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 // -*- 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_STOP_STATE_H
 #define _LIBCPP___STOP_TOKEN_STOP_STATE_H
 
 #include <__assert>
 #include <__config>
 #include <__stop_token/atomic_unique_lock.h>
 #include <__stop_token/intrusive_list_view.h>
 #include <__thread/id.h>
 #include <atomic>
 #include <cstdint>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_THREADS
 
 struct __stop_callback_base : __intrusive_node_base<__stop_callback_base> {
   using __callback_fn_t _LIBCPP_NODEBUG = void(__stop_callback_base*) noexcept;
   _LIBCPP_HIDE_FROM_ABI explicit __stop_callback_base(__callback_fn_t* __callback_fn) : __callback_fn_(__callback_fn) {}
 
   _LIBCPP_HIDE_FROM_ABI void __invoke() noexcept { __callback_fn_(this); }
 
   __callback_fn_t* __callback_fn_;
   atomic<bool> __completed_ = false;
   bool* __destroyed_        = nullptr;
 };
 
 class __stop_state {
   static constexpr uint32_t __stop_requested_bit        = 1;
   static constexpr uint32_t __callback_list_locked_bit  = 1 << 1;
   static constexpr uint32_t __stop_source_counter_shift = 2;
 
   // The "stop_source counter" is not used for lifetime reference counting.
   // When the number of stop_source reaches 0, the remaining stop_tokens's
   // stop_possible will return false. We need this counter to track this.
   //
   // The "callback list locked" bit implements the atomic_unique_lock to
   // guard the operations on the callback list
   //
   //       31 - 2          |  1                   |    0           |
   //  stop_source counter  | callback list locked | stop_requested |
   atomic<uint32_t> __state_ = 0;
 
   // Reference count for stop_token + stop_callback + stop_source
   // When the counter reaches zero, the state is destroyed
   // It is used by __intrusive_shared_ptr, but it is stored here for better layout
   atomic<uint32_t> __ref_count_ = 0;
 
   using __state_t _LIBCPP_NODEBUG            = uint32_t;
   using __callback_list_lock _LIBCPP_NODEBUG = __atomic_unique_lock<__state_t, __callback_list_locked_bit>;
   using __callback_list _LIBCPP_NODEBUG      = __intrusive_list_view<__stop_callback_base>;
 
   __callback_list __callback_list_;
   __thread_id __requesting_thread_;
 
 public:
   _LIBCPP_HIDE_FROM_ABI __stop_state() noexcept = default;
 
   _LIBCPP_HIDE_FROM_ABI void __increment_stop_source_counter() noexcept {
     _LIBCPP_ASSERT_UNCATEGORIZED(
         __state_.load(std::memory_order_relaxed) <= static_cast<__state_t>(~(1 << __stop_source_counter_shift)),
         "stop_source's counter reaches the maximum. Incrementing the counter will overflow");
     __state_.fetch_add(1 << __stop_source_counter_shift, std::memory_order_relaxed);
   }
 
   // We are not destroying the object after counter decrements to zero, nor do we have
   // operations depend on the ordering of decrementing the counter. relaxed is enough.
   _LIBCPP_HIDE_FROM_ABI void __decrement_stop_source_counter() noexcept {
     _LIBCPP_ASSERT_UNCATEGORIZED(
         __state_.load(std::memory_order_relaxed) >= static_cast<__state_t>(1 << __stop_source_counter_shift),
         "stop_source's counter is 0. Decrementing the counter will underflow");
     __state_.fetch_sub(1 << __stop_source_counter_shift, std::memory_order_relaxed);
   }
 
   _LIBCPP_HIDE_FROM_ABI bool __stop_requested() const noexcept {
     // acquire because [thread.stoptoken.intro] A call to request_stop that returns true
     // synchronizes with a call to stop_requested on an associated stop_token or stop_source
     // object that returns true.
     // request_stop's compare_exchange_weak has release which syncs with this acquire
     return (__state_.load(std::memory_order_acquire) & __stop_requested_bit) != 0;
   }
 
   _LIBCPP_HIDE_FROM_ABI bool __stop_possible_for_stop_token() const noexcept {
     // [stoptoken.mem] false if "a stop request was not made and there are no associated stop_source objects"
     // Todo: Can this be std::memory_order_relaxed as the standard does not say anything except not to introduce data
     // race?
     __state_t __curent_state = __state_.load(std::memory_order_acquire);
     return ((__curent_state & __stop_requested_bit) != 0) || ((__curent_state >> __stop_source_counter_shift) != 0);
   }
 
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __request_stop() noexcept {
     auto __cb_list_lock = __try_lock_for_request_stop();
     if (!__cb_list_lock.__owns_lock()) {
       return false;
     }
     __requesting_thread_ = this_thread::get_id();
 
     while (!__callback_list_.__empty()) {
       auto __cb = __callback_list_.__pop_front();
 
