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 #ifndef _LIBCPP___ATOMIC_ATOMIC_SYNC_H 10 #define _LIBCPP___ATOMIC_ATOMIC_SYNC_H 11 12 #include <__atomic/contention_t.h> 13 #include <__atomic/memory_order.h> 14 #include <__atomic/to_gcc_order.h> 15 #include <__chrono/duration.h> 16 #include <__config> 17 #include <__memory/addressof.h> 18 #include <__thread/poll_with_backoff.h> 19 #include <__type_traits/conjunction.h> 20 #include <__type_traits/decay.h> 21 #include <__type_traits/invoke.h> 22 #include <__type_traits/void_t.h> 23 #include <__utility/declval.h> 24 #include <cstring> 25 26 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 27 # pragma GCC system_header 28 #endif 29 30 _LIBCPP_BEGIN_NAMESPACE_STD 31 32 // The customisation points to enable the following functions: 33 // - __atomic_wait 34 // - __atomic_wait_unless 35 // - __atomic_notify_one 36 // - __atomic_notify_all 37 // Note that std::atomic<T>::wait was back-ported to C++03 38 // The below implementations look ugly to support C++03 39 template <class _Tp, class = void> 40 struct __atomic_waitable_traits { 41 template <class _AtomicWaitable> 42 static void __atomic_load(_AtomicWaitable&&, memory_order) = delete; 43 44 template <class _AtomicWaitable> 45 static void __atomic_contention_address(_AtomicWaitable&&) = delete; 46 }; 47 48 template <class _Tp, class = void> 49 struct __atomic_waitable : false_type {}; 50 51 template <class _Tp> 52 struct __atomic_waitable< _Tp, 53 __void_t<decltype(__atomic_waitable_traits<__decay_t<_Tp> >::__atomic_load( 54 std::declval<const _Tp&>(), std::declval<memory_order>())), 55 decltype(__atomic_waitable_traits<__decay_t<_Tp> >::__atomic_contention_address( 56 std::declval<const _Tp&>()))> > : true_type {}; 57 58 #if _LIBCPP_STD_VER >= 20 59 # if _LIBCPP_HAS_THREADS 60 61 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_one(void const volatile*) _NOEXCEPT; 62 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void __cxx_atomic_notify_all(void const volatile*) _NOEXCEPT; 63 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t 64 __libcpp_atomic_monitor(void const volatile*) _NOEXCEPT; 65 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void 66 __libcpp_atomic_wait(void const volatile*, __cxx_contention_t) _NOEXCEPT; 67 68 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void 69 __cxx_atomic_notify_one(__cxx_atomic_contention_t const volatile*) _NOEXCEPT; 70 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void 71 __cxx_atomic_notify_all(__cxx_atomic_contention_t const volatile*) _NOEXCEPT; 72 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI __cxx_contention_t 73 __libcpp_atomic_monitor(__cxx_atomic_contention_t const volatile*) _NOEXCEPT; 74 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI void 75 __libcpp_atomic_wait(__cxx_atomic_contention_t const volatile*, __cxx_contention_t) _NOEXCEPT; 76 77 template <class _AtomicWaitable, class _Poll> 78 struct __atomic_wait_backoff_impl { 79 const _AtomicWaitable& __a_; 80 _Poll __poll_; 81 memory_order __order_; 82 83 using __waitable_traits _LIBCPP_NODEBUG = __atomic_waitable_traits<__decay_t<_AtomicWaitable> >; 84 85 _LIBCPP_AVAILABILITY_SYNC 86 _LIBCPP_HIDE_FROM_ABI bool 87 __update_monitor_val_and_poll(__cxx_atomic_contention_t const volatile*, __cxx_contention_t& __monitor_val) const { 88 // In case the contention type happens to be __cxx_atomic_contention_t, i.e. __cxx_atomic_impl<int64_t>, 89 // the platform wait is directly monitoring the atomic value itself. 90 // `__poll_` takes the current value of the atomic as an in-out argument 91 // to potentially modify it. After it returns, `__monitor` has a value 92 // which can be safely waited on by `std::__libcpp_atomic_wait` without any 93 // ABA style issues. 