atomic_base.h

Go to the documentation of this file.

// -*- C++ -*- header.
 
// Copyright (C) 2008-2024 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/atomic_base.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{atomic}
 */
 
#ifndef _GLIBCXX_ATOMIC_BASE_H
#define _GLIBCXX_ATOMIC_BASE_H 1
 
#pragma GCC system_header
 
#include <bits/c++config.h>
#include <new> // For placement new
#include <stdint.h>
#include <bits/atomic_lockfree_defines.h>
#include <bits/move.h>
 
#if __cplusplus > 201703L && _GLIBCXX_HOSTED
#include <bits/atomic_wait.h>
#endif
 
#ifndef _GLIBCXX_ALWAYS_INLINE
#define _GLIBCXX_ALWAYS_INLINE inline __attribute__((__always_inline__))
#endif
 
#include <bits/version.h>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   * @defgroup atomics Atomics
   *
   * Components for performing atomic operations.
   * @{
   */
 
  /// Enumeration for memory_order
#if __cplusplus > 201703L
  enum class memory_order : int
    {
      relaxed,
      consume,
      acquire,
      release,
      acq_rel,
      seq_cst
    };
 
  inline constexpr memory_order memory_order_relaxed = memory_order::relaxed;
  inline constexpr memory_order memory_order_consume = memory_order::consume;
  inline constexpr memory_order memory_order_acquire = memory_order::acquire;
  inline constexpr memory_order memory_order_release = memory_order::release;
  inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel;
  inline constexpr memory_order memory_order_seq_cst = memory_order::seq_cst;
#else
  enum memory_order : int
    {
      memory_order_relaxed,
      memory_order_consume,
      memory_order_acquire,
      memory_order_release,
      memory_order_acq_rel,
      memory_order_seq_cst
    };
#endif
 
  /// @cond undocumented
  enum __memory_order_modifier
    {
      __memory_order_mask          = 0x0ffff,
      __memory_order_modifier_mask = 0xffff0000,
      __memory_order_hle_acquire   = 0x10000,
      __memory_order_hle_release   = 0x20000
    };
  /// @endcond
 
  constexpr memory_order
  operator|(memory_order __m, __memory_order_modifier __mod) noexcept
  {
    return memory_order(int(__m) | int(__mod));
  }
 
  constexpr memory_order
  operator&(memory_order __m, __memory_order_modifier __mod) noexcept
  {
    return memory_order(int(__m) & int(__mod));
  }
 
  /// @cond undocumented
 
  // Drop release ordering as per [atomics.types.operations.req]/21
  constexpr memory_order
  __cmpexch_failure_order2(memory_order __m) noexcept
  {
    return __m == memory_order_acq_rel ? memory_order_acquire
      : __m == memory_order_release ? memory_order_relaxed : __m;
  }
 
  constexpr memory_order
  __cmpexch_failure_order(memory_order __m) noexcept
  {
    return memory_order(__cmpexch_failure_order2(__m & __memory_order_mask)
      | __memory_order_modifier(__m & __memory_order_modifier_mask));
  }
 
  constexpr bool
  __is_valid_cmpexch_failure_order(memory_order __m) noexcept
  {
    return (__m & __memory_order_mask) != memory_order_release
        && (__m & __memory_order_mask) != memory_order_acq_rel;
  }
 
  // Base types for atomics.
  template<typename _IntTp>
    struct __atomic_base;
 
  /// @endcond
 
  _GLIBCXX_ALWAYS_INLINE void
  atomic_thread_fence(memory_order __m) noexcept
  { __atomic_thread_fence(int(__m)); }
 
  _GLIBCXX_ALWAYS_INLINE void
  atomic_signal_fence(memory_order __m) noexcept
  { __atomic_signal_fence(int(__m)); }
 
  /// kill_dependency
  template<typename _Tp>
    inline _Tp
    kill_dependency(_Tp __y) noexcept
    {
      _Tp __ret(__y);
      return __ret;
    }
 
/// @cond undocumented
#if __glibcxx_atomic_value_initialization
# define _GLIBCXX20_INIT(I) = I
#else
# define _GLIBCXX20_INIT(I)
#endif
/// @endcond
 
#define ATOMIC_VAR_INIT(_VI) { _VI }
 
  template<typename _Tp>
    struct atomic;
 
  template<typename _Tp>
    struct atomic<_Tp*>;
 
    /* The target's "set" value for test-and-set may not be exactly 1.  */
#if __GCC_ATOMIC_TEST_AND_SET_TRUEVAL == 1
    typedef bool __atomic_flag_data_type;
#else
    typedef unsigned char __atomic_flag_data_type;
#endif
 
  /// @cond undocumented
 
  /*
   *  Base type for atomic_flag.
   *
   *  Base type is POD with data, allowing atomic_flag to derive from
   *  it and meet the standard layout type requirement. In addition to
   *  compatibility with a C interface, this allows different
   *  implementations of atomic_flag to use the same atomic operation
   *  functions, via a standard conversion to the __atomic_flag_base
   *  argument.
  */
  _GLIBCXX_BEGIN_EXTERN_C
 
  struct __atomic_flag_base
  {
    __atomic_flag_data_type _M_i _GLIBCXX20_INIT({});
  };
 
  _GLIBCXX_END_EXTERN_C
 
  /// @endcond
 
#define ATOMIC_FLAG_INIT { 0 }
 
  /// atomic_flag
  struct atomic_flag : public __atomic_flag_base
  {
    atomic_flag() noexcept = default;
    ~atomic_flag() noexcept = default;
    atomic_flag(const atomic_flag&) = delete;
    atomic_flag& operator=(const atomic_flag&) = delete;
    atomic_flag& operator=(const atomic_flag&) volatile = delete;
 
    // Conversion to ATOMIC_FLAG_INIT.
    constexpr atomic_flag(bool __i) noexcept
      : __atomic_flag_base{ _S_init(__i) }
    { }
 
    _GLIBCXX_ALWAYS_INLINE bool
    test_and_set(memory_order __m = memory_order_seq_cst) noexcept
    {
      return __atomic_test_and_set (&_M_i, int(__m));
    }
 
    _GLIBCXX_ALWAYS_INLINE bool
    test_and_set(memory_order __m = memory_order_seq_cst) volatile noexcept
    {
      return __atomic_test_and_set (&_M_i, int(__m));
    }
 
#ifdef __glibcxx_atomic_flag_test // C++ >= 20
    _GLIBCXX_ALWAYS_INLINE bool
    test(memory_order __m = memory_order_seq_cst) const noexcept
    {
      __atomic_flag_data_type __v;
      __atomic_load(&_M_i, &__v, int(__m));
      return __v == __GCC_ATOMIC_TEST_AND_SET_TRUEVAL;
    }
 
    _GLIBCXX_ALWAYS_INLINE bool
    test(memory_order __m = memory_order_seq_cst) const volatile noexcept
    {
      __atomic_flag_data_type __v;
      __atomic_load(&_M_i, &__v, int(__m));
      return __v == __GCC_ATOMIC_TEST_AND_SET_TRUEVAL;
    }
#endif
 
#if __glibcxx_atomic_wait // C++ >= 20 && (linux_futex || gthread)
    _GLIBCXX_ALWAYS_INLINE void
    wait(bool __old,
        memory_order __m = memory_order_seq_cst) const noexcept
    {
      const __atomic_flag_data_type __v
        = __old ? __GCC_ATOMIC_TEST_AND_SET_TRUEVAL : 0;
 
      std::__atomic_wait_address_v(&_M_i, __v,
          [__m, this] { return __atomic_load_n(&_M_i, int(__m)); });
    }
 