       // allow other callbacks to be removed while invoking the current callback
       __cb_list_lock.__unlock();
 
       bool __destroyed   = false;
       __cb->__destroyed_ = &__destroyed;
 
       __cb->__invoke();
 
       // __cb's invoke function could potentially delete itself. We need to check before accessing __cb's member
       if (!__destroyed) {
         // needs to set __destroyed_ pointer to nullptr, otherwise it points to a local variable
         // which is to be destroyed at the end of the loop
         __cb->__destroyed_ = nullptr;
 
         // [stopcallback.cons] If callback is concurrently executing on another thread, then the return
         // from the invocation of callback strongly happens before ([intro.races]) callback is destroyed.
         // this release syncs with the acquire in the remove_callback
         __cb->__completed_.store(true, std::memory_order_release);
         __cb->__completed_.notify_all();
       }
 
       __cb_list_lock.__lock();
     }
 
     return true;
   }
 
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __add_callback(__stop_callback_base* __cb) noexcept {
     // If it is already stop_requested. Do not try to request it again.
     const auto __give_up_trying_to_lock_condition = [__cb](__state_t __state) {
       if ((__state & __stop_requested_bit) != 0) {
         // already stop requested, synchronously run the callback and no need to lock the list again
         __cb->__invoke();
         return true;
       }
       // no stop source. no need to lock the list to add the callback as it can never be invoked
       return (__state >> __stop_source_counter_shift) == 0;
     };
 
     __callback_list_lock __cb_list_lock(__state_, __give_up_trying_to_lock_condition);
 
     if (!__cb_list_lock.__owns_lock()) {
       return false;
     }
 
     __callback_list_.__push_front(__cb);
 
     return true;
     // unlock here: [thread.stoptoken.intro] Registration of a callback synchronizes with the invocation of
     // that callback.
     // Note: this release sync with the acquire in the request_stop' __try_lock_for_request_stop
   }
 
   // called by the destructor of stop_callback
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void __remove_callback(__stop_callback_base* __cb) noexcept {
     __callback_list_lock __cb_list_lock(__state_);
 
     // under below condition, the request_stop call just popped __cb from the list and could execute it now
     bool __potentially_executing_now = __cb->__prev_ == nullptr && !__callback_list_.__is_head(__cb);
 
     if (__potentially_executing_now) {
       auto __requested_thread = __requesting_thread_;
       __cb_list_lock.__unlock();
 
       if (std::this_thread::get_id() != __requested_thread) {
         // [stopcallback.cons] If callback is concurrently executing on another thread, then the return
         // from the invocation of callback strongly happens before ([intro.races]) callback is destroyed.
         __cb->__completed_.wait(false, std::memory_order_acquire);
       } else {
         // The destructor of stop_callback runs on the same thread of the thread that invokes the callback.
         // The callback is potentially invoking its own destuctor. Set the flag to avoid accessing destroyed
         // members on the invoking side
         if (__cb->__destroyed_) {
           *__cb->__destroyed_ = true;
         }
       }
     } else {
       __callback_list_.__remove(__cb);
     }
   }
 
 private:
   _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI __callback_list_lock __try_lock_for_request_stop() noexcept {
     // If it is already stop_requested, do not try to request stop or lock the list again.
     const auto __lock_fail_condition = [](__state_t __state) { return (__state & __stop_requested_bit) != 0; };
 
     // set locked and requested bit at the same time
     const auto __after_lock_state = [](__state_t __state) {
       return __state | __callback_list_locked_bit | __stop_requested_bit;
     };
 
     // acq because [thread.stoptoken.intro] Registration of a callback synchronizes with the invocation of that
     //     callback. We are going to invoke the callback after getting the lock, acquire so that we can see the
     //     registration of a callback (and other writes that happens-before the add_callback)
     //     Note: the rel (unlock) in the add_callback syncs with this acq
     // rel because [thread.stoptoken.intro] A call to request_stop that returns true synchronizes with a call
     //     to stop_requested on an associated stop_token or stop_source object that returns true.
     //     We need to make sure that all writes (including user code) before request_stop will be made visible
     //     to the threads that waiting for `stop_requested == true`
     //     Note: this rel syncs with the acq in `stop_requested`
     const auto __locked_ordering = std::memory_order_acq_rel;
 
     return __callback_list_lock(__state_, __lock_fail_condition, __after_lock_state, __locked_ordering);
   }
 
   template <class _Tp>
   friend struct __intrusive_shared_ptr_traits;
 };
 
 template <class _Tp>
 struct __intrusive_shared_ptr_traits;
 
 template <>
 struct __intrusive_shared_ptr_traits<__stop_state> {
   _LIBCPP_HIDE_FROM_ABI static atomic<uint32_t>& __get_atomic_ref_count(__stop_state& __state) {
     return __state.__ref_count_;
   }
 };
 
 #endif // _LIBCPP_STD_VER >= 20 && _LIBCPP_HAS_THREADS
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___STOP_TOKEN_STOP_STATE_H

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