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diff --git a/Source/JavaScriptCore/bytecode/ArrayAllocationProfile.cpp b/Source/JavaScriptCore/bytecode/ArrayAllocationProfile.cpp
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+++ b/Source/JavaScriptCore/bytecode/ArrayAllocationProfile.cpp
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+/*
+ * Copyright (C) 2012, 2013 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 "ArrayAllocationProfile.h"
+
+#include "JSCInlines.h"
+
+namespace JSC {
+
+void ArrayAllocationProfile::updateIndexingType()
+{
+    // This is awkwardly racy but totally sound even when executed concurrently. The
+    // worst cases go something like this:
+    //
+    // - Two threads race to execute this code; one of them succeeds in updating the
+    //   m_currentIndexingType and the other either updates it again, or sees a null
+    //   m_lastArray; if it updates it again then at worst it will cause the profile
+    //   to "forget" some array. That's still sound, since we don't promise that
+    //   this profile is a reflection of any kind of truth.
+    //
+    // - A concurrent thread reads m_lastArray, but that array is now dead. While
+    //   it's possible for that array to no longer be reachable, it cannot actually
+    //   be freed, since we require the GC to wait until all concurrent JITing
+    //   finishes.
+    
+    JSArray* lastArray = m_lastArray;
+    if (!lastArray)
+        return;
+    m_currentIndexingType = leastUpperBoundOfIndexingTypes(m_currentIndexingType, lastArray->indexingType());
+    m_lastArray = 0;
+}
+
+} // namespace JSC
+