Add an Atomic.h to the System library, for providing a platform independent API

to low-level sync operations.

The only one present at the moment is MemoryFence(), and only for the platforms
for which I could easily discern the proper way to do it.  If your favorite platform
isn't represented, patches are welcome!

llvm-svn: 71770

llvm/include/llvm/System/Atomic.h

@@ -0,0 +1,45 @@
+//===- llvm/System/Atomic.h - Atomic Operations -----------------*- 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 atomic operations.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Config/config.h"
+
+#ifdef __APPLE__
+#include <libkern/OSAtomic.h>
+#elif LLVM_ON_WIN32
+#include <windows.h>
+#endif
+
+
+#ifndef LLVM_SYSTEM_ATOMIC_H
+#define LLVM_SYSTEM_ATOMIC_H
+
+namespace llvm {
+  namespace sys {
+    inline void MemoryFence() {
+#if !defined(ENABLE_THREADS) || ENABLE_THREADS == 0
+      return;
+#elif __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 1)
+      __sync_synchronize();
+#elif defined(__APPLE__)
+      OSMemoryBarrier();
+#elif defined(LLVM_ON_WIN32)
+#warning Memory fence implementation requires Windows 2003 or later.
+      MemoryBarrier();
+#else
+#warning No memory fence implementation found for you platform!
+#endif
+    }
+  }
+}
+
+#endif