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diff --git a/Source/JavaScriptCore/tools/TieredMMapArray.h b/Source/JavaScriptCore/tools/TieredMMapArray.h
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+++ b/Source/JavaScriptCore/tools/TieredMMapArray.h
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+/*
+ * Copyright (C) 2012 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. 
+ */
+
+#ifndef TieredMMapArray_h
+#define TieredMMapArray_h
+
+#include "OSAllocator.h"
+
+namespace JSC {
+
+// This class implements a simple array class that can be grown by appending items to the end.
+// This class is implemented purely in terms of system allocations, with no malloc/free, so that
+// it can safely be used from a secondary thread whilst the main thrad is paused (potentially
+// holding the fast malloc heap lock).
+template<typename T>
+class TieredMMapArray {
+    static const size_t entriesPerBlock = 4096;
+
+public:
+    TieredMMapArray()
+        : m_directoryCount(4096)
+        , m_directory(static_cast<T**>(OSAllocator::reserveAndCommit(m_directoryCount * sizeof(T*))))
+        , m_size(0)
+    {
+        for (size_t block = 0; block < m_directoryCount; ++block)
+            m_directory[block] = 0;
+    }
+
+    ~TieredMMapArray()
+    {
+        size_t usedCount = (m_size + (entriesPerBlock - 1)) / entriesPerBlock;
+        ASSERT(usedCount == m_directoryCount || !m_directory[usedCount]);
+
+        for (size_t block = 0; block < usedCount; ++block) {
+            ASSERT(m_directory[block]);
+            OSAllocator::decommitAndRelease(m_directory[block], entriesPerBlock * sizeof(T));
+        }
+
+        OSAllocator::decommitAndRelease(m_directory, m_directoryCount * sizeof(T*));
+    }
+
+    T& operator[](size_t index)
+    {
+        ASSERT(index < m_size);
+        size_t block = index / entriesPerBlock;
+        size_t offset = index % entriesPerBlock;
+
+        ASSERT(m_directory[block]);
+        return m_directory[block][offset];
+    }
+
+    void append(const T& value)
+    {
+        // Check if the array is completely full, if so create more capacity in the directory.
+        if (m_size == m_directoryCount * entriesPerBlock) {
+            // Reallocate the directory.
+            size_t oldDirectorySize = m_directoryCount * sizeof(T*);
+            size_t newDirectorySize = oldDirectorySize * 2;
+            if (newDirectorySize < oldDirectorySize)
+                CRASH();
+            m_directory = OSAllocator::reallocateCommitted(m_directory, oldDirectorySize, newDirectorySize);
+
+            // 
+            size_t newDirectoryCount = m_directoryCount * 2;
+            for (size_t block = m_directoryCount; block < newDirectoryCount; ++block)
+                m_directory[block] = 0;
+            m_directoryCount = newDirectoryCount;
+        }
+
+        size_t index = m_size;
+        size_t block = index / entriesPerBlock;
+        size_t offset = index % entriesPerBlock;
+
+        if (!offset) {
+            ASSERT(!m_directory[block]);
+            m_directory[block] = static_cast<T*>(OSAllocator::reserveAndCommit(entriesPerBlock * sizeof(T)));
+        }
+
+        ASSERT(m_directory[block]);
+        ++m_size;
+        m_directory[block][offset] = value;
+    }
+
+    size_t size() const { return m_size; }
+
+private:
+    size_t m_directoryCount;
+    T** m_directory;
+    size_t m_size;
+};
+
+}
+
+#endif // TieredMMapArray_h
+