diff --git a/libcxx/src/mutex.cpp b/libcxx/src/mutex.cpp
--- a/libcxx/src/mutex.cpp
+++ b/libcxx/src/mutex.cpp
@@ -200,61 +200,65 @@
 _LIBCPP_SAFE_STATIC static __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
 #endif
 
-void __call_once(volatile once_flag::_State_type& flag, void* arg,
-                 void (*func)(void*))
-{
-#if defined(_LIBCPP_HAS_NO_THREADS)
-    if (flag == 0)
-    {
-#ifndef _LIBCPP_NO_EXCEPTIONS
-        try
-        {
-#endif // _LIBCPP_NO_EXCEPTIONS
-            flag = 1;
-            func(arg);
-            flag = ~once_flag::_State_type(0);
-#ifndef _LIBCPP_NO_EXCEPTIONS
-        }
-        catch (...)
-        {
-            flag = 0;
-            throw;
-        }
-#endif // _LIBCPP_NO_EXCEPTIONS
-    }
+/// Returns true if it's already been called, and false otherwise
+static inline bool call_once_prep(volatile once_flag::_State_type& flag) {
+#ifdef _LIBCPP_HAS_NO_THREADS
+  if (flag != 0)
+    return true;
+  flag = 1;
+  return false;
 #else // !_LIBCPP_HAS_NO_THREADS
-    __libcpp_mutex_lock(&mut);
-    while (flag == 1)
-        __libcpp_condvar_wait(&cv, &mut);
-    if (flag == 0)
-    {
-#ifndef _LIBCPP_NO_EXCEPTIONS
-        try
-        {
-#endif // _LIBCPP_NO_EXCEPTIONS
-            __libcpp_relaxed_store(&flag, once_flag::_State_type(1));
-            __libcpp_mutex_unlock(&mut);
-            func(arg);
-            __libcpp_mutex_lock(&mut);
-            __libcpp_atomic_store(&flag, ~once_flag::_State_type(0),
-                                  _AO_Release);
-            __libcpp_mutex_unlock(&mut);
-            __libcpp_condvar_broadcast(&cv);
-#ifndef _LIBCPP_NO_EXCEPTIONS
-        }
-        catch (...)
-        {
-            __libcpp_mutex_lock(&mut);
-            __libcpp_relaxed_store(&flag, once_flag::_State_type(0));
-            __libcpp_mutex_unlock(&mut);
-            __libcpp_condvar_broadcast(&cv);
-            throw;
-        }
-#endif // _LIBCPP_NO_EXCEPTIONS
-    }
-    else
-        __libcpp_mutex_unlock(&mut);
+  __libcpp_mutex_lock(&mut);
+  while (flag == 1)
+    __libcpp_condvar_wait(&cv, &mut);
+
+  if (flag != 0) {
+    __libcpp_mutex_unlock(&mut);
+    return true;
+  }
+
+  __libcpp_relaxed_store(&flag, once_flag::_State_type(1));
+  __libcpp_mutex_unlock(&mut);
+  return false;
 #endif // !_LIBCPP_HAS_NO_THREADS
 }
 
+static inline void call_once_finish(volatile once_flag::_State_type& flag) {
+#ifdef _LIBCPP_HAS_NO_THREADS
+  flag = ~once_flag::_State_type(0);
+#else // !_LIBCPP_HAS_NO_THREADS
+  __libcpp_mutex_lock(&mut);
+  __libcpp_atomic_store(&flag, ~once_flag::_State_type(0), _AO_Release);
+  __libcpp_mutex_unlock(&mut);
+  __libcpp_condvar_broadcast(&cv);
+#endif // !_LIBCPP_HAS_NO_THREADS
+}
+
+static inline void call_once_abort(volatile once_flag::_State_type& flag) {
+#ifdef _LIBCPP_HAS_NO_THREADS
+  flag = 0;
+#else // !_LIBCPP_HAS_NO_THREADS
+  __libcpp_mutex_lock(&mut);
+  __libcpp_relaxed_store(&flag, once_flag::_State_type(0));
+  __libcpp_mutex_unlock(&mut);
+  __libcpp_condvar_broadcast(&cv);
+#endif // !_LIBCPP_HAS_NO_THREADS
+}
+
+void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(void*)) {
+  if (call_once_prep(flag))
+    return;
+#ifdef _LIBCPP_NO_EXCEPTIONS
+  func(arg);
+#else // !_LIBCPP_NO_EXCEPTIONS
+  try {
+    func(arg);
+  } catch (...) {
+    call_once_abort(flag);
+    throw;
+  }
+#endif // !_LIBCPP_NO_EXCEPTIONS
+  call_once_finish(flag);
+}
+
 _LIBCPP_END_NAMESPACE_STD