Preview: atomic_wait.h
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//opt/rh/gcc-toolset-14/root/usr/include/c++/14/bits/atomic_wait.h
// -*- C++ -*- header.
// Copyright (C) 2020-2024 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_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
#pragma GCC system_header
#include <bits/version.h>
#if __glibcxx_atomic_wait
#include <cstdint>
#include <bits/functional_hash.h>
#include <bits/gthr.h>
#include <ext/numeric_traits.h>
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
# include <cerrno>
# include <climits>
# include <unistd.h>
# include <syscall.h>
# include <bits/functexcept.h>
#endif
# include <bits/std_mutex.h> // std::mutex, std::__condvar
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
{
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX
enum class __futex_wait_flags : int
{
#ifdef _GLIBCXX_HAVE_LINUX_FUTEX_PRIVATE
__private_flag = 128,
#else
__private_flag = 0,
#endif
__wait = 0,
__wake = 1,
__wait_bitset = 9,
__wake_bitset = 10,
__wait_private = __wait | __private_flag,
__wake_private = __wake | __private_flag,
__wait_bitset_private = __wait_bitset | __private_flag,
__wake_bitset_private = __wake_bitset | __private_flag,
__bitset_match_any = -1
};
template<typename _Tp>
void
__platform_wait(const _Tp* __addr, __platform_wait_t __val) noexcept
{
auto __e = syscall (SYS_futex, static_cast<const void*>(__addr),
static_cast<int>(__futex_wait_flags::__wait_private),
__val, nullptr);
if (!__e || errno == EAGAIN)
return;
if (errno != EINTR)
__throw_system_error(errno);
}
template<typename _Tp>
void
__platform_notify(const _Tp* __addr, bool __all) noexcept
{
syscall (SYS_futex, static_cast<const void*>(__addr),
static_cast<int>(__futex_wait_flags::__wake_private),
__all ? INT_MAX : 1);
}
#endif
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
}
inline constexpr auto __atomic_spin_count_relax = 12;
inline constexpr auto __atomic_spin_count = 16;
struct __default_spin_policy
{
bool
operator()() const noexcept
{ return false; }
};
template<typename _Pred,
typename _Spin = __default_spin_policy>
bool
__atomic_spin(_Pred& __pred, _Spin __spin = _Spin{ }) noexcept
{
for (auto __i = 0; __i < __atomic_spin_count; ++__i)
{
if (__pred())
return true;
if (__i < __atomic_spin_count_relax)
__detail::__thread_relax();
else
__detail::__thread_yield();
}
while (__spin())
{
if (__pred())
return true;
}
return false;
}
// return true if equal
template<typename _Tp>
bool __atomic_compare(const _Tp& __a, const _Tp& __b)
{
// TODO make this do the correct padding bit ignoring comparison
return __builtin_memcmp(&__a, &__b, sizeof(_Tp)) == 0;
}
struct __waiter_pool_base
{
// Don't use std::hardware_destructive_interference_size here because we
// don't want the layout of library types to depend on compiler options.
static constexpr auto _S_align = 64;
alignas(_S_align) __platform_wait_t _M_wait = 0;
#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
mutex _M_mtx;
#endif
alignas(_S_align) __platform_wait_t _M_ver = 0;
#ifndef _GLIBCXX_HAVE_PLATFORM_WAIT
__condvar _M_cv;
#endif
__waiter_pool_base() = default;
void
_M_enter_wait() noexcept
{ __atomic_fetch_add(&_M_wait, 1, __ATOMIC_SEQ_CST); }
void
_M_leave_wait() noexcept
{ __atomic_fetch_sub(&_M_wait, 1, __ATOMIC_RELEASE); }
bool
_M_waiting() const noexcept
{
__platform_wait_t __res;
__atomic_load(&_M_wait, &__res, __ATOMIC_SEQ_CST);
return __res != 0;
}
void
_M_notify(__platform_wait_t* __addr, [[maybe_unused]] bool __all,
bool __bare) noexcept
{
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
if (__addr == &_M_ver)
{
__atomic_fetch_add(__addr, 1, __ATOMIC_SEQ_CST);
__all = true;
}
if (__bare || _M_waiting())
__platform_notify(__addr, __all);
#else
{
lock_guard<mutex> __l(_M_mtx);
__atomic_fetch_add(__addr, 1, __ATOMIC_RELAXED);
}
if (__bare || _M_waiting())
_M_cv.notify_all();
#endif
}
static __waiter_pool_base&
_S_for(const void* __addr) noexcept
{
constexpr uintptr_t __ct = 16;
static __waiter_pool_base __w[__ct];
auto __key = (uintptr_t(__addr) >> 2) % __ct;
return __w[__key];
}
};
struct __waiter_pool : __waiter_pool_base
{
void
_M_do_wait(const __platform_wait_t* __addr, __platform_wait_t __old) noexcept
{
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
__platform_wait(__addr, __old);
#else
__platform_wait_t __val;
__atomic_load(__addr, &__val, __ATOMIC_SEQ_CST);
if (__val == __old)
{
lock_guard<mutex> __l(_M_mtx);
__atomic_load(__addr, &__val, __ATOMIC_RELAXED);
if (__val == __old)
_M_cv.wait(_M_mtx);
}
#endif // __GLIBCXX_HAVE_PLATFORM_WAIT
}
};
template<typename _Tp>