    // TODO add const volatile overload
 
    _GLIBCXX_ALWAYS_INLINE void
    notify_one() noexcept
    { std::__atomic_notify_address(&_M_i, false); }
 
    // TODO add const volatile overload
 
    _GLIBCXX_ALWAYS_INLINE void
    notify_all() noexcept
    { std::__atomic_notify_address(&_M_i, true); }
 
    // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
    _GLIBCXX_ALWAYS_INLINE void
    clear(memory_order __m = memory_order_seq_cst) noexcept
    {
      memory_order __b __attribute__ ((__unused__))
        = __m & __memory_order_mask;
      __glibcxx_assert(__b != memory_order_consume);
      __glibcxx_assert(__b != memory_order_acquire);
      __glibcxx_assert(__b != memory_order_acq_rel);
 
      __atomic_clear (&_M_i, int(__m));
    }
 
    _GLIBCXX_ALWAYS_INLINE void
    clear(memory_order __m = memory_order_seq_cst) volatile noexcept
    {
      memory_order __b __attribute__ ((__unused__))
        = __m & __memory_order_mask;
      __glibcxx_assert(__b != memory_order_consume);
      __glibcxx_assert(__b != memory_order_acquire);
      __glibcxx_assert(__b != memory_order_acq_rel);
 
      __atomic_clear (&_M_i, int(__m));
    }
 
  private:
    static constexpr __atomic_flag_data_type
    _S_init(bool __i)
    { return __i ? __GCC_ATOMIC_TEST_AND_SET_TRUEVAL : 0; }
  };
 
  /// @cond undocumented
 
  /// Base class for atomic integrals.
  //
  // For each of the integral types, define atomic_[integral type] struct
  //
  // atomic_bool     bool
  // atomic_char     char
  // atomic_schar    signed char
  // atomic_uchar    unsigned char
  // atomic_short    short
  // atomic_ushort   unsigned short
  // atomic_int      int
  // atomic_uint     unsigned int
  // atomic_long     long
  // atomic_ulong    unsigned long
  // atomic_llong    long long
  // atomic_ullong   unsigned long long
  // atomic_char8_t  char8_t
  // atomic_char16_t char16_t
  // atomic_char32_t char32_t
  // atomic_wchar_t  wchar_t
  //
  // NB: Assuming _ITp is an integral scalar type that is 1, 2, 4, or
  // 8 bytes, since that is what GCC built-in functions for atomic
  // memory access expect.
  template<typename _ITp>
    struct __atomic_base
    {
      using value_type = _ITp;
      using difference_type = value_type;
 
    private:
      typedef _ITp      __int_type;
 
      static constexpr int _S_alignment =
        sizeof(_ITp) > alignof(_ITp) ? sizeof(_ITp) : alignof(_ITp);
 
      alignas(_S_alignment) __int_type _M_i _GLIBCXX20_INIT(0);
 
    public:
      __atomic_base() noexcept = default;
      ~__atomic_base() noexcept = default;
      __atomic_base(const __atomic_base&) = delete;
      __atomic_base& operator=(const __atomic_base&) = delete;
      __atomic_base& operator=(const __atomic_base&) volatile = delete;
 
      // Requires __int_type convertible to _M_i.
      constexpr __atomic_base(__int_type __i) noexcept : _M_i (__i) { }
 
      operator __int_type() const noexcept
      { return load(); }
 
      operator __int_type() const volatile noexcept
      { return load(); }
 
      __int_type
      operator=(__int_type __i) noexcept
      {
        store(__i);
        return __i;
      }
 
      __int_type
      operator=(__int_type __i) volatile noexcept
      {
        store(__i);
        return __i;
      }
 
      __int_type
      operator++(int) noexcept
      { return fetch_add(1); }
 
      __int_type
      operator++(int) volatile noexcept
      { return fetch_add(1); }
 
      __int_type
      operator--(int) noexcept
      { return fetch_sub(1); }
 
      __int_type
      operator--(int) volatile noexcept
      { return fetch_sub(1); }
 
      __int_type
      operator++() noexcept
      { return __atomic_add_fetch(&_M_i, 1, int(memory_order_seq_cst)); }
 
      __int_type
      operator++() volatile noexcept
      { return __atomic_add_fetch(&_M_i, 1, int(memory_order_seq_cst)); }
 
      __int_type
      operator--() noexcept
      { return __atomic_sub_fetch(&_M_i, 1, int(memory_order_seq_cst)); }
 
      __int_type
      operator--() volatile noexcept
      { return __atomic_sub_fetch(&_M_i, 1, int(memory_order_seq_cst)); }
 
      __int_type
      operator+=(__int_type __i) noexcept
      { return __atomic_add_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator+=(__int_type __i) volatile noexcept
      { return __atomic_add_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator-=(__int_type __i) noexcept
      { return __atomic_sub_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator-=(__int_type __i) volatile noexcept
      { return __atomic_sub_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator&=(__int_type __i) noexcept
      { return __atomic_and_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator&=(__int_type __i) volatile noexcept
      { return __atomic_and_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator|=(__int_type __i) noexcept
      { return __atomic_or_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator|=(__int_type __i) volatile noexcept
      { return __atomic_or_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator^=(__int_type __i) noexcept
      { return __atomic_xor_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      __int_type
      operator^=(__int_type __i) volatile noexcept
      { return __atomic_xor_fetch(&_M_i, __i, int(memory_order_seq_cst)); }
 
      bool
      is_lock_free() const noexcept
      {
        // Use a fake, minimally aligned pointer.
        return __atomic_is_lock_free(sizeof(_M_i),
            reinterpret_cast<void *>(-_S_alignment));
      }
 
      bool
      is_lock_free() const volatile noexcept
      {
        // Use a fake, minimally aligned pointer.
        return __atomic_is_lock_free(sizeof(_M_i),
            reinterpret_cast<void *>(-_S_alignment));
      }
 
      _GLIBCXX_ALWAYS_INLINE void
      store(__int_type __i, memory_order __m = memory_order_seq_cst) noexcept
      {
        memory_order __b __attribute__ ((__unused__))
          = __m & __memory_order_mask;
        __glibcxx_assert(__b != memory_order_acquire);
        __glibcxx_assert(__b != memory_order_acq_rel);
        __glibcxx_assert(__b != memory_order_consume);
 
        __atomic_store_n(&_M_i, __i, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE void
      store(__int_type __i,
            memory_order __m = memory_order_seq_cst) volatile noexcept
      {
        memory_order __b __attribute__ ((__unused__))
          = __m & __memory_order_mask;
        __glibcxx_assert(__b != memory_order_acquire);
        __glibcxx_assert(__b != memory_order_acq_rel);
        __glibcxx_assert(__b != memory_order_consume);
 
        __atomic_store_n(&_M_i, __i, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      load(memory_order __m = memory_order_seq_cst) const noexcept
      {
        memory_order __b __attribute__ ((__unused__))
          = __m & __memory_order_mask;
        __glibcxx_assert(__b != memory_order_release);
        __glibcxx_assert(__b != memory_order_acq_rel);
 
        return __atomic_load_n(&_M_i, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      load(memory_order __m = memory_order_seq_cst) const volatile noexcept
      {
        memory_order __b __attribute__ ((__unused__))
          = __m & __memory_order_mask;
        __glibcxx_assert(__b != memory_order_release);
        __glibcxx_assert(__b != memory_order_acq_rel);
 
        return __atomic_load_n(&_M_i, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      exchange(__int_type __i,
               memory_order __m = memory_order_seq_cst) noexcept
      {
        return __atomic_exchange_n(&_M_i, __i, int(__m));
      }
 
