shared_ptr_atomic.h

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// shared_ptr atomic access -*- C++ -*-
 
// Copyright (C) 2014-2023 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file bits/shared_ptr_atomic.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{memory}
 */
 
#ifndef _SHARED_PTR_ATOMIC_H
#define _SHARED_PTR_ATOMIC_H 1
 
#include <bits/atomic_base.h>
 
// Annotations for the custom locking in atomic<shared_ptr<T>>.
#if defined _GLIBCXX_TSAN && __has_include(<sanitizer/tsan_interface.h>)
#include <sanitizer/tsan_interface.h>
#define _GLIBCXX_TSAN_MUTEX_DESTROY(X) \
  __tsan_mutex_destroy(X, __tsan_mutex_not_static)
#define _GLIBCXX_TSAN_MUTEX_TRY_LOCK(X) \
  __tsan_mutex_pre_lock(X, __tsan_mutex_not_static|__tsan_mutex_try_lock)
#define _GLIBCXX_TSAN_MUTEX_TRY_LOCK_FAILED(X) __tsan_mutex_post_lock(X, \
    __tsan_mutex_not_static|__tsan_mutex_try_lock_failed, 0)
#define _GLIBCXX_TSAN_MUTEX_LOCKED(X) \
  __tsan_mutex_post_lock(X, __tsan_mutex_not_static, 0)
#define _GLIBCXX_TSAN_MUTEX_PRE_UNLOCK(X) __tsan_mutex_pre_unlock(X, 0)
#define _GLIBCXX_TSAN_MUTEX_POST_UNLOCK(X) __tsan_mutex_post_unlock(X, 0)
#define _GLIBCXX_TSAN_MUTEX_PRE_SIGNAL(X) __tsan_mutex_pre_signal(X, 0)
#define _GLIBCXX_TSAN_MUTEX_POST_SIGNAL(X) __tsan_mutex_post_signal(X, 0)
#else
#define _GLIBCXX_TSAN_MUTEX_DESTROY(X)
#define _GLIBCXX_TSAN_MUTEX_TRY_LOCK(X)
#define _GLIBCXX_TSAN_MUTEX_TRY_LOCK_FAILED(X)
#define _GLIBCXX_TSAN_MUTEX_LOCKED(X)
#define _GLIBCXX_TSAN_MUTEX_PRE_UNLOCK(X)
#define _GLIBCXX_TSAN_MUTEX_POST_UNLOCK(X)
#define _GLIBCXX_TSAN_MUTEX_PRE_SIGNAL(X)
#define _GLIBCXX_TSAN_MUTEX_POST_SIGNAL(X)
#endif
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   * @addtogroup pointer_abstractions
   * @relates shared_ptr
   * @{
   */
 
  /// @cond undocumented
 
  struct _Sp_locker
  {
    _Sp_locker(const _Sp_locker&) = delete;
    _Sp_locker& operator=(const _Sp_locker&) = delete;
 
#ifdef __GTHREADS
    explicit
    _Sp_locker(const void*) noexcept;
    _Sp_locker(const void*, const void*) noexcept;
    ~_Sp_locker();
 
  private:
    unsigned char _M_key1;
    unsigned char _M_key2;
#else
    explicit _Sp_locker(const void*, const void* = nullptr) { }
#endif
  };
 
  /// @endcond
 
  /**
   *  @brief  Report whether shared_ptr atomic operations are lock-free.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @return True if atomic access to @c *__p is lock-free, false otherwise.
   *  @{
  */
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    atomic_is_lock_free(const __shared_ptr<_Tp, _Lp>* __p)
    {
#ifdef __GTHREADS
      return __gthread_active_p() == 0;
#else
      return true;
#endif
    }
 
  template<typename _Tp>
    inline bool
    atomic_is_lock_free(const shared_ptr<_Tp>* __p)
    { return std::atomic_is_lock_free<_Tp, __default_lock_policy>(__p); }
 