struct __waiter_base
{
using __waiter_type = _Tp;
__waiter_type& _M_w;
__platform_wait_t* _M_addr;
template<typename _Up>
static __platform_wait_t*
_S_wait_addr(const _Up* __a, __platform_wait_t* __b)
{
if constexpr (__platform_wait_uses_type<_Up>)
return reinterpret_cast<__platform_wait_t*>(const_cast<_Up*>(__a));
else
return __b;
}
static __waiter_type&
_S_for(const void* __addr) noexcept
{
static_assert(sizeof(__waiter_type) == sizeof(__waiter_pool_base));
auto& res = __waiter_pool_base::_S_for(__addr);
return reinterpret_cast<__waiter_type&>(res);
}
template<typename _Up>
explicit __waiter_base(const _Up* __addr) noexcept
: _M_w(_S_for(__addr))
, _M_addr(_S_wait_addr(__addr, &_M_w._M_ver))
{ }
void
_M_notify(bool __all, bool __bare = false) noexcept
{ _M_w._M_notify(_M_addr, __all, __bare); }
template<typename _Up, typename _ValFn,
typename _Spin = __default_spin_policy>
static bool
_S_do_spin_v(__platform_wait_t* __addr,
const _Up& __old, _ValFn __vfn,
__platform_wait_t& __val,
_Spin __spin = _Spin{ })
{
auto const __pred = [=]
{ return !__detail::__atomic_compare(__old, __vfn()); };
if constexpr (__platform_wait_uses_type<_Up>)
{
__builtin_memcpy(&__val, &__old, sizeof(__val));
}
else
{
__atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
}
return __atomic_spin(__pred, __spin);
}
template<typename _Up, typename _ValFn,
typename _Spin = __default_spin_policy>
bool
_M_do_spin_v(const _Up& __old, _ValFn __vfn,
__platform_wait_t& __val,
_Spin __spin = _Spin{ })
{ return _S_do_spin_v(_M_addr, __old, __vfn, __val, __spin); }
template<typename _Pred,
typename _Spin = __default_spin_policy>
static bool
_S_do_spin(const __platform_wait_t* __addr,
_Pred __pred,
__platform_wait_t& __val,
_Spin __spin = _Spin{ })
{
__atomic_load(__addr, &__val, __ATOMIC_ACQUIRE);
return __atomic_spin(__pred, __spin);
}
template<typename _Pred,
typename _Spin = __default_spin_policy>
bool
_M_do_spin(_Pred __pred, __platform_wait_t& __val,
_Spin __spin = _Spin{ })
{ return _S_do_spin(_M_addr, __pred, __val, __spin); }
};
template<typename _EntersWait>
struct __waiter : __waiter_base<__waiter_pool>
{
using __base_type = __waiter_base<__waiter_pool>;
template<typename _Tp>
explicit __waiter(const _Tp* __addr) noexcept
: __base_type(__addr)
{
if constexpr (_EntersWait::value)
_M_w._M_enter_wait();
}
~__waiter()
{
if constexpr (_EntersWait::value)
_M_w._M_leave_wait();
}
template<typename _Tp, typename _ValFn>
void
_M_do_wait_v(_Tp __old, _ValFn __vfn)
{
do
{
__platform_wait_t __val;
if (__base_type::_M_do_spin_v(__old, __vfn, __val))
return;
__base_type::_M_w._M_do_wait(__base_type::_M_addr, __val);
}
while (__detail::__atomic_compare(__old, __vfn()));
}
template<typename _Pred>
void
_M_do_wait(_Pred __pred) noexcept
{
do
{
__platform_wait_t __val;
if (__base_type::_M_do_spin(__pred, __val))
return;
__base_type::_M_w._M_do_wait(__base_type::_M_addr, __val);
}
while (!__pred());
}
};
using __enters_wait = __waiter<std::true_type>;
using __bare_wait = __waiter<std::false_type>;
} // namespace __detail
template<typename _Tp, typename _ValFn>
void
__atomic_wait_address_v(const _Tp* __addr, _Tp __old,
_ValFn __vfn) noexcept
{
__detail::__enters_wait __w(__addr);
__w._M_do_wait_v(__old, __vfn);
}
template<typename _Tp, typename _Pred>
void
__atomic_wait_address(const _Tp* __addr, _Pred __pred) noexcept
{
__detail::__enters_wait __w(__addr);
__w._M_do_wait(__pred);
}
// This call is to be used by atomic types which track contention externally
template<typename _Pred>
void
__atomic_wait_address_bare(const __detail::__platform_wait_t* __addr,
_Pred __pred) noexcept
{
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
do
{
__detail::__platform_wait_t __val;
if (__detail::__bare_wait::_S_do_spin(__addr, __pred, __val))
return;
__detail::__platform_wait(__addr, __val);
}
while (!__pred());
#else // !_GLIBCXX_HAVE_PLATFORM_WAIT
__detail::__bare_wait __w(__addr);
__w._M_do_wait(__pred);
#endif
}
template<typename _Tp>
void
__atomic_notify_address(const _Tp* __addr, bool __all) noexcept
{
__detail::__bare_wait __w(__addr);
__w._M_notify(__all);
}
// This call is to be used by atomic types which track contention externally
inline void
__atomic_notify_address_bare(const __detail::__platform_wait_t* __addr,
bool __all) noexcept
{
#ifdef _GLIBCXX_HAVE_PLATFORM_WAIT
__detail::__platform_notify(__addr, __all);
#else
__detail::__bare_wait __w(__addr);
__w._M_notify(__all, true);
#endif
}
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
#endif // __glibcxx_atomic_wait
#endif // _GLIBCXX_ATOMIC_WAIT_H
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