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diff --git a/Source/JavaScriptCore/b3/air/AirUseCounts.h b/Source/JavaScriptCore/b3/air/AirUseCounts.h
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+++ b/Source/JavaScriptCore/b3/air/AirUseCounts.h
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+/*
+ * Copyright (C) 2015-2016 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(B3_JIT)
+
+#include "AirArgInlines.h"
+#include "AirBlockWorklist.h"
+#include "AirCode.h"
+#include "AirInstInlines.h"
+#include <wtf/HashMap.h>
+#include <wtf/ListDump.h>
+
+namespace JSC { namespace B3 { namespace Air {
+
+class Code;
+
+// Computes the number of uses of a variable based on frequency of execution. The frequency of blocks
+// that are only reachable by rare edges is scaled by Options::rareBlockPenalty().
+
+// Thing can be either Tmp or Arg.
+template<typename Thing>
+class UseCounts {
+public:
+    struct Counts {
+        void dump(PrintStream& out) const
+        {
+            out.print(
+                "{numWarmUses = ", numWarmUses, ", numColdUses = ", numColdUses, ", numDefs = ",
+                numDefs, "}");
+        }
+        
+        double numWarmUses { 0 };
+        double numColdUses { 0 };
+        double numDefs { 0 };
+        double numConstDefs { 0 };
+    };
+    
+    UseCounts(Code& code)
+    {
+        // Find non-rare blocks.
+        BlockWorklist fastWorklist;
+        fastWorklist.push(code[0]);
+        while (BasicBlock* block = fastWorklist.pop()) {
+            for (FrequentedBlock& successor : block->successors()) {
+                if (!successor.isRare())
+                    fastWorklist.push(successor.block());
+            }
+        }
+        
+        for (BasicBlock* block : code) {
+            double frequency = block->frequency();
+            if (!fastWorklist.saw(block))
+                frequency *= Options::rareBlockPenalty();
+            for (Inst& inst : *block) {
+                inst.forEach<Thing>(
+                    [&] (Thing& arg, Arg::Role role, Arg::Type, Arg::Width) {
+                        Counts& counts = m_counts.add(arg, Counts()).iterator->value;
+
+                        if (Arg::isWarmUse(role))
+                            counts.numWarmUses += frequency;
+                        if (Arg::isColdUse(role))
+                            counts.numColdUses += frequency;
+                        if (Arg::isAnyDef(role))
+                            counts.numDefs += frequency;
+                    });
+
+                if ((inst.kind.opcode == Move || inst.kind.opcode == Move32)
+                    && inst.args[0].isSomeImm()
+                    && inst.args[1].is<Thing>())
+                    m_counts.add(inst.args[1].as<Thing>(), Counts()).iterator->value.numConstDefs++;
+            }
+        }
+    }
+
+    const Counts* operator[](const Thing& arg) const
+    {
+        auto iter = m_counts.find(arg);
+        if (iter == m_counts.end())
+            return nullptr;
+        return &iter->value;
+    }
+
+    void dump(PrintStream& out) const
+    {
+        out.print(mapDump(m_counts));
+    }
+
+private:
+    HashMap<Thing, Counts> m_counts;
+};
+
+} } } // namespace JSC::B3::Air
+
+#endif // ENABLE(B3_JIT)