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Editing: barrier

// -*- 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;

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 <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

#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, std::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(std::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

#endif //_LIBCPP_BARRIER

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