Preview: barrier
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//opt/rh/gcc-toolset-14/root/usr/include/c++/14/barrier
// <barrier> -*- C++ -*-
// 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/>.
// This implementation is based on libcxx/include/barrier
//===-- barrier.h --------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===---------------------------------------------------------------===//
/** @file include/barrier
* This is a Standard C++ Library header.
*/
#ifndef _GLIBCXX_BARRIER
#define _GLIBCXX_BARRIER 1
#pragma GCC system_header
#include <bits/requires_hosted.h> // threading primitive
#define __glibcxx_want_barrier
#include <bits/version.h>
#ifdef __cpp_lib_barrier // C++ >= 20 && __cpp_aligned_new && lib_atomic_wait
#include <bits/atomic_base.h>
#include <bits/std_thread.h>
#include <bits/unique_ptr.h>
#include <array>
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
struct __empty_completion
{
_GLIBCXX_ALWAYS_INLINE void
operator()() noexcept
{ }
};
/*
The default implementation of __tree_barrier is a classic tree barrier.
It looks different from literature pseudocode for two main reasons:
1. Threads that call into std::barrier functions do not provide indices,
so a numbering step is added before the actual barrier algorithm,
appearing as an N+1 round to the N rounds of the tree barrier.
2. A great deal of attention has been paid to avoid cache line thrashing
by flattening the tree structure into cache-line sized arrays, that
are indexed in an efficient way.
*/
enum class __barrier_phase_t : unsigned char { };
template<typename _CompletionF>
class __tree_barrier
{
using __atomic_phase_ref_t = std::__atomic_ref<__barrier_phase_t>;
using __atomic_phase_const_ref_t = std::__atomic_ref<const __barrier_phase_t>;
static constexpr auto __phase_alignment =
__atomic_phase_ref_t::required_alignment;
using __tickets_t = std::array<__barrier_phase_t, 64>;
struct alignas(64) /* naturally-align the heap state */ __state_t
{
alignas(__phase_alignment) __tickets_t __tickets;
};
ptrdiff_t _M_expected;
unique_ptr<__state_t[]> _M_state;
__atomic_base<ptrdiff_t> _M_expected_adjustment;
_CompletionF _M_completion;
alignas(__phase_alignment) __barrier_phase_t _M_phase;
bool
_M_arrive(__barrier_phase_t __old_phase, size_t __current)
{
const auto __old_phase_val = static_cast<unsigned char>(__old_phase);
const auto __half_step =
static_cast<__barrier_phase_t>(__old_phase_val + 1);
const auto __full_step =
static_cast<__barrier_phase_t>(__old_phase_val + 2);
size_t __current_expected = _M_expected;
__current %= ((_M_expected + 1) >> 1);
for (int __round = 0; ; ++__round)
{
if (__current_expected <= 1)
return true;
size_t const __end_node = ((__current_expected + 1) >> 1),
__last_node = __end_node - 1;
for ( ; ; ++__current)
{
if (__current == __end_node)
__current = 0;
auto __expect = __old_phase;
__atomic_phase_ref_t __phase(_M_state[__current]
.__tickets[__round]);
if (__current == __last_node && (__current_expected & 1))
{
if (__phase.compare_exchange_strong(__expect, __full_step,
memory_order_acq_rel))
break; // I'm 1 in 1, go to next __round
}
else if (__phase.compare_exchange_strong(__expect, __half_step,
memory_order_acq_rel))
{
return false; // I'm 1 in 2, done with arrival
}
else if (__expect == __half_step)
{
if (__phase.compare_exchange_strong(__expect, __full_step,
memory_order_acq_rel))
break; // I'm 2 in 2, go to next __round
}
}
__current_expected = __last_node + 1;
__current >>= 1;
}
}
public:
using arrival_token = __barrier_phase_t;
static constexpr ptrdiff_t
max() noexcept
{ return __PTRDIFF_MAX__; }
__tree_barrier(ptrdiff_t __expected, _CompletionF __completion)
: _M_expected(__expected), _M_expected_adjustment(0),
_M_completion(move(__completion)),
_M_phase(static_cast<__barrier_phase_t>(0))
{
size_t const __count = (_M_expected + 1) >> 1;
_M_state = std::make_unique<__state_t[]>(__count);
}
[[nodiscard]] arrival_token
arrive(ptrdiff_t __update)
{
std::hash<std::thread::id> __hasher;
size_t __current = __hasher(std::this_thread::get_id());
__atomic_phase_ref_t __phase(_M_phase);
const auto __old_phase = __phase.load(memory_order_relaxed);
const auto __cur = static_cast<unsigned char>(__old_phase);
for(; __update; --__update)
{
if(_M_arrive(__old_phase, __current))
{
_M_completion();
_M_expected += _M_expected_adjustment.load(memory_order_relaxed);
_M_expected_adjustment.store(0, memory_order_relaxed);
auto __new_phase = static_cast<__barrier_phase_t>(__cur + 2);
__phase.store(__new_phase, memory_order_release);
__phase.notify_all();
}
}
return __old_phase;
}
void
wait(arrival_token&& __old_phase) const
{
__atomic_phase_const_ref_t __phase(_M_phase);
auto const __test_fn = [=]
{
return __phase.load(memory_order_acquire) != __old_phase;
};
std::__atomic_wait_address(&_M_phase, __test_fn);
}
void
arrive_and_drop()
{
_M_expected_adjustment.fetch_sub(1, memory_order_relaxed);
(void)arrive(1);
}
};
template<typename _CompletionF = __empty_completion>
class barrier
{
// Note, we may introduce a "central" barrier algorithm at some point
// for more space constrained targets
using __algorithm_t = __tree_barrier<_CompletionF>;
__algorithm_t _M_b;
public:
class arrival_token final
{
public:
arrival_token(arrival_token&&) = default;
arrival_token& operator=(arrival_token&&) = default;
~arrival_token() = default;
private:
friend class barrier;
using __token = typename __algorithm_t::arrival_token;
explicit arrival_token(__token __tok) noexcept : _M_tok(__tok) { }
__token _M_tok;
};
static constexpr ptrdiff_t
max() noexcept
{ return __algorithm_t::max(); }
explicit
barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
: _M_b(__count, std::move(__completion))
{ }
barrier(barrier const&) = delete;
barrier& operator=(barrier const&) = delete;
[[nodiscard]] arrival_token
arrive(ptrdiff_t __update = 1)
{ return arrival_token{_M_b.arrive(__update)}; }
void
wait(arrival_token&& __phase) const
{ _M_b.wait(std::move(__phase._M_tok)); }
void
arrive_and_wait()
{ wait(arrive()); }
void
arrive_and_drop()
{ _M_b.arrive_and_drop(); }
};
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // __cpp_lib_barrier
#endif // _GLIBCXX_BARRIER
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