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      exchange(__int_type __i,
               memory_order __m = memory_order_seq_cst) volatile noexcept
      {
        return __atomic_exchange_n(&_M_i, __i, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_weak(__int_type& __i1, __int_type __i2,
                            memory_order __m1, memory_order __m2) noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
        return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 1,
                                           int(__m1), int(__m2));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_weak(__int_type& __i1, __int_type __i2,
                            memory_order __m1,
                            memory_order __m2) volatile noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
        return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 1,
                                           int(__m1), int(__m2));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_weak(__int_type& __i1, __int_type __i2,
                            memory_order __m = memory_order_seq_cst) noexcept
      {
        return compare_exchange_weak(__i1, __i2, __m,
                                     __cmpexch_failure_order(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_weak(__int_type& __i1, __int_type __i2,
                   memory_order __m = memory_order_seq_cst) volatile noexcept
      {
        return compare_exchange_weak(__i1, __i2, __m,
                                     __cmpexch_failure_order(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_strong(__int_type& __i1, __int_type __i2,
                              memory_order __m1, memory_order __m2) noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
        return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 0,
                                           int(__m1), int(__m2));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_strong(__int_type& __i1, __int_type __i2,
                              memory_order __m1,
                              memory_order __m2) volatile noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
        return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 0,
                                           int(__m1), int(__m2));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_strong(__int_type& __i1, __int_type __i2,
                              memory_order __m = memory_order_seq_cst) noexcept
      {
        return compare_exchange_strong(__i1, __i2, __m,
                                       __cmpexch_failure_order(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_strong(__int_type& __i1, __int_type __i2,
                 memory_order __m = memory_order_seq_cst) volatile noexcept
      {
        return compare_exchange_strong(__i1, __i2, __m,
                                       __cmpexch_failure_order(__m));
      }
 
#if __glibcxx_atomic_wait
      _GLIBCXX_ALWAYS_INLINE void
      wait(__int_type __old,
          memory_order __m = memory_order_seq_cst) const noexcept
      {
        std::__atomic_wait_address_v(&_M_i, __old,
                           [__m, this] { return this->load(__m); });
      }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_one() noexcept
      { std::__atomic_notify_address(&_M_i, false); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_all() noexcept
      { std::__atomic_notify_address(&_M_i, true); }
 
      // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_add(__int_type __i,
                memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_fetch_add(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_add(__int_type __i,
                memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_fetch_add(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_sub(__int_type __i,
                memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_fetch_sub(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_sub(__int_type __i,
                memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_fetch_sub(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_and(__int_type __i,
                memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_fetch_and(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_and(__int_type __i,
                memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_fetch_and(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_or(__int_type __i,
               memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_fetch_or(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_or(__int_type __i,
               memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_fetch_or(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_xor(__int_type __i,
                memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_fetch_xor(&_M_i, __i, int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __int_type
      fetch_xor(__int_type __i,
                memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_fetch_xor(&_M_i, __i, int(__m)); }
    };
 
 
  /// Partial specialization for pointer types.
  template<typename _PTp>
    struct __atomic_base<_PTp*>
    {
    private:
      typedef _PTp*     __pointer_type;
 
      __pointer_type    _M_p _GLIBCXX20_INIT(nullptr);
 
      static constexpr ptrdiff_t
      _S_type_size(ptrdiff_t __d)
      { return __d * sizeof(_PTp); }
 
    public:
      __atomic_base() noexcept = default;
      ~__atomic_base() noexcept = default;
      __atomic_base(const __atomic_base&) = delete;
      __atomic_base& operator=(const __atomic_base&) = delete;
      __atomic_base& operator=(const __atomic_base&) volatile = delete;
 
      // Requires __pointer_type convertible to _M_p.
      constexpr __atomic_base(__pointer_type __p) noexcept : _M_p (__p) { }
 
      operator __pointer_type() const noexcept
      { return load(); }
 
      operator __pointer_type() const volatile noexcept
      { return load(); }
 
      __pointer_type
      operator=(__pointer_type __p) noexcept
      {
        store(__p);
        return __p;
      }
 
      __pointer_type
      operator=(__pointer_type __p) volatile noexcept
      {
        store(__p);
        return __p;
      }
 
      __pointer_type
      operator++(int) noexcept
      { return fetch_add(1); }
 
      __pointer_type
      operator++(int) volatile noexcept
      { return fetch_add(1); }
 
      __pointer_type
      operator--(int) noexcept
      { return fetch_sub(1); }
 
      __pointer_type
      operator--(int) volatile noexcept
      { return fetch_sub(1); }
 
      __pointer_type
      operator++() noexcept
      { return __atomic_add_fetch(&_M_p, _S_type_size(1),
                                  int(memory_order_seq_cst)); }
 
      __pointer_type
      operator++() volatile noexcept
      { return __atomic_add_fetch(&_M_p, _S_type_size(1),
                                  int(memory_order_seq_cst)); }
 
      __pointer_type
      operator--() noexcept
      { return __atomic_sub_fetch(&_M_p, _S_type_size(1),
                                  int(memory_order_seq_cst)); }
 
      __pointer_type
      operator--() volatile noexcept
      { return __atomic_sub_fetch(&_M_p, _S_type_size(1),
                                  int(memory_order_seq_cst)); }
 
      __pointer_type
      operator+=(ptrdiff_t __d) noexcept
      { return __atomic_add_fetch(&_M_p, _S_type_size(__d),
                                  int(memory_order_seq_cst)); }
 
      __pointer_type
      operator+=(ptrdiff_t __d) volatile noexcept
      { return __atomic_add_fetch(&_M_p, _S_type_size(__d),
                                  int(memory_order_seq_cst)); }
 
      __pointer_type
      operator-=(ptrdiff_t __d) noexcept
      { return __atomic_sub_fetch(&_M_p, _S_type_size(__d),
                                  int(memory_order_seq_cst)); }
 
      __pointer_type
      operator-=(ptrdiff_t __d) volatile noexcept
      { return __atomic_sub_fetch(&_M_p, _S_type_size(__d),
                                  int(memory_order_seq_cst)); }
 
      bool
      is_lock_free() const noexcept
      {
        // Produce a fake, minimally aligned pointer.
        return __atomic_is_lock_free(sizeof(_M_p),
            reinterpret_cast<void *>(-__alignof(_M_p)));
      }
 
      bool
      is_lock_free() const volatile noexcept
      {
        // Produce a fake, minimally aligned pointer.
        return __atomic_is_lock_free(sizeof(_M_p),
            reinterpret_cast<void *>(-__alignof(_M_p)));
      }
 
      _GLIBCXX_ALWAYS_INLINE void
      store(__pointer_type __p,
            memory_order __m = memory_order_seq_cst) noexcept
      {
        memory_order __b __attribute__ ((__unused__))
          = __m & __memory_order_mask;
 
        __glibcxx_assert(__b != memory_order_acquire);
        __glibcxx_assert(__b != memory_order_acq_rel);
        __glibcxx_assert(__b != memory_order_consume);
 
        __atomic_store_n(&_M_p, __p, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE void
      store(__pointer_type __p,
            memory_order __m = memory_order_seq_cst) volatile noexcept
      {
        memory_order __b __attribute__ ((__unused__))
          = __m & __memory_order_mask;
        __glibcxx_assert(__b != memory_order_acquire);
        __glibcxx_assert(__b != memory_order_acq_rel);
        __glibcxx_assert(__b != memory_order_consume);
 
