[libcxx-commits] [libcxx] [libc++] Replace mutex+condvar with atomics in __call_once (PR #192433)

[Nikolas Klauser via libcxx-commits]

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@@ -42,27 +45,38 @@ void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(
 
 #else // !_LIBCPP_HAS_THREADS
 
-  __libcpp_mutex_lock(&mut);
-  while (flag == once_flag::_Pending)
-    __libcpp_condvar_wait(&cv, &mut);
-  if (flag == once_flag::_Unset) {
-    auto guard = std::__make_exception_guard([&flag] {
-      __libcpp_mutex_lock(&mut);
-      __libcpp_relaxed_store(&flag, once_flag::_Unset);
-      __libcpp_mutex_unlock(&mut);
-      __libcpp_condvar_broadcast(&cv);
-    });
-
-    __libcpp_relaxed_store(&flag, once_flag::_Pending);
-    __libcpp_mutex_unlock(&mut);
-    func(arg);
-    __libcpp_mutex_lock(&mut);
-    __libcpp_atomic_store(&flag, once_flag::_Complete, _AO_Release);
-    __libcpp_mutex_unlock(&mut);
-    __libcpp_condvar_broadcast(&cv);
-    guard.__complete();
-  } else {
-    __libcpp_mutex_unlock(&mut);
+  auto flag_read = __atomic_load_n(&flag, __ATOMIC_ACQUIRE);
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philnik777 wrote:

Can we just use `std::atomic` instead? I don't think there is a reason we can't and that's way nicer to read and easier to understand.

https://github.com/llvm/llvm-project/pull/192433