1 files changed, 139 insertions, 0 deletions
diff --git a/contrib/llvm-project/libcxx/include/__stop_token/atomic_unique_lock.h b/contrib/llvm-project/libcxx/include/__stop_token/atomic_unique_lock.h
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index 000000000000..6c63a254eab9
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+++ b/contrib/llvm-project/libcxx/include/__stop_token/atomic_unique_lock.h
@@ -0,0 +1,139 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___STOP_TOKEN_ATOMIC_UNIQUE_GUARD_H
+#define _LIBCPP___STOP_TOKEN_ATOMIC_UNIQUE_GUARD_H
+
+#include <__bit/popcount.h>
+#include <__config>
+#include <atomic>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 20
+
+// This class implements an RAII unique_lock without a mutex.
+// It uses std::atomic<State>,
+// where State contains a lock bit and might contain other data,
+// and LockedBit is the value of State when the lock bit is set, e.g  1 << 2
+template <class _State, _State _LockedBit>
+class _LIBCPP_AVAILABILITY_SYNC __atomic_unique_lock {
+  static_assert(std::popcount(_LockedBit) == 1, "LockedBit must be an integer where only one bit is set");
+
+  std::atomic<_State>& __state_;
+  bool __is_locked_;
+
+public:
+  _LIBCPP_HIDE_FROM_ABI explicit __atomic_unique_lock(std::atomic<_State>& __state) noexcept
+      : __state_(__state), __is_locked_(true) {
+    __lock();
+  }
+
+  template <class _Pred>
+  _LIBCPP_HIDE_FROM_ABI __atomic_unique_lock(std::atomic<_State>& __state, _Pred&& __give_up_locking) noexcept
+      : __state_(__state), __is_locked_(false) {
+    __is_locked_ = __lock_impl(__give_up_locking, __set_locked_bit, std::memory_order_acquire);
+  }
+
+  template <class _Pred, class _UnaryFunction>
+  _LIBCPP_HIDE_FROM_ABI __atomic_unique_lock(
+      std::atomic<_State>& __state,
+      _Pred&& __give_up_locking,
+      _UnaryFunction&& __state_after_lock,
+      std::memory_order __locked_ordering) noexcept
+      : __state_(__state), __is_locked_(false) {
+    __is_locked_ = __lock_impl(__give_up_locking, __state_after_lock, __locked_ordering);
+  }
+
+  __atomic_unique_lock(const __atomic_unique_lock&)            = delete;
+  __atomic_unique_lock(__atomic_unique_lock&&)                 = delete;
+  __atomic_unique_lock& operator=(const __atomic_unique_lock&) = delete;
+  __atomic_unique_lock& operator=(__atomic_unique_lock&&)      = delete;
+
+  _LIBCPP_HIDE_FROM_ABI ~__atomic_unique_lock() {
+    if (__is_locked_) {
+      __unlock();
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI bool __owns_lock() const noexcept { return __is_locked_; }
+
+  _LIBCPP_HIDE_FROM_ABI void __lock() noexcept {
+    const auto __never_give_up_locking = [](_State) { return false; };
+    // std::memory_order_acquire because we'd like to make sure that all the read operations after the lock can read the
+    // up-to-date values.
+    __lock_impl(__never_give_up_locking, __set_locked_bit, std::memory_order_acquire);
+    __is_locked_ = true;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI void __unlock() noexcept {
+    // unset the _LockedBit. `memory_order_release` because we need to make sure all the write operations before calling
+    // `__unlock` will be made visible to other threads
+    __state_.fetch_and(static_cast<_State>(~_LockedBit), std::memory_order_release);
+    __state_.notify_all();
+    __is_locked_ = false;
+  }
+
+private:
+  template <class _Pred, class _UnaryFunction>
+  _LIBCPP_HIDE_FROM_ABI bool
+  __lock_impl(_Pred&& __give_up_locking, // while trying to lock the state, if the predicate returns true, give up
+                                         // locking and return
+              _UnaryFunction&& __state_after_lock,
+              std::memory_order __locked_ordering) noexcept {
+    // At this stage, until we exit the inner while loop, other than the atomic state, we are not reading any order
+    // dependent values that is written on other threads, or writing anything that needs to be seen on other threads.
+    // Therefore `memory_order_relaxed` is enough.
+    _State __current_state = __state_.load(std::memory_order_relaxed);
+    do {
+      while (true) {
+        if (__give_up_locking(__current_state)) {
+          // user provided early return condition. fail to lock
+          return false;
+        } else if ((__current_state & _LockedBit) != 0) {
+          // another thread has locked the state, we need to wait
+          __state_.wait(__current_state, std::memory_order_relaxed);
+          // when it is woken up by notifyAll or spuriously, the __state_
+          // might have changed. reload the state
+          // Note that the new state's _LockedBit may or may not equal to 0
+          __current_state = __state_.load(std::memory_order_relaxed);
+        } else {
+          // at least for now, it is not locked. we can try `compare_exchange_weak` to lock it.
+          // Note that the variable `__current_state`'s lock bit has to be 0 at this point.
+          break;
+        }
+      }
+    } while (!__state_.compare_exchange_weak(
+        __current_state, // if __state_ has the same value of __current_state, lock bit must be zero before exchange and
+                         // we are good to lock/exchange and return. If _state has a different value, because other
+                         // threads locked it between the `break` statement above and this statement, exchange will fail
+                         // and go back to the inner while loop above.
+        __state_after_lock(__current_state), // state after lock. Usually it should be __current_state | _LockedBit.
+                                             // Some use cases need to set other bits at the same time as an atomic
+                                             // operation therefore we accept a function
+        __locked_ordering,        // sucessful exchange order. Usually it should be std::memory_order_acquire.
+                                  // Some use cases need more strict ordering therefore we accept it as a parameter
+        std::memory_order_relaxed // fail to exchange order. We don't need any ordering as we are going back to the
+                                  // inner while loop
+        ));
+    return true;
+  }
+
+  _LIBCPP_HIDE_FROM_ABI static constexpr auto __set_locked_bit = [](_State __state) { return __state | _LockedBit; };
+};
+
+#endif // _LIBCPP_STD_VER >= 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___STOP_TOKEN_ATOMIC_UNIQUE_GUARD_H