+};
+
+} // anonymous namespace
+
+ExitMode mayExit(Graph& graph, Node* node)
+{
+    ExitMode result = DoesNotExit;
+    
+    switch (node->op()) {
+    // This is a carefully curated list of nodes that definitely do not exit. We try to be very
+    // conservative when maintaining this list, because adding new node types to it doesn't
+    // generally make things a lot better but it might introduce subtle bugs.
+    case SetArgument:
+    case JSConstant:
+    case DoubleConstant:
+    case Int52Constant:
+    case MovHint:
+    case SetLocal:
+    case Flush:
+    case Phantom:
+    case Check:
+    case Identity:
+    case GetLocal:
+    case LoopHint:
+    case Phi:
+    case Upsilon:
+    case ZombieHint:
+    case ExitOK:
+    case BottomValue:
+    case PutHint:
+    case PhantomNewObject:
+    case PutStack:
+    case KillStack:
+    case GetStack:
+    case GetCallee:
+    case GetArgumentCount:
+    case GetRestLength:
+    case GetScope:
+    case PhantomLocal:
+    case CountExecution:
+    case Jump:
+    case Branch:
+    case Unreachable:
+    case DoubleRep:
+    case Int52Rep:
+    case ValueRep:
+    case ExtractOSREntryLocal:
+    case LogicalNot:
+    case NotifyWrite:
+    case PutStructure:
+    case StoreBarrier:
+    case PutByOffset:
+    case PutClosureVar:
+        break;
+
+    case StrCat:
+    case Call:
+    case Construct:
+    case CallVarargs:
+    case ConstructVarargs:
+    case CallForwardVarargs:
+    case ConstructForwardVarargs:
+    case MaterializeCreateActivation:
+    case MaterializeNewObject:
+    case NewFunction:
+    case NewArrowFunction:
+    case NewGeneratorFunction:
+    case NewStringObject:
+    case CreateActivation:
+        result = ExitsForExceptions;
+        break;
+
+    default:
+        // If in doubt, return true.
+        return Exits;
+    }
+
+    EdgeMayExit functor;
+    DFG_NODE_DO_TO_CHILDREN(graph, node, functor);
+    if (functor.result())
+        result = Exits;
+    
+    return result;
+}
+
+} } // namespace JSC::DFG
+
+namespace WTF {
+
+using namespace JSC::DFG;
+
+void printInternal(PrintStream& out, ExitMode mode)
+{
+    switch (mode) {
+    case DoesNotExit:
+        out.print("DoesNotExit");
+        return;
+    case ExitsForExceptions:
+        out.print("ExitsForExceptions");
+        return;
+    case Exits:
+        out.print("Exits");
+        return;
+    }
+    RELEASE_ASSERT_NOT_REACHED();
+}
+
+} // namespace WTF
+
+#endif // ENABLE(DFG_JIT)