diff --git a/include/llvm/System/RWMutex.h b/include/llvm/System/RWMutex.h
new file mode 100644
index 00000000000..eb56eec75f6
--- /dev/null
+++ b/include/llvm/System/RWMutex.h
@@ -0,0 +1,84 @@
+//===- 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_MUTEX_H
+#define LLVM_SYSTEM_MUTEX_H
+
+namespace llvm
+{
+  namespace sys
+  {
+    /// @brief Platform agnostic Mutex class.
+    class RWMutex
+    {
+    /// @name Constructors
+    /// @{
+    public:
+
+      /// Initializes the lock but doesn't acquire it.
+      /// @brief Default Constructor.
+      explicit RWMutex();
+
+      /// Releases and removes the lock
+      /// @brief Destructor
+      ~RWMutex();
+
+    /// @}
+    /// @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:
+#ifdef ENABLE_THREADS
+      void* data_; ///< We don't know what the data will be
+#endif
+
+    /// @}
+    /// @name Do Not Implement
+    /// @{
+    private:
+      RWMutex(const RWMutex & original);
+      void operator=(const RWMutex &);
+    /// @}
+    };
+  }
+}
+
+#endif
diff --git a/lib/System/RWMutex.cpp b/lib/System/RWMutex.cpp
new file mode 100644
index 00000000000..a781a379ed4
--- /dev/null
+++ b/lib/System/RWMutex.cpp
@@ -0,0 +1,169 @@
+//===- RWMutex.cpp - 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 implements the llvm::sys::RWMutex class.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Config/config.h"
+#include "llvm/System/RWMutex.h"
+
+//===----------------------------------------------------------------------===//
+//=== WARNING: Implementation here must contain only TRULY operating system
+//===          independent code.
+//===----------------------------------------------------------------------===//
+
+#if !defined(ENABLE_THREADS) || ENABLE_THREADS == 0
+// Define all methods as no-ops if threading is explicitly disabled
+namespace llvm {
+using namespace sys;
+RWMutex::RWMutex( bool recursive) { }
+RWMutex::~RWMutex() { }
+bool RWMutex::reader_acquire() { return true; }
+bool RWMutex::reader_release() { return true; }
+bool RWMutex::writer_acquire() { return true; }
+bool RWMutex::writer_release() { return true; }
+}
+#else
+
+#if defined(HAVE_PTHREAD_H) && defined(HAVE_PTHREAD_RWLOCK_INIT)
+
+#include <cassert>
+#include <pthread.h>
+#include <stdlib.h>
+
+namespace llvm {
+using namespace sys;
+
+
+// This variable is useful for situations where the pthread library has been
+// compiled with weak linkage for its interface symbols. This allows the
+// threading support to be turned off by simply not linking against -lpthread.
+// In that situation, the value of pthread_mutex_init will be 0 and
+// consequently pthread_enabled will be false. In such situations, all the
+// pthread operations become no-ops and the functions all return false. If
+// pthread_rwlock_init does have an address, then rwlock support is enabled.
+// Note: all LLVM tools will link against -lpthread if its available since it
+//       is configured into the LIBS variable.
+// Note: this line of code generates a warning if pthread_rwlock_init is not
+//       declared with weak linkage. It's safe to ignore the warning.
+static const bool pthread_enabled = true;
+
+// Construct a RWMutex using pthread calls
+RWMutex::RWMutex()
+  : data_(0)
+{
+  if (pthread_enabled)
+  {
+    // Declare the pthread_rwlock data structures
+    pthread_rwlock_t* rwlock =
+      static_cast<pthread_rwlock_t*>(malloc(sizeof(pthread_rwlock_t)));
+    pthread_rwlockattr_t attr;
+
+    // Initialize the rwlock attributes
+    int errorcode = pthread_rwlockattr_init(&attr);
+    assert(errorcode == 0);
+
+#if !defined(__FreeBSD__) && !defined(__OpenBSD__) && !defined(__NetBSD__) && !defined(__DragonFly__)
+    // Make it a process local rwlock
+    errorcode = pthread_rwlockattr_setpshared(&attr, PTHREAD_PROCESS_PRIVATE);
+#endif
+
+    // Initialize the rwlock
+    errorcode = pthread_rwlock_init(rwlock, &attr);
+    assert(errorcode == 0);
+
+    // Destroy the attributes
+    errorcode = pthread_rwlockattr_destroy(&attr);
+    assert(errorcode == 0);
+
+    // Assign the data member
+    data_ = rwlock;
+  }
+}
+
+// Destruct a RWMutex
+RWMutex::~RWMutex()
+{
+  if (pthread_enabled)
+  {
+    pthread_rwlock_t* rwlock = static_cast<pthread_rwlock_t*>(data_);
+    assert(rwlock != 0);
+    pthread_rwlock_destroy(rwlock);
+    free(rwlock);
+  }
+}
+
+bool
+RWMutex::reader_acquire()