        __atomic_store_n(&_M_p, __p, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE __pointer_type
      load(memory_order __m = memory_order_seq_cst) const noexcept
      {
        memory_order __b __attribute__ ((__unused__))
          = __m & __memory_order_mask;
        __glibcxx_assert(__b != memory_order_release);
        __glibcxx_assert(__b != memory_order_acq_rel);
 
        return __atomic_load_n(&_M_p, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE __pointer_type
      load(memory_order __m = memory_order_seq_cst) const volatile noexcept
      {
        memory_order __b __attribute__ ((__unused__))
          = __m & __memory_order_mask;
        __glibcxx_assert(__b != memory_order_release);
        __glibcxx_assert(__b != memory_order_acq_rel);
 
        return __atomic_load_n(&_M_p, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE __pointer_type
      exchange(__pointer_type __p,
               memory_order __m = memory_order_seq_cst) noexcept
      {
        return __atomic_exchange_n(&_M_p, __p, int(__m));
      }
 
 
      _GLIBCXX_ALWAYS_INLINE __pointer_type
      exchange(__pointer_type __p,
               memory_order __m = memory_order_seq_cst) volatile noexcept
      {
        return __atomic_exchange_n(&_M_p, __p, int(__m));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_weak(__pointer_type& __p1, __pointer_type __p2,
                            memory_order __m1,
                            memory_order __m2) noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
        return __atomic_compare_exchange_n(&_M_p, &__p1, __p2, 1,
                                           int(__m1), int(__m2));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_weak(__pointer_type& __p1, __pointer_type __p2,
                            memory_order __m1,
                            memory_order __m2) volatile noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
        return __atomic_compare_exchange_n(&_M_p, &__p1, __p2, 1,
                                           int(__m1), int(__m2));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_strong(__pointer_type& __p1, __pointer_type __p2,
                              memory_order __m1,
                              memory_order __m2) noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
        return __atomic_compare_exchange_n(&_M_p, &__p1, __p2, 0,
                                           int(__m1), int(__m2));
      }
 
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_strong(__pointer_type& __p1, __pointer_type __p2,
                              memory_order __m1,
                              memory_order __m2) volatile noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__m2));
 
        return __atomic_compare_exchange_n(&_M_p, &__p1, __p2, 0,
                                           int(__m1), int(__m2));
      }
 
#if __glibcxx_atomic_wait
      _GLIBCXX_ALWAYS_INLINE void
      wait(__pointer_type __old,
           memory_order __m = memory_order_seq_cst) const noexcept
      {
        std::__atomic_wait_address_v(&_M_p, __old,
                                     [__m, this]
                                     { return this->load(__m); });
      }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_one() const noexcept
      { std::__atomic_notify_address(&_M_p, false); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_all() const noexcept
      { std::__atomic_notify_address(&_M_p, true); }
 
      // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
      _GLIBCXX_ALWAYS_INLINE __pointer_type
      fetch_add(ptrdiff_t __d,
                memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_fetch_add(&_M_p, _S_type_size(__d), int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __pointer_type
      fetch_add(ptrdiff_t __d,
                memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_fetch_add(&_M_p, _S_type_size(__d), int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __pointer_type
      fetch_sub(ptrdiff_t __d,
                memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_fetch_sub(&_M_p, _S_type_size(__d), int(__m)); }
 
      _GLIBCXX_ALWAYS_INLINE __pointer_type
      fetch_sub(ptrdiff_t __d,
                memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_fetch_sub(&_M_p, _S_type_size(__d), int(__m)); }
    };
 
  namespace __atomic_impl
  {
    // Implementation details of atomic padding handling
 
    template<typename _Tp>
      constexpr bool
      __maybe_has_padding()
      {
#if ! __has_builtin(__builtin_clear_padding)
        return false;
#elif __has_builtin(__has_unique_object_representations)
        return !__has_unique_object_representations(_Tp)
          && !is_same<_Tp, float>::value && !is_same<_Tp, double>::value;
#else
        return true;
#endif
      }
 
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wc++17-extensions"
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _GLIBCXX14_CONSTEXPR _Tp*
      __clear_padding(_Tp& __val) noexcept
      {
        auto* __ptr = std::__addressof(__val);
#if __has_builtin(__builtin_clear_padding)
        if constexpr (__atomic_impl::__maybe_has_padding<_Tp>())
          __builtin_clear_padding(__ptr);
#endif
        return __ptr;
      }
 
    // Remove volatile and create a non-deduced context for value arguments.
    template<typename _Tp>
      using _Val = typename remove_volatile<_Tp>::type;
 
    template<bool _AtomicRef = false, typename _Tp>
      _GLIBCXX_ALWAYS_INLINE bool
      __compare_exchange(_Tp& __val, _Val<_Tp>& __e, _Val<_Tp>& __i,
                         bool __is_weak,
                         memory_order __s, memory_order __f) noexcept
      {
        __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
 
        using _Vp = _Val<_Tp>;
        _Tp* const __pval = std::__addressof(__val);
 
        if constexpr (!__atomic_impl::__maybe_has_padding<_Vp>())
          {
            return __atomic_compare_exchange(__pval, std::__addressof(__e),
                                             std::__addressof(__i), __is_weak,
                                             int(__s), int(__f));
          }
        else if constexpr (!_AtomicRef) // std::atomic<T>
          {
            // Clear padding of the value we want to set:
            _Vp* const __pi = __atomic_impl::__clear_padding(__i);
            // Only allowed to modify __e on failure, so make a copy:
            _Vp __exp = __e;
            // Clear padding of the expected value:
            _Vp* const __pexp = __atomic_impl::__clear_padding(__exp);
 
            // For std::atomic<T> we know that the contained value will already
            // have zeroed padding, so trivial memcmp semantics are OK.
            if (__atomic_compare_exchange(__pval, __pexp, __pi,
                                          __is_weak, int(__s), int(__f)))
              return true;
            // Value bits must be different, copy from __exp back to __e:
            __builtin_memcpy(std::__addressof(__e), __pexp, sizeof(_Vp));
            return false;
          }
        else // std::atomic_ref<T> where T has padding bits.
          {
            // Clear padding of the value we want to set:
            _Vp* const __pi = __atomic_impl::__clear_padding(__i);
 
            // Only allowed to modify __e on failure, so make a copy:
            _Vp __exp = __e;
            // Optimistically assume that a previous store had zeroed padding
            // so that zeroing it in the expected value will match first time.
            _Vp* const __pexp = __atomic_impl::__clear_padding(__exp);
 
            // compare_exchange is specified to compare value representations.
            // Need to check whether a failure is 'real' or just due to
            // differences in padding bits. This loop should run no more than
            // three times, because the worst case scenario is:
            // First CAS fails because the actual value has non-zero padding.
            // Second CAS fails because another thread stored the same value,
            // but now with padding cleared. Third CAS succeeds.
            // We will never need to loop a fourth time, because any value
            // written by another thread (whether via store, exchange or
            // compare_exchange) will have had its padding cleared.
            while (true)
              {
                // Copy of the expected value so we can clear its padding.
                _Vp __orig = __exp;
 
                if (__atomic_compare_exchange(__pval, __pexp, __pi,
                                              __is_weak, int(__s), int(__f)))
                  return true;
 
                // Copy of the actual value so we can clear its padding.
                _Vp __curr = __exp;
 