94 __monitor_val = __waitable_traits::__atomic_load(__a_, __order_); 95 return __poll_(__monitor_val); 96 } 97 98 _LIBCPP_AVAILABILITY_SYNC 99 _LIBCPP_HIDE_FROM_ABI bool 100 __update_monitor_val_and_poll(void const volatile* __contention_address, __cxx_contention_t& __monitor_val) const { 101 // In case the contention type is anything else, platform wait is monitoring a __cxx_atomic_contention_t 102 // from the global pool, the monitor comes from __libcpp_atomic_monitor 103 __monitor_val = std::__libcpp_atomic_monitor(__contention_address); 104 auto __current_val = __waitable_traits::__atomic_load(__a_, __order_); 105 return __poll_(__current_val); 106 } 107 108 _LIBCPP_AVAILABILITY_SYNC 109 _LIBCPP_HIDE_FROM_ABI bool operator()(chrono::nanoseconds __elapsed) const { 110 if (__elapsed > chrono::microseconds(4)) { 111 auto __contention_address = __waitable_traits::__atomic_contention_address(__a_); 112 __cxx_contention_t __monitor_val; 113 if (__update_monitor_val_and_poll(__contention_address, __monitor_val)) 114 return true; 115 std::__libcpp_atomic_wait(__contention_address, __monitor_val); 116 } else { 117 } // poll 118 return false; 119 } 120 }; 121 122 // The semantics of this function are similar to `atomic`'s 123 // `.wait(T old, std::memory_order order)`, but instead of having a hardcoded 124 // predicate (is the loaded value unequal to `old`?), the predicate function is 125 // specified as an argument. The loaded value is given as an in-out argument to 126 // the predicate. If the predicate function returns `true`, 127 // `__atomic_wait_unless` will return. If the predicate function returns 128 // `false`, it must set the argument to its current understanding of the atomic 129 // value. The predicate function must not return `false` spuriously. 130 template <class _AtomicWaitable, class _Poll> 131 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void 132 __atomic_wait_unless(const _AtomicWaitable& __a, memory_order __order, _Poll&& __poll) { 133 static_assert(__atomic_waitable<_AtomicWaitable>::value, ""); 134 __atomic_wait_backoff_impl<_AtomicWaitable, __decay_t<_Poll> > __backoff_fn = {__a, __poll, __order}; 135 std::__libcpp_thread_poll_with_backoff( 136 /* poll */ 137 [&]() { 138 auto __current_val = __atomic_waitable_traits<__decay_t<_AtomicWaitable> >::__atomic_load(__a, __order); 139 return __poll(__current_val); 140 }, 141 /* backoff */ __backoff_fn); 142 } 143 144 template <class _AtomicWaitable> 145 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void __atomic_notify_one(const _AtomicWaitable& __a) { 146 static_assert(__atomic_waitable<_AtomicWaitable>::value, ""); 147 std::__cxx_atomic_notify_one(__atomic_waitable_traits<__decay_t<_AtomicWaitable> >::__atomic_contention_address(__a)); 148 } 149 150 template <class _AtomicWaitable> 151 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void __atomic_notify_all(const _AtomicWaitable& __a) { 152 static_assert(__atomic_waitable<_AtomicWaitable>::value, ""); 153 std::__cxx_atomic_notify_all(__atomic_waitable_traits<__decay_t<_AtomicWaitable> >::__atomic_contention_address(__a)); 154 } 155 156 # else // _LIBCPP_HAS_THREADS 157 158 template <class _AtomicWaitable, class _Poll> 159 _LIBCPP_HIDE_FROM_ABI void __atomic_wait_unless(const _AtomicWaitable& __a, memory_order __order, _Poll&& __poll) { 160 std::__libcpp_thread_poll_with_backoff( 161 /* poll */ 162 [&]() { 163 auto __current_val = __atomic_waitable_traits<__decay_t<_AtomicWaitable> >::__atomic_load(__a, __order); 164 return __poll(__current_val); 165 }, 166 /* backoff */ __spinning_backoff_policy()); 167 } 168 169 template <class _AtomicWaitable> 170 _LIBCPP_HIDE_FROM_ABI void __atomic_notify_one(const _AtomicWaitable&) {} 171 172 template <class _AtomicWaitable> 173 _LIBCPP_HIDE_FROM_ABI void __atomic_notify_all(const _AtomicWaitable&) {} 174 175 # endif // _LIBCPP_HAS_THREADS 176 177 template <typename _Tp> 178 _LIBCPP_HIDE_FROM_ABI bool __cxx_nonatomic_compare_equal(_Tp const& __lhs, _Tp const& __rhs) { 179 return std::memcmp(std::addressof(__lhs), std::addressof(__rhs), sizeof(_Tp)) == 0; 180 } 181 182 template <class _AtomicWaitable, class _Tp> 183 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void 184 __atomic_wait(_AtomicWaitable& __a, _Tp __val, memory_order __order) { 185 static_assert(__atomic_waitable<_AtomicWaitable>::value, ""); 186 std::__atomic_wait_unless(__a, __order, [&](_Tp const& __current) { 187 return !std::__cxx_nonatomic_compare_equal(__current, __val); 188 }); 189 } 190 191 #endif // C++20 192 193 _LIBCPP_END_NAMESPACE_STD 194 195 #endif // _LIBCPP___ATOMIC_ATOMIC_SYNC_H 196