  /// @}
 
  /**
   *  @brief  Atomic load for shared_ptr objects.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @return @c *__p
   *
   *  The memory order shall not be `memory_order_release` or
   *  `memory_order_acq_rel`.
   *  @{
  */
  template<typename _Tp>
    inline shared_ptr<_Tp>
    atomic_load_explicit(const shared_ptr<_Tp>* __p, memory_order)
    {
      _Sp_locker __lock{__p};
      return *__p;
    }
 
  template<typename _Tp>
    inline shared_ptr<_Tp>
    atomic_load(const shared_ptr<_Tp>* __p)
    { return std::atomic_load_explicit(__p, memory_order_seq_cst); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    atomic_load_explicit(const __shared_ptr<_Tp, _Lp>* __p, memory_order)
    {
      _Sp_locker __lock{__p};
      return *__p;
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    atomic_load(const __shared_ptr<_Tp, _Lp>* __p)
    { return std::atomic_load_explicit(__p, memory_order_seq_cst); }
  /// @}
 
  /**
   *  @brief  Atomic store for shared_ptr objects.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @param  __r The value to store.
   *
   *  The memory order shall not be `memory_order_acquire` or
   *  `memory_order_acq_rel`.
   *  @{
  */
  template<typename _Tp>
    inline void
    atomic_store_explicit(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r,
                          memory_order)
    {
      _Sp_locker __lock{__p};
      __p->swap(__r); // use swap so that **__p not destroyed while lock held
    }
 
  template<typename _Tp>
    inline void
    atomic_store(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r)
    { std::atomic_store_explicit(__p, std::move(__r), memory_order_seq_cst); }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    atomic_store_explicit(__shared_ptr<_Tp, _Lp>* __p,
                          __shared_ptr<_Tp, _Lp> __r,
                          memory_order)
    {
      _Sp_locker __lock{__p};
      __p->swap(__r); // use swap so that **__p not destroyed while lock held
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline void
    atomic_store(__shared_ptr<_Tp, _Lp>* __p, __shared_ptr<_Tp, _Lp> __r)
    { std::atomic_store_explicit(__p, std::move(__r), memory_order_seq_cst); }
  /// @}
 
  /**
   *  @brief  Atomic exchange for shared_ptr objects.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @param  __r New value to store in `*__p`.
   *  @return The original value of `*__p`
   *  @{
  */
  template<typename _Tp>
    inline shared_ptr<_Tp>
    atomic_exchange_explicit(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r,
                             memory_order)
    {
      _Sp_locker __lock{__p};
      __p->swap(__r);
      return __r;
    }
 
  template<typename _Tp>
    inline shared_ptr<_Tp>
    atomic_exchange(shared_ptr<_Tp>* __p, shared_ptr<_Tp> __r)
    {
      return std::atomic_exchange_explicit(__p, std::move(__r),
                                           memory_order_seq_cst);
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    atomic_exchange_explicit(__shared_ptr<_Tp, _Lp>* __p,
                             __shared_ptr<_Tp, _Lp> __r,
                             memory_order)
    {
      _Sp_locker __lock{__p};
      __p->swap(__r);
      return __r;
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline __shared_ptr<_Tp, _Lp>
    atomic_exchange(__shared_ptr<_Tp, _Lp>* __p, __shared_ptr<_Tp, _Lp> __r)
    {
      return std::atomic_exchange_explicit(__p, std::move(__r),
                                           memory_order_seq_cst);
    }
  /// @}
 
