barrier.hpp

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/*
 * Copyright (c) 2019-2020 Arm Limited.
 *
 * SPDX-License-Identifier: MIT
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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 */
#pragma once
 
#ifndef NO_MULTI_THREADING
 
#include <atomic>
 
namespace arm_gemm {
 
class barrier {
private:
    unsigned int        m_threads;
 
    std::atomic<unsigned int> m_waiters;
    std::atomic<unsigned int> m_leavers;
 
public:
    barrier(unsigned int threads) : m_threads(threads), m_waiters(0), m_leavers(0) { }
 
    /* This isn't safe if any thread is waiting... */
    void set_nthreads(unsigned int nthreads) {
        m_threads = nthreads;
    }
 
    void arrive_and_wait() {
        m_waiters++;
 
        while (m_waiters != m_threads) {
            ; /* spin */
        }
 
        unsigned int v = m_leavers.fetch_add(1);
 
        if (v == (m_threads - 1)) {
            m_waiters -= m_threads;
            m_leavers = 0;
        } else {
            while (m_leavers > 0) {
                ; /* spin */
            }
        }
    }
};
 
} // namespace arm_gemm
 
#else
 
namespace arm_gemm {
 
class barrier {
public:
    barrier(unsigned int) { }
 
    void arrive_and_wait() { }
    void set_nthreads(unsigned int ) { }
};
 
} // namespace arm_gemm
 
#endif