Index: include/llvm/CodeGen/Passes.h
===================================================================
--- include/llvm/CodeGen/Passes.h
+++ include/llvm/CodeGen/Passes.h
@@ -418,7 +418,7 @@
   Registry.registerPass(*PI, true);                                            \
   return PI;                                                                   \
   }                                                                            \
-  LLVM_DEFINE_ONCE_FLAG(Initialize##passName##PassFlag);                       \
+  static llvm::once_flag Initialize##passName##PassFlag;                       \
   void llvm::initialize##passName##Pass(PassRegistry &Registry) {              \
     llvm::call_once(Initialize##passName##PassFlag,                            \
                     initialize##passName##PassOnce, std::ref(Registry));       \
Index: include/llvm/PassSupport.h
===================================================================
--- include/llvm/PassSupport.h
+++ include/llvm/PassSupport.h
@@ -41,7 +41,7 @@
     Registry.registerPass(*PI, true);                                          \
     return PI;                                                                 \
   }                                                                            \
-  LLVM_DEFINE_ONCE_FLAG(Initialize##passName##PassFlag);                       \
+  static llvm::once_flag Initialize##passName##PassFlag;                       \
   void llvm::initialize##passName##Pass(PassRegistry &Registry) {              \
     llvm::call_once(Initialize##passName##PassFlag,                            \
                     initialize##passName##PassOnce, std::ref(Registry));       \
@@ -61,7 +61,7 @@
   Registry.registerPass(*PI, true);                                            \
   return PI;                                                                   \
   }                                                                            \
-  LLVM_DEFINE_ONCE_FLAG(Initialize##passName##PassFlag);                       \
+  static llvm::once_flag Initialize##passName##PassFlag;                       \
   void llvm::initialize##passName##Pass(PassRegistry &Registry) {              \
     llvm::call_once(Initialize##passName##PassFlag,                            \
                     initialize##passName##PassOnce, std::ref(Registry));       \
@@ -152,7 +152,7 @@
     Registry.registerAnalysisGroup(&agName::ID, 0, *AI, false, true);          \
     return AI;                                                                 \
   }                                                                            \
-  LLVM_DEFINE_ONCE_FLAG(Initialize##agName##AnalysisGroupFlag);                \
+  static llvm::once_flag Initialize##agName##AnalysisGroupFlag;                \
   void llvm::initialize##agName##AnalysisGroup(PassRegistry &Registry) {       \
     llvm::call_once(Initialize##agName##AnalysisGroupFlag,                     \
                     initialize##agName##AnalysisGroupOnce,                     \
@@ -173,7 +173,7 @@
                                    true);                                      \
     return AI;                                                                 \
   }                                                                            \
-  LLVM_DEFINE_ONCE_FLAG(Initialize##passName##PassFlag);                       \
+  static llvm::once_flag Initialize##passName##PassFlag;                       \
   void llvm::initialize##passName##Pass(PassRegistry &Registry) {              \
     llvm::call_once(Initialize##passName##PassFlag,                            \
                     initialize##passName##PassOnce, std::ref(Registry));       \
@@ -194,7 +194,7 @@
   Registry.registerAnalysisGroup(&agName::ID, &passName::ID, *AI, def, true);  \
   return AI;                                                                   \
   }                                                                            \
-  LLVM_DEFINE_ONCE_FLAG(Initialize##passName##PassFlag);                       \
+  static llvm::once_flag Initialize##passName##PassFlag;                       \
   void llvm::initialize##passName##Pass(PassRegistry &Registry) {              \
     llvm::call_once(Initialize##passName##PassFlag,                            \
                     initialize##passName##PassOnce, std::ref(Registry));       \
Index: include/llvm/Support/Threading.h
===================================================================
--- include/llvm/Support/Threading.h
+++ include/llvm/Support/Threading.h
@@ -60,19 +60,18 @@
 
   typedef std::once_flag once_flag;
 
-  /// This macro is the only way you should define your once flag for LLVM's
-  /// call_once.
-#define LLVM_DEFINE_ONCE_FLAG(flag) static once_flag flag
-
 #else
 
   enum InitStatus { Uninitialized = 0, Wait = 1, Done = 2 };
-  typedef volatile sys::cas_flag once_flag;
 