  /**
   *  @brief  Atomic compare-and-swap for shared_ptr objects.
   *  @param  __p A non-null pointer to a shared_ptr object.
   *  @param  __v A non-null pointer to a shared_ptr object.
   *  @param  __w A non-null pointer to a shared_ptr object.
   *  @return True if `*__p` was equivalent to `*__v`, false otherwise.
   *
   *  The memory order for failure shall not be `memory_order_release` or
   *  `memory_order_acq_rel`.
   *  @{
  */
  template<typename _Tp>
    bool
    atomic_compare_exchange_strong_explicit(shared_ptr<_Tp>* __p,
                                            shared_ptr<_Tp>* __v,
                                            shared_ptr<_Tp> __w,
                                            memory_order,
                                            memory_order)
    {
      shared_ptr<_Tp> __x; // goes out of scope after __lock
      _Sp_locker __lock{__p, __v};
      owner_less<shared_ptr<_Tp>> __less;
      if (*__p == *__v && !__less(*__p, *__v) && !__less(*__v, *__p))
        {
          __x = std::move(*__p);
          *__p = std::move(__w);
          return true;
        }
      __x = std::move(*__v);
      *__v = *__p;
      return false;
    }
 
  template<typename _Tp>
    inline bool
    atomic_compare_exchange_strong(shared_ptr<_Tp>* __p, shared_ptr<_Tp>* __v,
                                 shared_ptr<_Tp> __w)
    {
      return std::atomic_compare_exchange_strong_explicit(__p, __v,
          std::move(__w), memory_order_seq_cst, memory_order_seq_cst);
    }
 
  template<typename _Tp>
    inline bool
    atomic_compare_exchange_weak_explicit(shared_ptr<_Tp>* __p,
                                          shared_ptr<_Tp>* __v,
                                          shared_ptr<_Tp> __w,
                                          memory_order __success,
                                          memory_order __failure)
    {
      return std::atomic_compare_exchange_strong_explicit(__p, __v,
          std::move(__w), __success, __failure);
    }
 
  template<typename _Tp>
    inline bool
    atomic_compare_exchange_weak(shared_ptr<_Tp>* __p, shared_ptr<_Tp>* __v,
                                 shared_ptr<_Tp> __w)
    {
      return std::atomic_compare_exchange_weak_explicit(__p, __v,
          std::move(__w), memory_order_seq_cst, memory_order_seq_cst);
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    bool
    atomic_compare_exchange_strong_explicit(__shared_ptr<_Tp, _Lp>* __p,
                                            __shared_ptr<_Tp, _Lp>* __v,
                                            __shared_ptr<_Tp, _Lp> __w,
                                            memory_order,
                                            memory_order)
    {
      __shared_ptr<_Tp, _Lp> __x; // goes out of scope after __lock
      _Sp_locker __lock{__p, __v};
      owner_less<__shared_ptr<_Tp, _Lp>> __less;
      if (*__p == *__v && !__less(*__p, *__v) && !__less(*__v, *__p))
        {
          __x = std::move(*__p);
          *__p = std::move(__w);
          return true;
        }
      __x = std::move(*__v);
      *__v = *__p;
      return false;
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    atomic_compare_exchange_strong(__shared_ptr<_Tp, _Lp>* __p,
                                   __shared_ptr<_Tp, _Lp>* __v,
                                   __shared_ptr<_Tp, _Lp> __w)
    {
      return std::atomic_compare_exchange_strong_explicit(__p, __v,
          std::move(__w), memory_order_seq_cst, memory_order_seq_cst);
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    atomic_compare_exchange_weak_explicit(__shared_ptr<_Tp, _Lp>* __p,
                                          __shared_ptr<_Tp, _Lp>* __v,
                                          __shared_ptr<_Tp, _Lp> __w,
                                          memory_order __success,
                                          memory_order __failure)
    {
      return std::atomic_compare_exchange_strong_explicit(__p, __v,
          std::move(__w), __success, __failure);
    }
 
  template<typename _Tp, _Lock_policy _Lp>
    inline bool
    atomic_compare_exchange_weak(__shared_ptr<_Tp, _Lp>* __p,
                                 __shared_ptr<_Tp, _Lp>* __v,
                                 __shared_ptr<_Tp, _Lp> __w)
    {
      return std::atomic_compare_exchange_weak_explicit(__p, __v,
          std::move(__w), memory_order_seq_cst, memory_order_seq_cst);
    }
  /// @}
 
