semaphore.h

Go to the documentation of this file.

// Extrapolated from https://vorbrodt.blog/2019/02/05/fast-semaphore/
#pragma once
 
#include <mutex>
#include <atomic>
#include <condition_variable>
#include <cassert>
 
class semaphore
{
public:
    semaphore(int count) noexcept
        : m_count(count) {
        assert(count > -1);
    }
 
    void post() noexcept
    {
        {
            std::unique_lock<std::mutex> lock(m_mutex);
            ++m_count;
        }
        m_cv.notify_one();
    }
 
    void wait() noexcept
    {
        std::unique_lock<std::mutex> lock(m_mutex);
        m_cv.wait(lock, [&]() { return m_count != 0; });
        --m_count;
    }
 
private:
    int m_count;
    std::mutex m_mutex;
    std::condition_variable m_cv;
};
 
class fast_semaphore
{
public:
    fast_semaphore(int count) noexcept
        : m_count(count), m_semaphore(0) {}
 
    void post()
    {
        std::atomic_thread_fence(std::memory_order_release);
        int count = m_count.fetch_add(1, std::memory_order_relaxed);
        if (count < 0)
            m_semaphore.post();
    }
 
    void wait()
    {
        int count = m_count.fetch_sub(1, std::memory_order_relaxed);
        if (count < 1)
            m_semaphore.wait();
        std::atomic_thread_fence(std::memory_order_acquire);
    }
 
private:
    std::atomic<int> m_count;
    semaphore m_semaphore;
};