+{
+  if (pthread_enabled)
+  {
+    pthread_rwlock_t* rwlock = static_cast<pthread_rwlock_t*>(data_);
+    assert(rwlock != 0);
+
+    int errorcode = pthread_rwlock_rdlock(rwlock);
+    return errorcode == 0;
+  }
+  return false;
+}
+
+bool
+RWMutex::reader_release()
+{
+  if (pthread_enabled)
+  {
+    pthread_rwlock_t* rwlock = static_cast<pthread_rwlock_t*>(data_);
+    assert(rwlock != 0);
+
+    int errorcode = pthread_rwlock_unlock(rwlock);
+    return errorcode == 0;
+  }
+  return false;
+}
+
+bool
+RWMutex::writer_acquire()
+{
+  if (pthread_enabled)
+  {
+    pthread_rwlock_t* rwlock = static_cast<pthread_rwlock_t*>(data_);
+    assert(rwlock != 0);
+
+    int errorcode = pthread_rwlock_wrlock(rwlock);
+    return errorcode == 0;
+  }
+  return false;
+}
+
+bool
+RWMutex::writer_release()
+{
+  if (pthread_enabled)
+  {
+    pthread_rwlock_t* rwlock = static_cast<pthread_rwlock_t*>(data_);
+    assert(rwlock != 0);
+
+    int errorcode = pthread_rwlock_unlock(rwlock);
+    return errorcode == 0;
+  }
+  return false;
+}
+
+}
+
+#elif defined(LLVM_ON_UNIX)
+#include "Unix/RWMutex.inc"
+#elif defined( LLVM_ON_WIN32)
+#include "Win32/RWMutex.inc"
+#else
+#warning Neither LLVM_ON_UNIX nor LLVM_ON_WIN32 was set in System/Mutex.cpp
+#endif
+#endif
+
diff --git a/lib/System/Unix/RWMutex.inc b/lib/System/Unix/RWMutex.inc
new file mode 100644
index 00000000000..4487d7f83da
--- /dev/null
+++ b/lib/System/Unix/RWMutex.inc
@@ -0,0 +1,43 @@
+//= llvm/System/Unix/RWMutex.inc - Unix Reader/Writer Mutual Exclusion Lock  =//
+// 
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+// 
+//===----------------------------------------------------------------------===//
+//
+// This file implements the Unix specific (non-pthread) RWMutex class.
+//
+//===----------------------------------------------------------------------===//
+
+//===----------------------------------------------------------------------===//
+//=== WARNING: Implementation here must contain only generic UNIX code that
+//===          is guaranteed to work on *all* UNIX variants.
+//===----------------------------------------------------------------------===//
+
+namespace llvm {
+
+using namespace sys;
+
+RWMutex::RWMutex() { }
+
+RWMutex::~RWMutex() { }
+
+bool RWMutex::reader_acquire() {
+  return true;
+}
+
+bool RWMutex::reader_release() {
+  return true;
+}
+
+bool RWMutex::writer_acquire() {
+  return true;
+}
+
+bool RWMutex::writer_release() {
+  return true;
+}
+
+}
diff --git a/lib/System/Win32/Mutex.inc b/lib/System/Win32/Mutex.inc
index 7c1723be73f..85cf0a7f9b5 100644
--- a/lib/System/Win32/Mutex.inc
+++ b/lib/System/Win32/Mutex.inc
@@ -22,7 +22,7 @@
 namespace llvm {
 using namespace sys;
 
-Mutex::Mutex(bool /*recursive*/)
+Mutex::Mutex()
 {
   data_ = new CRITICAL_SECTION;
   InitializeCriticalSection((LPCRITICAL_SECTION)data_);
diff --git a/lib/System/Win32/RWMutex.inc b/lib/System/Win32/RWMutex.inc
new file mode 100644
index 00000000000..08c31f1eb7d
--- /dev/null
+++ b/lib/System/Win32/RWMutex.inc
@@ -0,0 +1,55 @@
+//= llvm/System/Win32/Mutex.inc - Win32 Reader/Writer Mutual Exclusion Lock  =//
+// 
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+// 
+//===----------------------------------------------------------------------===//
+//
+// This file implements the Win32 specific (non-pthread) RWMutex class.
+//
+//===----------------------------------------------------------------------===//
+
+//===----------------------------------------------------------------------===//
+//=== WARNING: Implementation here must contain only generic Win32 code that
+//===          is guaranteed to work on *all* Win32 variants.
+//===----------------------------------------------------------------------===//
+
+#include "Win32.h"
+
+namespace llvm {
+using namespace sys;
+
+RWMutex::RWMutex() {
+  data_ = new PSRWLOCK;
+  InitializeSRWLock((PSRWLOCK*)data_);
+}
+
+RWMutex::~RWMutex() {
+  delete (PSRWLOCK*)data_;
+  data_ = 0;
+}
+
+bool RWMutex::reader_acquire() {
+  AcquireSRWLockShared((PSRWLOCK*)data_);
+  return true;
+}
+
+bool RWMutex::reader_release() {
+  ReleaseSRWLockShared((PSRWLOCK*)data_);
+  return true;
+}
+
+bool RWMutex::writer_acquire() {
+  AcquireSRWLockExclusive((PSRWLOCK*)data_);
+  return true;
+}
+
+bool RWMutex::writer_release() {
+  ReleaseSRWLockExclusive((PSRWLOCK*)data_);
+  return true;
+}
+
+
+}