  /// @} group pointer_abstractions
 
#if __cplusplus >= 202002L
# define __cpp_lib_atomic_shared_ptr 201711L
  template<typename _Tp>
    class atomic;
 
  /**
   * @addtogroup pointer_abstractions
   * @relates shared_ptr
   * @{
   */
 
  template<typename _Up>
    static constexpr bool __is_shared_ptr = false;
  template<typename _Up>
    static constexpr bool __is_shared_ptr<shared_ptr<_Up>> = true;
 
  template<typename _Tp>
    class _Sp_atomic
    {
      using value_type = _Tp;
 
      friend class atomic<_Tp>;
 
      // An atomic version of __shared_count<> and __weak_count<>.
      // Stores a _Sp_counted_base<>* but uses the LSB as a lock.
      struct _Atomic_count
      {
        // Either __shared_count<> or __weak_count<>
        using __count_type = decltype(_Tp::_M_refcount);
 
        // _Sp_counted_base<>*
        using pointer = decltype(__count_type::_M_pi);
 
        // Ensure we can use the LSB as the lock bit.
        static_assert(alignof(remove_pointer_t<pointer>) > 1);
 
        constexpr _Atomic_count() noexcept = default;
 
        explicit
        _Atomic_count(__count_type&& __c) noexcept
        : _M_val(reinterpret_cast<uintptr_t>(__c._M_pi))
        {
          __c._M_pi = nullptr;
        }
 
        ~_Atomic_count()
        {
          auto __val = _M_val.load(memory_order_relaxed);
          _GLIBCXX_TSAN_MUTEX_DESTROY(&_M_val);
          __glibcxx_assert(!(__val & _S_lock_bit));
          if (auto __pi = reinterpret_cast<pointer>(__val))
            {
              if constexpr (__is_shared_ptr<_Tp>)
                __pi->_M_release();
              else
                __pi->_M_weak_release();
            }
        }
 
        _Atomic_count(const _Atomic_count&) = delete;
        _Atomic_count& operator=(const _Atomic_count&) = delete;
 
        // Precondition: Caller does not hold lock!
        // Returns the raw pointer value without the lock bit set.
        pointer
        lock(memory_order __o) const noexcept
        {
          // To acquire the lock we flip the LSB from 0 to 1.
 
          auto __current = _M_val.load(memory_order_relaxed);
          while (__current & _S_lock_bit)
            {
#if __cpp_lib_atomic_wait
              __detail::__thread_relax();
#endif
              __current = _M_val.load(memory_order_relaxed);
            }
 
          _GLIBCXX_TSAN_MUTEX_TRY_LOCK(&_M_val);
 
          while (!_M_val.compare_exchange_strong(__current,
                                                 __current | _S_lock_bit,
                                                 __o,
                                                 memory_order_relaxed))
            {
              _GLIBCXX_TSAN_MUTEX_TRY_LOCK_FAILED(&_M_val);
#if __cpp_lib_atomic_wait
              __detail::__thread_relax();
#endif
              __current = __current & ~_S_lock_bit;
              _GLIBCXX_TSAN_MUTEX_TRY_LOCK(&_M_val);
            }
          _GLIBCXX_TSAN_MUTEX_LOCKED(&_M_val);
          return reinterpret_cast<pointer>(__current);
        }
 
        // Precondition: caller holds lock!
        void
        unlock(memory_order __o) const noexcept
        {
          _GLIBCXX_TSAN_MUTEX_PRE_UNLOCK(&_M_val);
          _M_val.fetch_sub(1, __o);
          _GLIBCXX_TSAN_MUTEX_POST_UNLOCK(&_M_val);
        }
 