                // Compare value representations (i.e. ignoring padding).
                if (__builtin_memcmp(__atomic_impl::__clear_padding(__orig),
                                     __atomic_impl::__clear_padding(__curr),
                                     sizeof(_Vp)))
                  {
                    // Value representations compare unequal, real failure.
                    __builtin_memcpy(std::__addressof(__e), __pexp,
                                     sizeof(_Vp));
                    return false;
                  }
              }
          }
      }
#pragma GCC diagnostic pop
  } // namespace __atomic_impl
 
#if __cplusplus > 201703L
  // Implementation details of atomic_ref and atomic<floating-point>.
  namespace __atomic_impl
  {
    // Like _Val<T> above, but for difference_type arguments.
    template<typename _Tp>
      using _Diff = __conditional_t<is_pointer_v<_Tp>, ptrdiff_t, _Val<_Tp>>;
 
    template<size_t _Size, size_t _Align>
      _GLIBCXX_ALWAYS_INLINE bool
      is_lock_free() noexcept
      {
        // Produce a fake, minimally aligned pointer.
        return __atomic_is_lock_free(_Size, reinterpret_cast<void *>(-_Align));
      }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE void
      store(_Tp* __ptr, _Val<_Tp> __t, memory_order __m) noexcept
      {
        __atomic_store(__ptr, __atomic_impl::__clear_padding(__t), int(__m));
      }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Val<_Tp>
      load(const _Tp* __ptr, memory_order __m) noexcept
      {
        alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
        auto* __dest = reinterpret_cast<_Val<_Tp>*>(__buf);
        __atomic_load(__ptr, __dest, int(__m));
        return *__dest;
      }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Val<_Tp>
      exchange(_Tp* __ptr, _Val<_Tp> __desired, memory_order __m) noexcept
      {
        alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
        auto* __dest = reinterpret_cast<_Val<_Tp>*>(__buf);
        __atomic_exchange(__ptr, __atomic_impl::__clear_padding(__desired),
                          __dest, int(__m));
        return *__dest;
      }
 
    template<bool _AtomicRef = false, typename _Tp>
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_weak(_Tp* __ptr, _Val<_Tp>& __expected,
                            _Val<_Tp> __desired, memory_order __success,
                            memory_order __failure,
                            bool __check_padding = false) noexcept
      {
        return __atomic_impl::__compare_exchange<_AtomicRef>(
                   *__ptr, __expected, __desired, true, __success, __failure);
      }
 
    template<bool _AtomicRef = false, typename _Tp>
      _GLIBCXX_ALWAYS_INLINE bool
      compare_exchange_strong(_Tp* __ptr, _Val<_Tp>& __expected,
                              _Val<_Tp> __desired, memory_order __success,
                              memory_order __failure,
                              bool __ignore_padding = false) noexcept
      {
        return __atomic_impl::__compare_exchange<_AtomicRef>(
                   *__ptr, __expected, __desired, false, __success, __failure);
      }
 
#if __glibcxx_atomic_wait
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE void
      wait(const _Tp* __ptr, _Val<_Tp> __old,
           memory_order __m = memory_order_seq_cst) noexcept
      {
        std::__atomic_wait_address_v(__ptr, __old,
            [__ptr, __m]() { return __atomic_impl::load(__ptr, __m); });
      }
 
      // TODO add const volatile overload
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE void
      notify_one(const _Tp* __ptr) noexcept
      { std::__atomic_notify_address(__ptr, false); }
 
      // TODO add const volatile overload
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE void
      notify_all(const _Tp* __ptr) noexcept
      { std::__atomic_notify_address(__ptr, true); }
 
      // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      fetch_add(_Tp* __ptr, _Diff<_Tp> __i, memory_order __m) noexcept
      { return __atomic_fetch_add(__ptr, __i, int(__m)); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      fetch_sub(_Tp* __ptr, _Diff<_Tp> __i, memory_order __m) noexcept
      { return __atomic_fetch_sub(__ptr, __i, int(__m)); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      fetch_and(_Tp* __ptr, _Val<_Tp> __i, memory_order __m) noexcept
      { return __atomic_fetch_and(__ptr, __i, int(__m)); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      fetch_or(_Tp* __ptr, _Val<_Tp> __i, memory_order __m) noexcept
      { return __atomic_fetch_or(__ptr, __i, int(__m)); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      fetch_xor(_Tp* __ptr, _Val<_Tp> __i, memory_order __m) noexcept
      { return __atomic_fetch_xor(__ptr, __i, int(__m)); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      __add_fetch(_Tp* __ptr, _Diff<_Tp> __i) noexcept
      { return __atomic_add_fetch(__ptr, __i, __ATOMIC_SEQ_CST); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      __sub_fetch(_Tp* __ptr, _Diff<_Tp> __i) noexcept
      { return __atomic_sub_fetch(__ptr, __i, __ATOMIC_SEQ_CST); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      __and_fetch(_Tp* __ptr, _Val<_Tp> __i) noexcept
      { return __atomic_and_fetch(__ptr, __i, __ATOMIC_SEQ_CST); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      __or_fetch(_Tp* __ptr, _Val<_Tp> __i) noexcept
      { return __atomic_or_fetch(__ptr, __i, __ATOMIC_SEQ_CST); }
 
    template<typename _Tp>
      _GLIBCXX_ALWAYS_INLINE _Tp
      __xor_fetch(_Tp* __ptr, _Val<_Tp> __i) noexcept
      { return __atomic_xor_fetch(__ptr, __i, __ATOMIC_SEQ_CST); }
 
    template<typename _Tp>
      _Tp
      __fetch_add_flt(_Tp* __ptr, _Val<_Tp> __i, memory_order __m) noexcept
      {
        _Val<_Tp> __oldval = load(__ptr, memory_order_relaxed);
        _Val<_Tp> __newval = __oldval + __i;
        while (!compare_exchange_weak(__ptr, __oldval, __newval, __m,
                                      memory_order_relaxed))
          __newval = __oldval + __i;
        return __oldval;
      }
 
    template<typename _Tp>
      _Tp
      __fetch_sub_flt(_Tp* __ptr, _Val<_Tp> __i, memory_order __m) noexcept
      {
        _Val<_Tp> __oldval = load(__ptr, memory_order_relaxed);
        _Val<_Tp> __newval = __oldval - __i;
        while (!compare_exchange_weak(__ptr, __oldval, __newval, __m,
                                      memory_order_relaxed))
          __newval = __oldval - __i;
        return __oldval;
      }
 
    template<typename _Tp>
      _Tp
      __add_fetch_flt(_Tp* __ptr, _Val<_Tp> __i) noexcept
      {
        _Val<_Tp> __oldval = load(__ptr, memory_order_relaxed);
        _Val<_Tp> __newval = __oldval + __i;
        while (!compare_exchange_weak(__ptr, __oldval, __newval,
                                      memory_order_seq_cst,
                                      memory_order_relaxed))
          __newval = __oldval + __i;
        return __newval;
      }
 
    template<typename _Tp>
      _Tp
      __sub_fetch_flt(_Tp* __ptr, _Val<_Tp> __i) noexcept
      {
        _Val<_Tp> __oldval = load(__ptr, memory_order_relaxed);
        _Val<_Tp> __newval = __oldval - __i;
        while (!compare_exchange_weak(__ptr, __oldval, __newval,
                                      memory_order_seq_cst,
                                      memory_order_relaxed))
          __newval = __oldval - __i;
        return __newval;
      }
  } // namespace __atomic_impl
 
  // base class for atomic<floating-point-type>
  template<typename _Fp>
    struct __atomic_float
    {
      static_assert(is_floating_point_v<_Fp>);
 
      static constexpr size_t _S_alignment = __alignof__(_Fp);
 
    public:
      using value_type = _Fp;
      using difference_type = value_type;
 
      static constexpr bool is_always_lock_free
        = __atomic_always_lock_free(sizeof(_Fp), 0);
 
