1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include <__config>
10#include <__mutex/once_flag.h>
11#include <__utility/exception_guard.h>
12
13#if _LIBCPP_HAS_THREADS
14# include <__thread/support.h>
15#endif
16
17#include "include/atomic_support.h"
18
19_LIBCPP_BEGIN_NAMESPACE_STD
20
21// If dispatch_once_f ever handles C++ exceptions, and if one can get to it
22// without illegal macros (unexpected macros not beginning with _UpperCase or
23// __lowercase), and if it stops spinning waiting threads, then call_once should
24// call into dispatch_once_f instead of here. Relevant radar this code needs to
25// keep in sync with: 7741191.
26
27#if _LIBCPP_HAS_THREADS
28static constinit __libcpp_mutex_t mut = _LIBCPP_MUTEX_INITIALIZER;
29static constinit __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
30#endif
31
32void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(void*)) {
33#if !_LIBCPP_HAS_THREADS
34
35 if (flag == once_flag::_Unset) {
36 auto guard = std::__make_exception_guard([&flag] { flag = once_flag::_Unset; });
37 flag = once_flag::_Pending;
38 func(arg);
39 flag = once_flag::_Complete;
40 guard.__complete();
41 }
42
43#else // !_LIBCPP_HAS_THREADS
44
45 __libcpp_mutex_lock(m: &mut);
46 while (flag == once_flag::_Pending)
47 __libcpp_condvar_wait(cv: &cv, m: &mut);
48 if (flag == once_flag::_Unset) {
49 auto guard = std::__make_exception_guard(rollback: [&flag] {
50 __libcpp_mutex_lock(m: &mut);
51 __libcpp_relaxed_store(dest: &flag, val: once_flag::_Unset);
52 __libcpp_mutex_unlock(m: &mut);
53 __libcpp_condvar_broadcast(cv: &cv);
54 });
55
56 __libcpp_relaxed_store(dest: &flag, val: once_flag::_Pending);
57 __libcpp_mutex_unlock(m: &mut);
58 func(arg);
59 __libcpp_mutex_lock(m: &mut);
60 __libcpp_atomic_store(dest: &flag, val: once_flag::_Complete, order: _AO_Release);
61 __libcpp_mutex_unlock(m: &mut);
62 __libcpp_condvar_broadcast(cv: &cv);
63 guard.__complete();
64 } else {
65 __libcpp_mutex_unlock(m: &mut);
66 }
67
68#endif // !_LIBCPP_HAS_THREADS
69}
70
71_LIBCPP_END_NAMESPACE_STD
72