        // Swaps the values of *this and __c, and unlocks *this.
        // Precondition: caller holds lock!
        void
        _M_swap_unlock(__count_type& __c, memory_order __o) noexcept
        {
          if (__o != memory_order_seq_cst)
            __o = memory_order_release;
          auto __x = reinterpret_cast<uintptr_t>(__c._M_pi);
          _GLIBCXX_TSAN_MUTEX_PRE_UNLOCK(&_M_val);
          __x = _M_val.exchange(__x, __o);
          _GLIBCXX_TSAN_MUTEX_POST_UNLOCK(&_M_val);
          __c._M_pi = reinterpret_cast<pointer>(__x & ~_S_lock_bit);
        }
 
#if __cpp_lib_atomic_wait
        // Precondition: caller holds lock!
        void
        _M_wait_unlock(memory_order __o) const noexcept
        {
          _GLIBCXX_TSAN_MUTEX_PRE_UNLOCK(&_M_val);
          auto __v = _M_val.fetch_sub(1, memory_order_relaxed);
          _GLIBCXX_TSAN_MUTEX_POST_UNLOCK(&_M_val);
          _M_val.wait(__v & ~_S_lock_bit, __o);
        }
 
        void
        notify_one() noexcept
        {
          _GLIBCXX_TSAN_MUTEX_PRE_SIGNAL(&_M_val);
          _M_val.notify_one();
          _GLIBCXX_TSAN_MUTEX_POST_SIGNAL(&_M_val);
        }
 
        void
        notify_all() noexcept
        {
          _GLIBCXX_TSAN_MUTEX_PRE_SIGNAL(&_M_val);
          _M_val.notify_all();
          _GLIBCXX_TSAN_MUTEX_POST_SIGNAL(&_M_val);
        }
#endif
 
      private:
        mutable __atomic_base<uintptr_t> _M_val{0};
        static constexpr uintptr_t _S_lock_bit{1};
      };
 
      typename _Tp::element_type* _M_ptr = nullptr;
      _Atomic_count _M_refcount;
 
      static typename _Atomic_count::pointer
      _S_add_ref(typename _Atomic_count::pointer __p)
      {
        if (__p)
          {
            if constexpr (__is_shared_ptr<_Tp>)
              __p->_M_add_ref_copy();
            else
              __p->_M_weak_add_ref();
          }
        return __p;
      }
 
      constexpr _Sp_atomic() noexcept = default;
 
      explicit
      _Sp_atomic(value_type __r) noexcept
      : _M_ptr(__r._M_ptr), _M_refcount(std::move(__r._M_refcount))
      { }
 
      ~_Sp_atomic() = default;
 
      _Sp_atomic(const _Sp_atomic&) = delete;
      void operator=(const _Sp_atomic&) = delete;
 
      value_type
      load(memory_order __o) const noexcept
      {
        __glibcxx_assert(__o != memory_order_release
                           && __o != memory_order_acq_rel);
        // Ensure that the correct value of _M_ptr is visible after locking.,
        // by upgrading relaxed or consume to acquire.
        if (__o != memory_order_seq_cst)
          __o = memory_order_acquire;
 
        value_type __ret;
        auto __pi = _M_refcount.lock(__o);
        __ret._M_ptr = _M_ptr;
        __ret._M_refcount._M_pi = _S_add_ref(__pi);
        _M_refcount.unlock(memory_order_relaxed);
        return __ret;
      }
 
      void
      swap(value_type& __r, memory_order __o) noexcept
      {
        _M_refcount.lock(memory_order_acquire);
        std::swap(_M_ptr, __r._M_ptr);
        _M_refcount._M_swap_unlock(__r._M_refcount, __o);
      }
 
      bool
      compare_exchange_strong(value_type& __expected, value_type __desired,
                              memory_order __o, memory_order __o2) noexcept
      {
        bool __result = true;
        auto __pi = _M_refcount.lock(memory_order_acquire);
        if (_M_ptr == __expected._M_ptr
              && __pi == __expected._M_refcount._M_pi)
          {
            _M_ptr = __desired._M_ptr;
            _M_refcount._M_swap_unlock(__desired._M_refcount, __o);
          }
        else
          {
            _Tp __sink = std::move(__expected);
            __expected._M_ptr = _M_ptr;
            __expected._M_refcount._M_pi = _S_add_ref(__pi);
            _M_refcount.unlock(__o2);
            __result = false;
          }
        return __result;
      }
 