      __atomic_float() = default;
 
      constexpr
      __atomic_float(_Fp __t) : _M_fp(__t)
      { 
        if (!std::__is_constant_evaluated())
          __atomic_impl::__clear_padding(_M_fp);
      }
 
      __atomic_float(const __atomic_float&) = delete;
      __atomic_float& operator=(const __atomic_float&) = delete;
      __atomic_float& operator=(const __atomic_float&) volatile = delete;
 
      _Fp
      operator=(_Fp __t) volatile noexcept
      {
        this->store(__t);
        return __t;
      }
 
      _Fp
      operator=(_Fp __t) noexcept
      {
        this->store(__t);
        return __t;
      }
 
      bool
      is_lock_free() const volatile noexcept
      { return __atomic_impl::is_lock_free<sizeof(_Fp), _S_alignment>(); }
 
      bool
      is_lock_free() const noexcept
      { return __atomic_impl::is_lock_free<sizeof(_Fp), _S_alignment>(); }
 
      void
      store(_Fp __t, memory_order __m = memory_order_seq_cst) volatile noexcept
      { __atomic_impl::store(&_M_fp, __t, __m); }
 
      void
      store(_Fp __t, memory_order __m = memory_order_seq_cst) noexcept
      { __atomic_impl::store(&_M_fp, __t, __m); }
 
      _Fp
      load(memory_order __m = memory_order_seq_cst) const volatile noexcept
      { return __atomic_impl::load(&_M_fp, __m); }
 
      _Fp
      load(memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::load(&_M_fp, __m); }
 
      operator _Fp() const volatile noexcept { return this->load(); }
      operator _Fp() const noexcept { return this->load(); }
 
      _Fp
      exchange(_Fp __desired,
               memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_impl::exchange(&_M_fp, __desired, __m); }
 
      _Fp
      exchange(_Fp __desired,
               memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_impl::exchange(&_M_fp, __desired, __m); }
 
      bool
      compare_exchange_weak(_Fp& __expected, _Fp __desired,
                            memory_order __success,
                            memory_order __failure) noexcept
      {
        return __atomic_impl::compare_exchange_weak(&_M_fp,
                                                    __expected, __desired,
                                                    __success, __failure);
      }
 
      bool
      compare_exchange_weak(_Fp& __expected, _Fp __desired,
                            memory_order __success,
                            memory_order __failure) volatile noexcept
      {
        return __atomic_impl::compare_exchange_weak(&_M_fp,
                                                    __expected, __desired,
                                                    __success, __failure);
      }
 
      bool
      compare_exchange_strong(_Fp& __expected, _Fp __desired,
                              memory_order __success,
                              memory_order __failure) noexcept
      {
        return __atomic_impl::compare_exchange_strong(&_M_fp,
                                                      __expected, __desired,
                                                      __success, __failure);
      }
 
      bool
      compare_exchange_strong(_Fp& __expected, _Fp __desired,
                              memory_order __success,
                              memory_order __failure) volatile noexcept
      {
        return __atomic_impl::compare_exchange_strong(&_M_fp,
                                                      __expected, __desired,
                                                      __success, __failure);
      }
 
      bool
      compare_exchange_weak(_Fp& __expected, _Fp __desired,
                            memory_order __order = memory_order_seq_cst)
      noexcept
      {
        return compare_exchange_weak(__expected, __desired, __order,
                                     __cmpexch_failure_order(__order));
      }
 
      bool
      compare_exchange_weak(_Fp& __expected, _Fp __desired,
                            memory_order __order = memory_order_seq_cst)
      volatile noexcept
      {
        return compare_exchange_weak(__expected, __desired, __order,
                                     __cmpexch_failure_order(__order));
      }
 
      bool
      compare_exchange_strong(_Fp& __expected, _Fp __desired,
                              memory_order __order = memory_order_seq_cst)
      noexcept
      {
        return compare_exchange_strong(__expected, __desired, __order,
                                       __cmpexch_failure_order(__order));
      }
 
      bool
      compare_exchange_strong(_Fp& __expected, _Fp __desired,
                              memory_order __order = memory_order_seq_cst)
      volatile noexcept
      {
        return compare_exchange_strong(__expected, __desired, __order,
                                       __cmpexch_failure_order(__order));
      }
 
#if __glibcxx_atomic_wait
      _GLIBCXX_ALWAYS_INLINE void
      wait(_Fp __old, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::wait(&_M_fp, __old, __m); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_one() const noexcept
      { __atomic_impl::notify_one(&_M_fp); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_all() const noexcept
      { __atomic_impl::notify_all(&_M_fp); }
 
      // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
      value_type
      fetch_add(value_type __i,
                memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_impl::__fetch_add_flt(&_M_fp, __i, __m); }
 
      value_type
      fetch_add(value_type __i,
                memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_impl::__fetch_add_flt(&_M_fp, __i, __m); }
 
      value_type
      fetch_sub(value_type __i,
                memory_order __m = memory_order_seq_cst) noexcept
      { return __atomic_impl::__fetch_sub_flt(&_M_fp, __i, __m); }
 
      value_type
      fetch_sub(value_type __i,
                memory_order __m = memory_order_seq_cst) volatile noexcept
      { return __atomic_impl::__fetch_sub_flt(&_M_fp, __i, __m); }
 
      value_type
      operator+=(value_type __i) noexcept
      { return __atomic_impl::__add_fetch_flt(&_M_fp, __i); }
 
      value_type
      operator+=(value_type __i) volatile noexcept
      { return __atomic_impl::__add_fetch_flt(&_M_fp, __i); }
 
      value_type
      operator-=(value_type __i) noexcept
      { return __atomic_impl::__sub_fetch_flt(&_M_fp, __i); }
 
      value_type
      operator-=(value_type __i) volatile noexcept
      { return __atomic_impl::__sub_fetch_flt(&_M_fp, __i); }
 
    private:
      alignas(_S_alignment) _Fp _M_fp _GLIBCXX20_INIT(0);
    };
#undef _GLIBCXX20_INIT
 
  template<typename _Tp,
           bool = is_integral_v<_Tp>, bool = is_floating_point_v<_Tp>>
    struct __atomic_ref;
 
  // base class for non-integral, non-floating-point, non-pointer types
  template<typename _Tp>
    struct __atomic_ref<_Tp, false, false>
    {
      static_assert(is_trivially_copyable_v<_Tp>);
 
      // 1/2/4/8/16-byte types must be aligned to at least their size.
      static constexpr int _S_min_alignment
        = (sizeof(_Tp) & (sizeof(_Tp) - 1)) || sizeof(_Tp) > 16
        ? 0 : sizeof(_Tp);
 
    public:
      using value_type = _Tp;
 
      static constexpr bool is_always_lock_free
        = __atomic_always_lock_free(sizeof(_Tp), 0);
 
      static constexpr size_t required_alignment
        = _S_min_alignment > alignof(_Tp) ? _S_min_alignment : alignof(_Tp);
 
      __atomic_ref& operator=(const __atomic_ref&) = delete;
 
      explicit
      __atomic_ref(_Tp& __t) : _M_ptr(std::__addressof(__t))
      { __glibcxx_assert(((uintptr_t)_M_ptr % required_alignment) == 0); }
 
      __atomic_ref(const __atomic_ref&) noexcept = default;
 
      _Tp
      operator=(_Tp __t) const noexcept
      {
        this->store(__t);
        return __t;
      }
 
      operator _Tp() const noexcept { return this->load(); }
 
      bool
      is_lock_free() const noexcept
      { return __atomic_impl::is_lock_free<sizeof(_Tp), required_alignment>(); }
 
      void
      store(_Tp __t, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::store(_M_ptr, __t, __m); }
 
