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diff --git a/Source/JavaScriptCore/b3/B3DuplicateTails.cpp b/Source/JavaScriptCore/b3/B3DuplicateTails.cpp
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+++ b/Source/JavaScriptCore/b3/B3DuplicateTails.cpp
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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. 
+ */
+
+#include "config.h"
+#include "B3DuplicateTails.h"
+
+#if ENABLE(B3_JIT)
+
+#include "B3BasicBlockInlines.h"
+#include "B3BreakCriticalEdges.h"
+#include "B3ControlValue.h"
+#include "B3Dominators.h"
+#include "B3FixSSA.h"
+#include "B3IndexSet.h"
+#include "B3InsertionSetInlines.h"
+#include "B3PhaseScope.h"
+#include "B3ProcedureInlines.h"
+#include "B3SwitchValue.h"
+#include "B3UpsilonValue.h"
+#include "B3ValueInlines.h"
+
+namespace JSC { namespace B3 {
+
+namespace {
+
+const bool verbose = false;
+
+class DuplicateTails {
+public:
+    DuplicateTails(Procedure& proc)
+        : m_proc(proc)
+        , m_insertionSet(proc)
+        , m_maxSize(Options::maxB3TailDupBlockSize())
+        , m_maxSuccessors(Options::maxB3TailDupBlockSuccessors())
+    {
+    }
+
+    void run()
+    {
+        // Breaking critical edges introduces blocks that jump to things. Those Jumps' successors
+        // become candidates for tail duplication. Prior to critical edge breaking, some of those
+        // Jumps would have been Branches, and so no tail duplication would have happened.
+        breakCriticalEdges(m_proc);
+        
+        // Find blocks that would be candidates for tail duplication. They must be small enough
+        // and they much not have too many successors.
+
+        m_proc.resetValueOwners();
+
+        IndexSet<BasicBlock> candidates;
+
+        for (BasicBlock* block : m_proc) {
+            if (block->size() > m_maxSize || block->numSuccessors() > m_maxSuccessors)
+                continue;
+
+            candidates.add(block);
+        }
+
+        // Collect the set of values that must be de-SSA'd.
+        IndexSet<Value> valuesToDemote;
+        for (BasicBlock* block : m_proc) {
+            for (Value* value : *block) {
+                if (value->opcode() == Phi && candidates.contains(block))
+                    valuesToDemote.add(value);
+                for (Value* child : value->children()) {
+                    if (child->owner != block && candidates.contains(child->owner))
+                        valuesToDemote.add(child);
+                }
+            }
+        }
+        demoteValues(m_proc, valuesToDemote);
+        if (verbose) {
+            dataLog("Procedure after value demotion:\n");
+            dataLog(m_proc);
+        }
+
+        for (BasicBlock* block : m_proc) {
+            ControlValue* jump = block->last()->as<ControlValue>();
+            if (jump->opcode() != Jump)
+                continue;
+
+            BasicBlock* tail = jump->successorBlock(0);
+            if (!candidates.contains(tail))
+                continue;
+
+            // Don't tail duplicate a trivial self-loop, because the code below can't handle block and
+            // tail being the same block.
+            if (block == tail)
+                continue;
+
+            // We're about to change 'block'. Make sure that nobody duplicates block after this
+            // point.
+            candidates.remove(block);
+
+            if (verbose)
+                dataLog("Duplicating ", *tail, " into ", *block, "\n");
+
+            block->removeLast(m_proc);
+
+            HashMap<Value*, Value*> map;
+            for (Value* value : *tail) {
+                Value* clone = m_proc.clone(value);
+                for (Value*& child : clone->children()) {
+                    if (Value* replacement = map.get(child))
+                        child = replacement;
+                }
+                if (value->type() != Void)
+                    map.add(value, clone);
+                block->append(clone);
+            }
+        }
+
+        m_proc.resetReachability();
+        m_proc.invalidateCFG();
+    }
+    
+private:
+
+    Procedure& m_proc;
+    InsertionSet m_insertionSet;
+    unsigned m_maxSize;
+    unsigned m_maxSuccessors;
+};
+
+} // anonymous namespace
+
+void duplicateTails(Procedure& proc)
+{
+    PhaseScope phaseScope(proc, "duplicateTails");
+    DuplicateTails duplicateTails(proc);
+    duplicateTails.run();
+}
+
+} } // namespace JSC::B3
+
+#endif // ENABLE(B3_JIT)
+