lightweightsemaphore.h

// Provides an efficient implementation of a semaphore (LightweightSemaphore).
// This is an extension of Jeff Preshing's sempahore implementation (licensed
// under the terms of its separate zlib license) that has been adapted and
// extended by Cameron Desrochers.
 
#pragma once
 
#include <cstddef> // For std::size_t
#include <atomic>
#include <type_traits> // For std::make_signed<T>
 
#if defined(_WIN32)
// Avoid including windows.h in a header; we only need a handful of
// items, so we'll redeclare them here (this is relatively safe since
// the API generally has to remain stable between Windows versions).
// I know this is an ugly hack but it still beats polluting the global
// namespace with thousands of generic names or adding a .cpp for nothing.
extern "C" {
struct _SECURITY_ATTRIBUTES;
__declspec(dllimport) void* __stdcall CreateSemaphoreW(_SECURITY_ATTRIBUTES* lpSemaphoreAttributes,
                                                       long lInitialCount,
                                                       long lMaximumCount,
                                                       const wchar_t* lpName);
__declspec(dllimport) int __stdcall CloseHandle(void* hObject);
__declspec(dllimport) unsigned long __stdcall WaitForSingleObject(void* hHandle, unsigned long dwMilliseconds);
__declspec(dllimport) int __stdcall ReleaseSemaphore(void* hSemaphore, long lReleaseCount, long* lpPreviousCount);
}
#elif defined(__MACH__)
#include <mach/mach.h>
#elif defined(__unix__) || defined(__wasm__)
#include <semaphore.h>
 
#if defined(__GLIBC_PREREQ) && defined(_GNU_SOURCE)
#if __GLIBC_PREREQ(2, 30)
#define MOODYCAMEL_LIGHTWEIGHTSEMAPHORE_MONOTONIC
#endif
#endif
#endif
 
namespace moodycamel {
namespace details {
 
// Code in the mpmc_sema namespace below is an adaptation of Jeff Preshing's
// portable + lightweight semaphore implementations, originally from
// https://github.com/preshing/cpp11-on-multicore/blob/master/common/sema.h
// LICENSE:
// Copyright (c) 2015 Jeff Preshing
//
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
//
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
//
// 1. The origin of this software must not be misrepresented; you must not
//  claim that you wrote the original software. If you use this software
//  in a product, an acknowledgement in the product documentation would be
//  appreciated but is not required.
// 2. Altered source versions must be plainly marked as such, and must not be
//  misrepresented as being the original software.
// 3. This notice may not be removed or altered from any source distribution.
#if defined(_WIN32)
class Semaphore {
  private:
    void* m_hSema;
 
    Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
    Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
 
  public:
    Semaphore(int initialCount = 0)
    {
        assert(initialCount >= 0);
        const long maxLong = 0x7fffffff;
        m_hSema = CreateSemaphoreW(nullptr, initialCount, maxLong, nullptr);
        assert(m_hSema);
    }
 
    ~Semaphore() { CloseHandle(m_hSema); }
 
    bool wait()
    {
        const unsigned long infinite = 0xffffffff;
        return WaitForSingleObject(m_hSema, infinite) == 0;
    }
 
    bool try_wait() { return WaitForSingleObject(m_hSema, 0) == 0; }
 
    bool timed_wait(std::uint64_t usecs) { return WaitForSingleObject(m_hSema, (unsigned long)(usecs / 1000)) == 0; }
 
    void signal(int count = 1)
    {
        while (!ReleaseSemaphore(m_hSema, count, nullptr))
            ;
    }
};
#elif defined(__MACH__)
//---------------------------------------------------------
// Semaphore (Apple iOS and OSX)
// Can't use POSIX semaphores due to http://lists.apple.com/archives/darwin-kernel/2009/Apr/msg00010.html
//---------------------------------------------------------
class Semaphore {
  private:
    semaphore_t m_sema;
 
    Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
    Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
 
  public:
    Semaphore(int initialCount = 0)
    {
        assert(initialCount >= 0);
        kern_return_t rc = semaphore_create(mach_task_self(), &m_sema, SYNC_POLICY_FIFO, initialCount);
        assert(rc == KERN_SUCCESS);
        (void)rc;
    }
 