      _Tp
      load(memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::load(_M_ptr, __m); }
 
      _Tp
      exchange(_Tp __desired, memory_order __m = memory_order_seq_cst)
      const noexcept
      { return __atomic_impl::exchange(_M_ptr, __desired, __m); }
 
      bool
      compare_exchange_weak(_Tp& __expected, _Tp __desired,
                            memory_order __success,
                            memory_order __failure) const noexcept
      {
        return __atomic_impl::compare_exchange_weak<true>(
                 _M_ptr, __expected, __desired, __success, __failure);
      }
 
      bool
      compare_exchange_strong(_Tp& __expected, _Tp __desired,
                            memory_order __success,
                            memory_order __failure) const noexcept
      {
        return __atomic_impl::compare_exchange_strong<true>(
                 _M_ptr, __expected, __desired, __success, __failure);
      }
 
      bool
      compare_exchange_weak(_Tp& __expected, _Tp __desired,
                            memory_order __order = memory_order_seq_cst)
      const noexcept
      {
        return compare_exchange_weak(__expected, __desired, __order,
                                     __cmpexch_failure_order(__order));
      }
 
      bool
      compare_exchange_strong(_Tp& __expected, _Tp __desired,
                              memory_order __order = memory_order_seq_cst)
      const noexcept
      {
        return compare_exchange_strong(__expected, __desired, __order,
                                       __cmpexch_failure_order(__order));
      }
 
#if __glibcxx_atomic_wait
      _GLIBCXX_ALWAYS_INLINE void
      wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::wait(_M_ptr, __old, __m); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_one() const noexcept
      { __atomic_impl::notify_one(_M_ptr); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_all() const noexcept
      { __atomic_impl::notify_all(_M_ptr); }
 
      // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
    private:
      _Tp* _M_ptr;
    };
 
  // base class for atomic_ref<integral-type>
  template<typename _Tp>
    struct __atomic_ref<_Tp, true, false>
    {
      static_assert(is_integral_v<_Tp>);
 
    public:
      using value_type = _Tp;
      using difference_type = value_type;
 
      static constexpr bool is_always_lock_free
        = __atomic_always_lock_free(sizeof(_Tp), 0);
 
      static constexpr size_t required_alignment
        = sizeof(_Tp) > alignof(_Tp) ? sizeof(_Tp) : alignof(_Tp);
 
      __atomic_ref() = delete;
      __atomic_ref& operator=(const __atomic_ref&) = delete;
 
      explicit
      __atomic_ref(_Tp& __t) : _M_ptr(&__t)
      { __glibcxx_assert(((uintptr_t)_M_ptr % required_alignment) == 0); }
 
      __atomic_ref(const __atomic_ref&) noexcept = default;
 
      _Tp
      operator=(_Tp __t) const noexcept
      {
        this->store(__t);
        return __t;
      }
 
      operator _Tp() const noexcept { return this->load(); }
 
      bool
      is_lock_free() const noexcept
      {
        return __atomic_impl::is_lock_free<sizeof(_Tp), required_alignment>();
      }
 
      void
      store(_Tp __t, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::store(_M_ptr, __t, __m); }
 
      _Tp
      load(memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::load(_M_ptr, __m); }
 
      _Tp
      exchange(_Tp __desired,
               memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::exchange(_M_ptr, __desired, __m); }
 
      bool
      compare_exchange_weak(_Tp& __expected, _Tp __desired,
                            memory_order __success,
                            memory_order __failure) const noexcept
      {
        return __atomic_impl::compare_exchange_weak<true>(
                 _M_ptr, __expected, __desired, __success, __failure);
      }
 
      bool
      compare_exchange_strong(_Tp& __expected, _Tp __desired,
                              memory_order __success,
                              memory_order __failure) const noexcept
      {
        return __atomic_impl::compare_exchange_strong<true>(
                 _M_ptr, __expected, __desired, __success, __failure);
      }
 
      bool
      compare_exchange_weak(_Tp& __expected, _Tp __desired,
                            memory_order __order = memory_order_seq_cst)
      const noexcept
      {
        return compare_exchange_weak(__expected, __desired, __order,
                                     __cmpexch_failure_order(__order));
      }
 
      bool
      compare_exchange_strong(_Tp& __expected, _Tp __desired,
                              memory_order __order = memory_order_seq_cst)
      const noexcept
      {
        return compare_exchange_strong(__expected, __desired, __order,
                                       __cmpexch_failure_order(__order));
      }
 
#if __glibcxx_atomic_wait
      _GLIBCXX_ALWAYS_INLINE void
      wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::wait(_M_ptr, __old, __m); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_one() const noexcept
      { __atomic_impl::notify_one(_M_ptr); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_all() const noexcept
      { __atomic_impl::notify_all(_M_ptr); }
 
      // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
      value_type
      fetch_add(value_type __i,
                memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::fetch_add(_M_ptr, __i, __m); }
 
      value_type
      fetch_sub(value_type __i,
                memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::fetch_sub(_M_ptr, __i, __m); }
 
      value_type
      fetch_and(value_type __i,
                memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::fetch_and(_M_ptr, __i, __m); }
 
      value_type
      fetch_or(value_type __i,
               memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::fetch_or(_M_ptr, __i, __m); }
 
      value_type
      fetch_xor(value_type __i,
                memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::fetch_xor(_M_ptr, __i, __m); }
 
      _GLIBCXX_ALWAYS_INLINE value_type
      operator++(int) const noexcept
      { return fetch_add(1); }
 
      _GLIBCXX_ALWAYS_INLINE value_type
      operator--(int) const noexcept
      { return fetch_sub(1); }
 
      value_type
      operator++() const noexcept
      { return __atomic_impl::__add_fetch(_M_ptr, value_type(1)); }
 
      value_type
      operator--() const noexcept
      { return __atomic_impl::__sub_fetch(_M_ptr, value_type(1)); }
 
      value_type
      operator+=(value_type __i) const noexcept
      { return __atomic_impl::__add_fetch(_M_ptr, __i); }
 
      value_type
      operator-=(value_type __i) const noexcept
      { return __atomic_impl::__sub_fetch(_M_ptr, __i); }
 
      value_type
      operator&=(value_type __i) const noexcept
      { return __atomic_impl::__and_fetch(_M_ptr, __i); }
 
      value_type
      operator|=(value_type __i) const noexcept
      { return __atomic_impl::__or_fetch(_M_ptr, __i); }
 
      value_type
      operator^=(value_type __i) const noexcept
      { return __atomic_impl::__xor_fetch(_M_ptr, __i); }
 
    private:
      _Tp* _M_ptr;
    };
 
  // base class for atomic_ref<floating-point-type>
  template<typename _Fp>
    struct __atomic_ref<_Fp, false, true>
    {
      static_assert(is_floating_point_v<_Fp>);
 
    public:
      using value_type = _Fp;
      using difference_type = value_type;
 
      static constexpr bool is_always_lock_free
        = __atomic_always_lock_free(sizeof(_Fp), 0);
 
      static constexpr size_t required_alignment = __alignof__(_Fp);
 
      __atomic_ref() = delete;
      __atomic_ref& operator=(const __atomic_ref&) = delete;
 
      explicit
      __atomic_ref(_Fp& __t) : _M_ptr(&__t)
      { __glibcxx_assert(((uintptr_t)_M_ptr % required_alignment) == 0); }
 