-  /// This macro is the only way you should define your once flag for LLVM's
-  /// call_once.
-#define LLVM_DEFINE_ONCE_FLAG(flag) \
-  static ::llvm::once_flag flag = ::llvm::Uninitialized
+  /// \brief The llvm::once_flag structure
+  ///
+  /// This type is modeled after std::once_flag to use with llvm::call_once.
+  /// This structure must be used as an opaque object. It is a struct to force
+  /// autoinitialization and behave like std::once_flag.
+  struct once_flag {
+    volatile sys::cas_flag status = Uninitialized;
+  };
 
 #endif
 
@@ -82,7 +81,7 @@
   /// \code
   ///   void foo() {...};
   ///   ...
-  ///   LLVM_DEFINE_ONCE_FLAG(flag);
+  ///   static once_flag flag;
   ///   call_once(flag, foo);
   /// \endcode
   ///
@@ -96,24 +95,24 @@
 #else
     // For other platforms we use a generic (if brittle) version based on our
     // atomics.
-    sys::cas_flag old_val = sys::CompareAndSwap(&flag, Wait, Uninitialized);
+    sys::cas_flag old_val = sys::CompareAndSwap(&flag.status, Wait, Uninitialized);
     if (old_val == Uninitialized) {
       std::forward<Function>(F)(std::forward<Args>(ArgList)...);
       sys::MemoryFence();
       TsanIgnoreWritesBegin();
-      TsanHappensBefore(&flag);
-      flag = Done;
+      TsanHappensBefore(&flag.status);
+      flag.status = Done;
       TsanIgnoreWritesEnd();
     } else {
       // Wait until any thread doing the call has finished.
-      sys::cas_flag tmp = flag;
+      sys::cas_flag tmp = flag.status;
       sys::MemoryFence();
       while (tmp != Done) {
-        tmp = flag;
+        tmp = flag.status;
         sys::MemoryFence();
       }
     }
-    TsanHappensAfter(&flag);
+    TsanHappensAfter(&flag.status);
 #endif
   }
 
Index: lib/CodeGen/TargetPassConfig.cpp
===================================================================
--- lib/CodeGen/TargetPassConfig.cpp
+++ lib/CodeGen/TargetPassConfig.cpp
@@ -733,7 +733,7 @@
 
 /// A dummy default pass factory indicates whether the register allocator is
 /// overridden on the command line.
-LLVM_DEFINE_ONCE_FLAG(InitializeDefaultRegisterAllocatorFlag);
+static llvm::once_flag InitializeDefaultRegisterAllocatorFlag;
 static FunctionPass *useDefaultRegisterAllocator() { return nullptr; }
 static RegisterRegAlloc
 defaultRegAlloc("default",
Index: lib/Support/ManagedStatic.cpp
===================================================================
--- lib/Support/ManagedStatic.cpp
+++ lib/Support/ManagedStatic.cpp
@@ -21,7 +21,7 @@
 
 static const ManagedStaticBase *StaticList = nullptr;
 static sys::Mutex *ManagedStaticMutex = nullptr;
-LLVM_DEFINE_ONCE_FLAG(mutex_init_flag);
+static llvm::once_flag mutex_init_flag;
 
 static void initializeMutex() {
   ManagedStaticMutex = new sys::Mutex();
Index: lib/Target/X86/X86InstrFMA3Info.cpp
===================================================================
--- lib/Target/X86/X86InstrFMA3Info.cpp
+++ lib/Target/X86/X86InstrFMA3Info.cpp
@@ -23,7 +23,7 @@
 
 /// This flag is used in the method llvm::call_once() used below to make the
 /// initialization of the map 'OpcodeToGroup' thread safe.
-LLVM_DEFINE_ONCE_FLAG(InitGroupsOnceFlag);
+static llvm::once_flag InitGroupsOnceFlag;
 
 static ManagedStatic<X86InstrFMA3Info> X86InstrFMA3InfoObj;
 X86InstrFMA3Info *X86InstrFMA3Info::getX86InstrFMA3Info() {