#if __cpp_lib_atomic_wait
      void
      wait(value_type __old, memory_order __o) const noexcept
      {
        auto __pi = _M_refcount.lock(memory_order_acquire);
        if (_M_ptr == __old._M_ptr && __pi == __old._M_refcount._M_pi)
          _M_refcount._M_wait_unlock(__o);
        else
          _M_refcount.unlock(memory_order_relaxed);
      }
 
      void
      notify_one() noexcept
      {
        _M_refcount.notify_one();
      }
 
      void
      notify_all() noexcept
      {
        _M_refcount.notify_all();
      }
#endif
    };
 
  template<typename _Tp>
    class atomic<shared_ptr<_Tp>>
    {
    public:
      using value_type = shared_ptr<_Tp>;
 
      static constexpr bool is_always_lock_free = false;
 
      bool
      is_lock_free() const noexcept
      { return false; }
 
      constexpr atomic() noexcept = default;
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3661. constinit atomic<shared_ptr<T>> a(nullptr); should work
      constexpr atomic(nullptr_t) noexcept : atomic() { }
 
      atomic(shared_ptr<_Tp> __r) noexcept
      : _M_impl(std::move(__r))
      { }
 
      atomic(const atomic&) = delete;
      void operator=(const atomic&) = delete;
 
      shared_ptr<_Tp>
      load(memory_order __o = memory_order_seq_cst) const noexcept
      { return _M_impl.load(__o); }
 
      operator shared_ptr<_Tp>() const noexcept
      { return _M_impl.load(memory_order_seq_cst); }
 
      void
      store(shared_ptr<_Tp> __desired,
            memory_order __o = memory_order_seq_cst) noexcept
      { _M_impl.swap(__desired, __o); }
 
      void
      operator=(shared_ptr<_Tp> __desired) noexcept
      { _M_impl.swap(__desired, memory_order_seq_cst); }
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 3893. LWG 3661 broke atomic<shared_ptr<T>> a; a = nullptr;
      void
      operator=(nullptr_t) noexcept
      { store(nullptr); }
 
      shared_ptr<_Tp>
      exchange(shared_ptr<_Tp> __desired,
               memory_order __o = memory_order_seq_cst) noexcept
      {
        _M_impl.swap(__desired, __o);
        return __desired;
      }
 
      bool
      compare_exchange_strong(shared_ptr<_Tp>& __expected,
                              shared_ptr<_Tp> __desired,
                              memory_order __o, memory_order __o2) noexcept
      {
        return _M_impl.compare_exchange_strong(__expected, __desired, __o, __o2);
      }
 
      bool
      compare_exchange_strong(value_type& __expected, value_type __desired,
                              memory_order __o = memory_order_seq_cst) noexcept
      {
        memory_order __o2;
        switch (__o)
        {
        case memory_order_acq_rel:
          __o2 = memory_order_acquire;
          break;
        case memory_order_release:
          __o2 = memory_order_relaxed;
          break;
        default:
          __o2 = __o;
        }
        return compare_exchange_strong(__expected, std::move(__desired),
                                       __o, __o2);
      }
 
      bool
      compare_exchange_weak(value_type& __expected, value_type __desired,
                            memory_order __o, memory_order __o2) noexcept
      {
        return compare_exchange_strong(__expected, std::move(__desired),
                                       __o, __o2);
      }
 
      bool
      compare_exchange_weak(value_type& __expected, value_type __desired,
                            memory_order __o = memory_order_seq_cst) noexcept
      {
        return compare_exchange_strong(__expected, std::move(__desired), __o);
      }
 
