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diff --git a/include/llvm/Support/RWMutex.h b/include/llvm/Support/RWMutex.h
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+++ b/include/llvm/Support/RWMutex.h
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+//===- RWMutex.h - Reader/Writer Mutual Exclusion Lock ----------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file declares the llvm::sys::RWMutex class.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_SYSTEM_RWMUTEX_H
+#define LLVM_SYSTEM_RWMUTEX_H
+
+#include "llvm/Support/Threading.h"
+#include <cassert>
+
+namespace llvm
+{
+  namespace sys
+  {
+    /// @brief Platform agnostic RWMutex class.
+    class RWMutexImpl
+    {
+    /// @name Constructors
+    /// @{
+    public:
+
+      /// Initializes the lock but doesn't acquire it.
+      /// @brief Default Constructor.
+      explicit RWMutexImpl();
+
+      /// Releases and removes the lock
+      /// @brief Destructor
+      ~RWMutexImpl();
+
+    /// @}
+    /// @name Methods
+    /// @{
+    public:
+
+      /// Attempts to unconditionally acquire the lock in reader mode. If the
+      /// lock is held by a writer, this method will wait until it can acquire
+      /// the lock.
+      /// @returns false if any kind of error occurs, true otherwise.
+      /// @brief Unconditionally acquire the lock in reader mode.
+      bool reader_acquire();
+
+      /// Attempts to release the lock in reader mode.
+      /// @returns false if any kind of error occurs, true otherwise.
+      /// @brief Unconditionally release the lock in reader mode.
+      bool reader_release();
+
+      /// Attempts to unconditionally acquire the lock in reader mode. If the
+      /// lock is held by any readers, this method will wait until it can
+      /// acquire the lock.
+      /// @returns false if any kind of error occurs, true otherwise.
+      /// @brief Unconditionally acquire the lock in writer mode.
+      bool writer_acquire();
+
+      /// Attempts to release the lock in writer mode.
+      /// @returns false if any kind of error occurs, true otherwise.
+      /// @brief Unconditionally release the lock in write mode.
+      bool writer_release();
+
+    //@}
+    /// @name Platform Dependent Data
+    /// @{
+    private:
+      void* data_; ///< We don't know what the data will be
+
+    /// @}
+    /// @name Do Not Implement
+    /// @{
+    private:
+      RWMutexImpl(const RWMutexImpl & original);
+      void operator=(const RWMutexImpl &);
+    /// @}
+    };
+
+    /// SmartMutex - An R/W mutex with a compile time constant parameter that
+    /// indicates whether this mutex should become a no-op when we're not
+    /// running in multithreaded mode.
+    template<bool mt_only>
+    class SmartRWMutex : public RWMutexImpl {
+      unsigned readers, writers;
+    public:
+      explicit SmartRWMutex() : RWMutexImpl(), readers(0), writers(0) { }
+
+      bool reader_acquire() {
+        if (!mt_only || llvm_is_multithreaded())
+          return RWMutexImpl::reader_acquire();
+
+        // Single-threaded debugging code.  This would be racy in multithreaded
+        // mode, but provides not sanity checks in single threaded mode.
+        ++readers;
+        return true;
+      }
+
+      bool reader_release() {
+        if (!mt_only || llvm_is_multithreaded())
+          return RWMutexImpl::reader_release();
+
+        // Single-threaded debugging code.  This would be racy in multithreaded
+        // mode, but provides not sanity checks in single threaded mode.
+        assert(readers > 0 && "Reader lock not acquired before release!");
+        --readers;
+        return true;
+      }
+
+      bool writer_acquire() {
+        if (!mt_only || llvm_is_multithreaded())
+          return RWMutexImpl::writer_acquire();
+
+        // Single-threaded debugging code.  This would be racy in multithreaded
+        // mode, but provides not sanity checks in single threaded mode.
+        assert(writers == 0 && "Writer lock already acquired!");
+        ++writers;
+        return true;
+      }
+
+      bool writer_release() {
+        if (!mt_only || llvm_is_multithreaded())
+          return RWMutexImpl::writer_release();
+
+        // Single-threaded debugging code.  This would be racy in multithreaded
+        // mode, but provides not sanity checks in single threaded mode.
+        assert(writers == 1 && "Writer lock not acquired before release!");
+        --writers;
+        return true;
+      }
+
+    private:
+      SmartRWMutex(const SmartRWMutex<mt_only> & original);
+      void operator=(const SmartRWMutex<mt_only> &);
+    };
+    typedef SmartRWMutex<false> RWMutex;
+
+    /// ScopedReader - RAII acquisition of a reader lock
+    template<bool mt_only>
+    struct SmartScopedReader {
+      SmartRWMutex<mt_only>& mutex;
+
+      explicit SmartScopedReader(SmartRWMutex<mt_only>& m) : mutex(m) {
+        mutex.reader_acquire();
+      }
+
+      ~SmartScopedReader() {
+        mutex.reader_release();
+      }
+    };
+    typedef SmartScopedReader<false> ScopedReader;
+
+    /// ScopedWriter - RAII acquisition of a writer lock
+    template<bool mt_only>
+    struct SmartScopedWriter {
+      SmartRWMutex<mt_only>& mutex;
+
+      explicit SmartScopedWriter(SmartRWMutex<mt_only>& m) : mutex(m) {
+        mutex.writer_acquire();
+      }
+
+      ~SmartScopedWriter() {
+        mutex.writer_release();
+      }
+    };
+    typedef SmartScopedWriter<false> ScopedWriter;
+  }
+}
+
+#endif