// -*- C++ -*-
//===--------------------------- barrier ----------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
#ifndef _LIBCPP_BARRIER
#define _LIBCPP_BARRIER
/*
barrier synopsis
namespace std
{
template<class CompletionFunction = see below>
class barrier
{
public:
using arrival_token = see below;
static constexpr ptrdiff_t max() noexcept;
constexpr explicit barrier(ptrdiff_t phase_count,
CompletionFunction f = CompletionFunction());
~barrier();
barrier(const barrier&) = delete;
barrier& operator=(const barrier&) = delete;
[[nodiscard]] arrival_token arrive(ptrdiff_t update = 1);
void wait(arrival_token&& arrival) const;
void arrive_and_wait();
void arrive_and_drop();
private:
CompletionFunction completion; // exposition only
};
}
*/
#include <__config>
#include <__availability>
#include <atomic>
#ifndef _LIBCPP_HAS_NO_TREE_BARRIER
# include <memory>
#endif
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif
#ifdef _LIBCPP_HAS_NO_THREADS
# error <barrier> is not supported on this single threaded system
#endif
_LIBCPP_PUSH_MACROS
#include <__undef_macros>
#if _LIBCPP_STD_VER >= 14
_LIBCPP_BEGIN_NAMESPACE_STD
struct __empty_completion
{
inline _LIBCPP_INLINE_VISIBILITY
void operator()() noexcept
{
}
};
#ifndef _LIBCPP_HAS_NO_TREE_BARRIER
/*
The default implementation of __barrier_base 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.
*/
using __barrier_phase_t = uint8_t;
class __barrier_algorithm_base;
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
__barrier_algorithm_base* __construct_barrier_algorithm_base(ptrdiff_t& __expected);
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
bool __arrive_barrier_algorithm_base(__barrier_algorithm_base* __barrier,
__barrier_phase_t __old_phase);
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_EXPORTED_FROM_ABI
void __destroy_barrier_algorithm_base(__barrier_algorithm_base* __barrier);
template<class _CompletionF>
class __barrier_base {
ptrdiff_t __expected;
unique_ptr<__barrier_algorithm_base,
void (*)(__barrier_algorithm_base*)> __base;
__atomic_base<ptrdiff_t> __expected_adjustment;
_CompletionF __completion;
__atomic_base<__barrier_phase_t> __phase;
public:
using arrival_token = __barrier_phase_t;
static constexpr ptrdiff_t max() noexcept {
return numeric_limits<ptrdiff_t>::max();
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
__barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
: __expected(__expected), __base(__construct_barrier_algorithm_base(this->__expected),
&__destroy_barrier_algorithm_base),
__expected_adjustment(0), __completion(move(__completion)), __phase(0)
{
}
[[nodiscard]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
arrival_token arrive(ptrdiff_t update)
{
auto const __old_phase = __phase.load(memory_order_relaxed);
for(; update; --update)
if(__arrive_barrier_algorithm_base(__base.get(), __old_phase)) {
__completion();
__expected += __expected_adjustment.load(memory_order_relaxed);
__expected_adjustment.store(0, memory_order_relaxed);
__phase.store(__old_phase + 2, memory_order_release);
__phase.notify_all();
}
return __old_phase;
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void wait(arrival_token&& __old_phase) const
{
auto const __test_fn = [=]() -> bool {
return __phase.load(memory_order_acquire) != __old_phase;
};
__libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy());
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void arrive_and_drop()
{
__expected_adjustment.fetch_sub(1, memory_order_relaxed);
(void)arrive(1);
}
};
#else
/*
The alternative implementation of __barrier_base is a central barrier.
Two versions of this algorithm are provided:
1. A fairly straightforward implementation of the litterature for the
general case where the completion function is not empty.
2. An optimized implementation that exploits 2's complement arithmetic
and well-defined overflow in atomic arithmetic, to handle the phase
roll-over for free.
*/
template<class _CompletionF>
class __barrier_base {
__atomic_base<ptrdiff_t> __expected;
__atomic_base<ptrdiff_t> __arrived;
_CompletionF __completion;
__atomic_base<bool> __phase;
public:
using arrival_token = bool;
static constexpr ptrdiff_t max() noexcept {
return numeric_limits<ptrdiff_t>::max();
}
_LIBCPP_INLINE_VISIBILITY
__barrier_base(ptrdiff_t __expected, _CompletionF __completion = _CompletionF())
: __expected(__expected), __arrived(__expected), __completion(move(__completion)), __phase(false)
{
}
[[nodiscard]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
arrival_token arrive(ptrdiff_t update)
{
auto const __old_phase = __phase.load(memory_order_relaxed);
auto const __result = __arrived.fetch_sub(update, memory_order_acq_rel) - update;
auto const new_expected = __expected.load(memory_order_relaxed);
if(0 == __result) {
__completion();
__arrived.store(new_expected, memory_order_relaxed);
__phase.store(!__old_phase, memory_order_release);
__phase.notify_all();
}
return __old_phase;
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void wait(arrival_token&& __old_phase) const
{
__phase.wait(__old_phase, memory_order_acquire);
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void arrive_and_drop()
{
__expected.fetch_sub(1, memory_order_relaxed);
(void)arrive(1);
}
};
template<>
class __barrier_base<__empty_completion> {
static constexpr uint64_t __expected_unit = 1ull;
static constexpr uint64_t __arrived_unit = 1ull << 32;
static constexpr uint64_t __expected_mask = __arrived_unit - 1;
static constexpr uint64_t __phase_bit = 1ull << 63;
static constexpr uint64_t __arrived_mask = (__phase_bit - 1) & ~__expected_mask;
__atomic_base<uint64_t> __phase_arrived_expected;
static _LIBCPP_INLINE_VISIBILITY
constexpr uint64_t __init(ptrdiff_t __count) _NOEXCEPT
{
return ((uint64_t(1u << 31) - __count) << 32)
| (uint64_t(1u << 31) - __count);
}
public:
using arrival_token = uint64_t;
static constexpr ptrdiff_t max() noexcept {
return ptrdiff_t(1u << 31) - 1;
}
_LIBCPP_INLINE_VISIBILITY
explicit inline __barrier_base(ptrdiff_t __count, __empty_completion = __empty_completion())
: __phase_arrived_expected(__init(__count))
{
}
[[nodiscard]] inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
arrival_token arrive(ptrdiff_t update)
{
auto const __inc = __arrived_unit * update;
auto const __old = __phase_arrived_expected.fetch_add(__inc, memory_order_acq_rel);
if((__old ^ (__old + __inc)) & __phase_bit) {
__phase_arrived_expected.fetch_add((__old & __expected_mask) << 32, memory_order_relaxed);
__phase_arrived_expected.notify_all();
}
return __old & __phase_bit;
}
inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void wait(arrival_token&& __phase) const
{
auto const __test_fn = [=]() -> bool {
uint64_t const __current = __phase_arrived_expected.load(memory_order_acquire);
return ((__current & __phase_bit) != __phase);
};
__libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy());
}
inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void arrive_and_drop()
{
__phase_arrived_expected.fetch_add(__expected_unit, memory_order_relaxed);
(void)arrive(1);
}
};
#endif //_LIBCPP_HAS_NO_TREE_BARRIER
template<class _CompletionF = __empty_completion>
class barrier {
__barrier_base<_CompletionF> __b;
public:
using arrival_token = typename __barrier_base<_CompletionF>::arrival_token;
static constexpr ptrdiff_t max() noexcept {
return __barrier_base<_CompletionF>::max();
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF())
: __b(__count, _VSTD::move(__completion)) {
}
barrier(barrier const&) = delete;
barrier& operator=(barrier const&) = delete;
[[nodiscard]] _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
arrival_token arrive(ptrdiff_t update = 1)
{
return __b.arrive(update);
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void wait(arrival_token&& __phase) const
{
__b.wait(_VSTD::move(__phase));
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void arrive_and_wait()
{
wait(arrive());
}
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
void arrive_and_drop()
{
__b.arrive_and_drop();
}
};
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP_STD_VER >= 14
_LIBCPP_POP_MACROS
#endif //_LIBCPP_BARRIER