#if __cpp_lib_atomic_wait
      void
      wait(value_type __old,
           memory_order __o = memory_order_seq_cst) const noexcept
      {
        _M_impl.wait(std::move(__old), __o);
      }
 
      void
      notify_one() noexcept
      {
        _M_impl.notify_one();
      }
 
      void
      notify_all() noexcept
      {
        _M_impl.notify_all();
      }
#endif
 
    private:
      _Sp_atomic<shared_ptr<_Tp>> _M_impl;
    };
 
  template<typename _Tp>
    class atomic<weak_ptr<_Tp>>
    {
    public:
      using value_type = weak_ptr<_Tp>;
 
      static constexpr bool is_always_lock_free = false;
 
      bool
      is_lock_free() const noexcept
      { return false; }
 
      constexpr atomic() noexcept = default;
 
      atomic(weak_ptr<_Tp> __r) noexcept
     : _M_impl(move(__r))
      { }
 
      atomic(const atomic&) = delete;
      void operator=(const atomic&) = delete;
 
      weak_ptr<_Tp>
      load(memory_order __o = memory_order_seq_cst) const noexcept
      { return _M_impl.load(__o); }
 
      operator weak_ptr<_Tp>() const noexcept
      { return _M_impl.load(memory_order_seq_cst); }
 
      void
      store(weak_ptr<_Tp> __desired,
            memory_order __o = memory_order_seq_cst) noexcept
      { _M_impl.swap(__desired, __o); }
 
      void
      operator=(weak_ptr<_Tp> __desired) noexcept
      { _M_impl.swap(__desired, memory_order_seq_cst); }
 
      weak_ptr<_Tp>
      exchange(weak_ptr<_Tp> __desired,
               memory_order __o = memory_order_seq_cst) noexcept
      {
        _M_impl.swap(__desired, __o);
        return __desired;
      }
 
      bool
      compare_exchange_strong(weak_ptr<_Tp>& __expected,
                              weak_ptr<_Tp> __desired,
                              memory_order __o, memory_order __o2) noexcept
      {
        return _M_impl.compare_exchange_strong(__expected, __desired, __o, __o2);
      }
 
      bool
      compare_exchange_strong(value_type& __expected, value_type __desired,
                              memory_order __o = memory_order_seq_cst) noexcept
      {
        memory_order __o2;
        switch (__o)
        {
        case memory_order_acq_rel:
          __o2 = memory_order_acquire;
          break;
        case memory_order_release:
          __o2 = memory_order_relaxed;
          break;
        default:
          __o2 = __o;
        }
        return compare_exchange_strong(__expected, std::move(__desired),
                                       __o, __o2);
      }
 
      bool
      compare_exchange_weak(value_type& __expected, value_type __desired,
                            memory_order __o, memory_order __o2) noexcept
      {
        return compare_exchange_strong(__expected, std::move(__desired),
                                       __o, __o2);
      }
 
      bool
      compare_exchange_weak(value_type& __expected, value_type __desired,
                            memory_order __o = memory_order_seq_cst) noexcept
      {
        return compare_exchange_strong(__expected, std::move(__desired), __o);
      }
 
#if __cpp_lib_atomic_wait
      void
      wait(value_type __old,
           memory_order __o = memory_order_seq_cst) const noexcept
      {
        _M_impl.wait(std::move(__old), __o);
      }
 
      void
      notify_one() noexcept
      {
        _M_impl.notify_one();
      }
 
      void
      notify_all() noexcept
      {
        _M_impl.notify_all();
      }
#endif
 
    private:
      _Sp_atomic<weak_ptr<_Tp>> _M_impl;
    };
  /// @} group pointer_abstractions
#endif // C++20
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
 
#endif // _SHARED_PTR_ATOMIC_H