      __atomic_ref(const __atomic_ref&) noexcept = default;
 
      _Fp
      operator=(_Fp __t) const noexcept
      {
        this->store(__t);
        return __t;
      }
 
      operator _Fp() const noexcept { return this->load(); }
 
      bool
      is_lock_free() const noexcept
      {
        return __atomic_impl::is_lock_free<sizeof(_Fp), required_alignment>();
      }
 
      void
      store(_Fp __t, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::store(_M_ptr, __t, __m); }
 
      _Fp
      load(memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::load(_M_ptr, __m); }
 
      _Fp
      exchange(_Fp __desired,
               memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::exchange(_M_ptr, __desired, __m); }
 
      bool
      compare_exchange_weak(_Fp& __expected, _Fp __desired,
                            memory_order __success,
                            memory_order __failure) const noexcept
      {
        return __atomic_impl::compare_exchange_weak<true>(
                 _M_ptr, __expected, __desired, __success, __failure);
      }
 
      bool
      compare_exchange_strong(_Fp& __expected, _Fp __desired,
                              memory_order __success,
                              memory_order __failure) const noexcept
      {
        return __atomic_impl::compare_exchange_strong<true>(
                 _M_ptr, __expected, __desired, __success, __failure);
      }
 
      bool
      compare_exchange_weak(_Fp& __expected, _Fp __desired,
                            memory_order __order = memory_order_seq_cst)
      const noexcept
      {
        return compare_exchange_weak(__expected, __desired, __order,
                                     __cmpexch_failure_order(__order));
      }
 
      bool
      compare_exchange_strong(_Fp& __expected, _Fp __desired,
                              memory_order __order = memory_order_seq_cst)
      const noexcept
      {
        return compare_exchange_strong(__expected, __desired, __order,
                                       __cmpexch_failure_order(__order));
      }
 
#if __glibcxx_atomic_wait
      _GLIBCXX_ALWAYS_INLINE void
      wait(_Fp __old, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::wait(_M_ptr, __old, __m); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_one() const noexcept
      { __atomic_impl::notify_one(_M_ptr); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_all() const noexcept
      { __atomic_impl::notify_all(_M_ptr); }
 
      // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
      value_type
      fetch_add(value_type __i,
                memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::__fetch_add_flt(_M_ptr, __i, __m); }
 
      value_type
      fetch_sub(value_type __i,
                memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::__fetch_sub_flt(_M_ptr, __i, __m); }
 
      value_type
      operator+=(value_type __i) const noexcept
      { return __atomic_impl::__add_fetch_flt(_M_ptr, __i); }
 
      value_type
      operator-=(value_type __i) const noexcept
      { return __atomic_impl::__sub_fetch_flt(_M_ptr, __i); }
 
    private:
      _Fp* _M_ptr;
    };
 
  // base class for atomic_ref<pointer-type>
  template<typename _Tp>
    struct __atomic_ref<_Tp*, false, false>
    {
    public:
      using value_type = _Tp*;
      using difference_type = ptrdiff_t;
 
      static constexpr bool is_always_lock_free = ATOMIC_POINTER_LOCK_FREE == 2;
 
      static constexpr size_t required_alignment = __alignof__(_Tp*);
 
      __atomic_ref() = delete;
      __atomic_ref& operator=(const __atomic_ref&) = delete;
 
      explicit
      __atomic_ref(_Tp*& __t) : _M_ptr(std::__addressof(__t))
      { __glibcxx_assert(((uintptr_t)_M_ptr % required_alignment) == 0); }
 
      __atomic_ref(const __atomic_ref&) noexcept = default;
 
      _Tp*
      operator=(_Tp* __t) const noexcept
      {
        this->store(__t);
        return __t;
      }
 
      operator _Tp*() const noexcept { return this->load(); }
 
      bool
      is_lock_free() const noexcept
      {
        return __atomic_impl::is_lock_free<sizeof(_Tp*), required_alignment>();
      }
 
      void
      store(_Tp* __t, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::store(_M_ptr, __t, __m); }
 
      _Tp*
      load(memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::load(_M_ptr, __m); }
 
      _Tp*
      exchange(_Tp* __desired,
               memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::exchange(_M_ptr, __desired, __m); }
 
      bool
      compare_exchange_weak(_Tp*& __expected, _Tp* __desired,
                            memory_order __success,
                            memory_order __failure) const noexcept
      {
        return __atomic_impl::compare_exchange_weak<true>(
                 _M_ptr, __expected, __desired, __success, __failure);
      }
 
      bool
      compare_exchange_strong(_Tp*& __expected, _Tp* __desired,
                            memory_order __success,
                            memory_order __failure) const noexcept
      {
        return __atomic_impl::compare_exchange_strong<true>(
                 _M_ptr, __expected, __desired, __success, __failure);
      }
 
      bool
      compare_exchange_weak(_Tp*& __expected, _Tp* __desired,
                            memory_order __order = memory_order_seq_cst)
      const noexcept
      {
        return compare_exchange_weak(__expected, __desired, __order,
                                     __cmpexch_failure_order(__order));
      }
 
      bool
      compare_exchange_strong(_Tp*& __expected, _Tp* __desired,
                              memory_order __order = memory_order_seq_cst)
      const noexcept
      {
        return compare_exchange_strong(__expected, __desired, __order,
                                       __cmpexch_failure_order(__order));
      }
 
#if __glibcxx_atomic_wait
      _GLIBCXX_ALWAYS_INLINE void
      wait(_Tp* __old, memory_order __m = memory_order_seq_cst) const noexcept
      { __atomic_impl::wait(_M_ptr, __old, __m); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_one() const noexcept
      { __atomic_impl::notify_one(_M_ptr); }
 
      // TODO add const volatile overload
 
      _GLIBCXX_ALWAYS_INLINE void
      notify_all() const noexcept
      { __atomic_impl::notify_all(_M_ptr); }
 
      // TODO add const volatile overload
#endif // __glibcxx_atomic_wait
 
      _GLIBCXX_ALWAYS_INLINE value_type
      fetch_add(difference_type __d,
                memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::fetch_add(_M_ptr, _S_type_size(__d), __m); }
 
      _GLIBCXX_ALWAYS_INLINE value_type
      fetch_sub(difference_type __d,
                memory_order __m = memory_order_seq_cst) const noexcept
      { return __atomic_impl::fetch_sub(_M_ptr, _S_type_size(__d), __m); }
 
      value_type
      operator++(int) const noexcept
      { return fetch_add(1); }
 
      value_type
      operator--(int) const noexcept
      { return fetch_sub(1); }
 
      value_type
      operator++() const noexcept
      {
        return __atomic_impl::__add_fetch(_M_ptr, _S_type_size(1));
      }
 
      value_type
      operator--() const noexcept
      {
        return __atomic_impl::__sub_fetch(_M_ptr, _S_type_size(1));
      }
 
      value_type
      operator+=(difference_type __d) const noexcept
      {
        return __atomic_impl::__add_fetch(_M_ptr, _S_type_size(__d));
      }
 
      value_type
      operator-=(difference_type __d) const noexcept
      {
        return __atomic_impl::__sub_fetch(_M_ptr, _S_type_size(__d));
      }
 
    private:
      static constexpr ptrdiff_t
      _S_type_size(ptrdiff_t __d) noexcept
      {
        static_assert(is_object_v<_Tp>);
        return __d * sizeof(_Tp);
      }
 
      _Tp** _M_ptr;
    };
#endif // C++2a
 
  /// @endcond
 
  /// @} group atomics
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif