atomic_wait.h

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// -*- C++ -*- header.
 
// Copyright (C) 2020-2025 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_wait.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{atomic}
 */
 
#ifndef _GLIBCXX_ATOMIC_WAIT_H
#define _GLIBCXX_ATOMIC_WAIT_H 1
 
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
 
#include <bits/version.h>
 
#if __glibcxx_atomic_wait
#include <bits/gthr.h>
#include <ext/numeric_traits.h>
 
#include <bits/stl_pair.h>
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
  namespace __detail
  {
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
#define _GLIBCXX_HAVE_PLATFORM_WAIT 1
    using __platform_wait_t = int;
    inline constexpr size_t __platform_wait_alignment = 4;
#else
// define _GLIBCX_HAVE_PLATFORM_WAIT and implement __platform_wait()
// and __platform_notify() if there is a more efficient primitive supported
// by the platform (e.g. __ulock_wait()/__ulock_wake()) which is better than
// a mutex/condvar based wait.
# if ATOMIC_LONG_LOCK_FREE == 2
    using __platform_wait_t = unsigned long;
# else
    using __platform_wait_t = unsigned int;
# endif
    inline constexpr size_t __platform_wait_alignment
      = __alignof__(__platform_wait_t);
#endif
  } // namespace __detail
 
  template<typename _Tp>
    inline constexpr bool __platform_wait_uses_type
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
      = is_scalar_v<_Tp>
        && ((sizeof(_Tp) == sizeof(__detail::__platform_wait_t))
        && (alignof(_Tp) >= __detail::__platform_wait_alignment));
#else
      = false;
#endif
 
  namespace __detail
  {
    inline void
    __thread_yield() noexcept
    {
#if defined _GLIBCXX_HAS_GTHREADS && defined _GLIBCXX_USE_SCHED_YIELD
      __gthread_yield();
#endif
    }
 
    inline void
    __thread_relax() noexcept
    {
#if defined __i386__ || defined __x86_64__
      __builtin_ia32_pause();
#else
      __thread_yield();
#endif
    }
 
    // return true if equal
    template<typename _Tp>
      inline bool
      __atomic_eq(const _Tp& __a, const _Tp& __b)
      {
        // TODO make this do the correct padding bit ignoring comparison
        return __builtin_memcmp(&__a, &__b, sizeof(_Tp)) == 0;
      }
 
    // lightweight std::optional<__platform_wait_t>
    struct __wait_result_type
    {
      __platform_wait_t _M_val;
      unsigned char _M_has_val : 1; // _M_val value was loaded before return.
      unsigned char _M_timeout : 1; // Waiting function ended with timeout.
      unsigned char _M_unused : 6;  // padding
    };
 
    enum class __wait_flags : __UINT_LEAST32_TYPE__
    {
       __abi_version = 0,
       __proxy_wait = 1,
       __track_contention = 2,
       __do_spin = 4,
       __spin_only = 8, // Ignored unless __do_spin is also set.
       // __abi_version_mask = 0xffff0000,
    };
 
    [[__gnu__::__always_inline__]]
    constexpr __wait_flags
    operator|(__wait_flags __l, __wait_flags __r) noexcept
    {
      using _Ut = underlying_type_t<__wait_flags>;
      return static_cast<__wait_flags>(static_cast<_Ut>(__l)
                                         | static_cast<_Ut>(__r));
    }
 
    [[__gnu__::__always_inline__]]
    constexpr __wait_flags&
    operator|=(__wait_flags& __l, __wait_flags __r) noexcept
    { return __l = __l | __r; }
 
    // Simple aggregate containing arguments used by implementation details.
    struct __wait_args_base
    {
      __wait_flags _M_flags;
      int _M_order = __ATOMIC_ACQUIRE;
      __platform_wait_t _M_old = 0;
      void* _M_wait_state = nullptr;
 
      // Test whether _M_flags & __flags is non-zero.
      bool
      operator&(__wait_flags __flags) const noexcept
      {
         using _Ut = underlying_type_t<__wait_flags>;
         return static_cast<_Ut>(_M_flags) & static_cast<_Ut>(__flags);
      }
    };
 
    // Utility for populating a __wait_args_base structure.
    struct __wait_args : __wait_args_base
    {
      template<typename _Tp> requires (!is_same_v<_Tp, __wait_args>)
        explicit
        __wait_args(const _Tp* __addr, bool __bare_wait = false) noexcept
        : __wait_args_base{ _S_flags_for(__addr, __bare_wait) }
        { }
 
