[C++11] Replace LLVM atomics with std::atomic.

With C++11 we finally have a standardized way to specify atomic operations. Use
them to replace the existing custom implemention. Sadly the translation is not
entirely trivial as std::atomic allows more fine-grained control over the
atomicity. I tried to preserve the old semantics as well as possible.

Differential Revision: http://llvm-reviews.chandlerc.com/D2915

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202730 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/PassSupport.h

@@ -24,8 +24,8 @@
 #include "Pass.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/PassRegistry.h"
-#include "llvm/Support/Atomic.h"
 #include "llvm/Support/Valgrind.h"
+#include <atomic>
 #include <vector>
 
 namespace llvm {
@@ -147,21 +147,21 @@ private:
 };
 
 #define CALL_ONCE_INITIALIZATION(function) \
-  static volatile sys::cas_flag initialized = 0; \
-  sys::cas_flag old_val = sys::CompareAndSwap(&initialized, 1, 0); \
-  if (old_val == 0) { \
+  static std::atomic<int> initialized; \
+  int old_val = 0; \
+  if (initialized.compare_exchange_strong(old_val, 1)) { \
     function(Registry); \
-    sys::MemoryFence(); \
+    std::atomic_thread_fence(std::memory_order_seq_cst); \
     TsanIgnoreWritesBegin(); \
     TsanHappensBefore(&initialized); \
     initialized = 2; \
     TsanIgnoreWritesEnd(); \
   } else { \
-    sys::cas_flag tmp = initialized; \
-    sys::MemoryFence(); \
+    int tmp = initialized.load(); \
+    std::atomic_thread_fence(std::memory_order_seq_cst); \
     while (tmp != 2) { \
-      tmp = initialized; \
-      sys::MemoryFence(); \
+      tmp = initialized.load(); \
+      std::atomic_thread_fence(std::memory_order_seq_cst); \
     } \
   } \
   TsanHappensAfter(&initialized);