      __wait_args(const __platform_wait_t* __addr, __platform_wait_t __old,
                  int __order, bool __bare_wait = false) noexcept
      : __wait_args_base{ _S_flags_for(__addr, __bare_wait), __order, __old }
      { }
 
      __wait_args(const __wait_args&) noexcept = default;
      __wait_args& operator=(const __wait_args&) noexcept = default;
 
      template<typename _ValFn,
               typename _Tp = decay_t<decltype(std::declval<_ValFn&>()())>>
        _Tp
        _M_setup_wait(const void* __addr, _ValFn __vfn,
                      __wait_result_type __res = {})
        {
          if constexpr (__platform_wait_uses_type<_Tp>)
            {
              // If the wait is not proxied, the value we check when waiting
              // is the value of the atomic variable itself.
 
              if (__res._M_has_val) // The previous wait loaded a recent value.
                {
                  _M_old = __res._M_val;
                  return __builtin_bit_cast(_Tp, __res._M_val);
                }
              else // Load the value from __vfn
                {
                  _Tp __val = __vfn();
                  _M_old = __builtin_bit_cast(__platform_wait_t, __val);
                  return __val;
                }
            }
          else // It's a proxy wait and the proxy's _M_ver is used.
            {
              if (__res._M_has_val) // The previous wait loaded a recent value.
                _M_old = __res._M_val;
              else // Load _M_ver from the proxy (must happen before __vfn()).
                _M_load_proxy_wait_val(__addr);
              return __vfn();
            }
        }
 
    private:
      // Populates _M_wait_state and _M_old from the proxy for __addr.
      void
      _M_load_proxy_wait_val(const void* __addr);
 
      template<typename _Tp>
        static constexpr __wait_flags
        _S_flags_for(const _Tp*, bool __bare_wait) noexcept
        {
          using enum __wait_flags;
          __wait_flags __res = __abi_version | __do_spin;
          if (!__bare_wait)
            __res |= __track_contention;
          if constexpr (!__platform_wait_uses_type<_Tp>)
            __res |= __proxy_wait;
          return __res;
        }
    };
 
    __wait_result_type
    __wait_impl(const void* __addr, __wait_args_base&);
 
    void
    __notify_impl(const void* __addr, bool __all, const __wait_args_base&);
  } // namespace __detail
 
  // Wait on __addr while __pred(__vfn()) is false.
  // If __bare_wait is false, increment a counter while waiting.
  // For callers that keep their own count of waiters, use __bare_wait=true.
  template<typename _Tp, typename _Pred, typename _ValFn>
    void
    __atomic_wait_address(const _Tp* __addr, _Pred&& __pred, _ValFn&& __vfn,
                          bool __bare_wait = false) noexcept
    {
      __detail::__wait_args __args{ __addr, __bare_wait };
      _Tp __val = __args._M_setup_wait(__addr, __vfn);
      while (!__pred(__val))
        {
          auto __res = __detail::__wait_impl(__addr, __args);
          __val = __args._M_setup_wait(__addr, __vfn, __res);
        }
      // C++26 will return __val
    }
 
  // Wait on __addr while *__addr == __old is true.
  inline void
  __atomic_wait_address_v(const __detail::__platform_wait_t* __addr,
                          __detail::__platform_wait_t __old,
                          int __order, bool __bare_wait = false)
  {
#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
    __glibcxx_assert(false); // This function can't be used for proxy wait.
#endif
    __detail::__wait_args __args{ __addr, __old, __order, __bare_wait };
    // C++26 will not ignore the return value here
    __detail::__wait_impl(__addr, __args);
  }
 
  // Wait on __addr while __vfn() == __old is true.
  template<typename _Tp, typename _ValFn>
    void
    __atomic_wait_address_v(const _Tp* __addr, _Tp __old,
                            _ValFn __vfn) noexcept
    {
      auto __pfn = [&](const _Tp& __val)
          { return !__detail::__atomic_eq(__old, __val); };
      std::__atomic_wait_address(__addr, __pfn, forward<_ValFn>(__vfn));
    }
 
  template<typename _Tp>
    void
    __atomic_notify_address(const _Tp* __addr, bool __all,
                            bool __bare_wait = false) noexcept
    {
      __detail::__wait_args __args{ __addr, __bare_wait };
      __detail::__notify_impl(__addr, __all, __args);
    }
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
#endif // __glibcxx_atomic_wait
#endif // _GLIBCXX_ATOMIC_WAIT_H