    ~Semaphore() { semaphore_destroy(mach_task_self(), m_sema); }
 
    bool wait() { return semaphore_wait(m_sema) == KERN_SUCCESS; }
 
    bool try_wait() { return timed_wait(0); }
 
    bool timed_wait(std::uint64_t timeout_usecs)
    {
        mach_timespec_t ts;
        ts.tv_sec = static_cast<unsigned int>(timeout_usecs / 1000000);
        ts.tv_nsec = static_cast<int>((timeout_usecs % 1000000) * 1000);
 
        // added in OSX 10.10:
        // https://developer.apple.com/library/prerelease/mac/documentation/General/Reference/APIDiffsMacOSX10_10SeedDiff/modules/Darwin.html
        kern_return_t rc = semaphore_timedwait(m_sema, ts);
        return rc == KERN_SUCCESS;
    }
 
    void signal()
    {
        while (semaphore_signal(m_sema) != KERN_SUCCESS)
            ;
    }
 
    void signal(int count)
    {
        while (count-- > 0) {
            while (semaphore_signal(m_sema) != KERN_SUCCESS)
                ;
        }
    }
};
#elif defined(__unix__) || defined(__wasm__)
//---------------------------------------------------------
// Semaphore (POSIX, Linux)
//---------------------------------------------------------
class Semaphore {
  private:
    sem_t m_sema;
 
    Semaphore(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
    Semaphore& operator=(const Semaphore& other) MOODYCAMEL_DELETE_FUNCTION;
 
  public:
    Semaphore(int initialCount = 0)
    {
        assert(initialCount >= 0);
        int rc = sem_init(&m_sema, 0, static_cast<unsigned int>(initialCount));
        assert(rc == 0);
        (void)rc;
    }
 
    ~Semaphore() { sem_destroy(&m_sema); }
 
    bool wait()
    {
        // http://stackoverflow.com/questions/2013181/gdb-causes-sem-wait-to-fail-with-eintr-error
        int rc;
        do {
            rc = sem_wait(&m_sema);
        } while (rc == -1 && errno == EINTR);
        return rc == 0;
    }
 
    bool try_wait()
    {
        int rc;
        do {
            rc = sem_trywait(&m_sema);
        } while (rc == -1 && errno == EINTR);
        return rc == 0;
    }
 
    bool timed_wait(std::uint64_t usecs)
    {
        struct timespec ts;
        const int usecs_in_1_sec = 1000000;
        const int nsecs_in_1_sec = 1000000000;
#ifdef MOODYCAMEL_LIGHTWEIGHTSEMAPHORE_MONOTONIC
        clock_gettime(CLOCK_MONOTONIC, &ts);
#else
        clock_gettime(CLOCK_REALTIME, &ts);
#endif
        ts.tv_sec += (time_t)(usecs / usecs_in_1_sec);
        ts.tv_nsec += (long)(usecs % usecs_in_1_sec) * 1000;
        // sem_timedwait bombs if you have more than 1e9 in tv_nsec
        // so we have to clean things up before passing it in
        if (ts.tv_nsec >= nsecs_in_1_sec) {
            ts.tv_nsec -= nsecs_in_1_sec;
            ++ts.tv_sec;
        }
 
        int rc;
        do {
#ifdef MOODYCAMEL_LIGHTWEIGHTSEMAPHORE_MONOTONIC
            rc = sem_clockwait(&m_sema, CLOCK_MONOTONIC, &ts);
#else
            rc = sem_timedwait(&m_sema, &ts);
#endif
        } while (rc == -1 && errno == EINTR);
        return rc == 0;
    }
 
    void signal()
    {
        while (sem_post(&m_sema) == -1)
            ;
    }
 
    void signal(int count)
    {
        while (count-- > 0) {
            while (sem_post(&m_sema) == -1)
                ;
        }
    }
};
#else
#error Unsupported platform! (No semaphore wrapper available)
#endif
 
} // end namespace details
 
//---------------------------------------------------------
// LightweightSemaphore
//---------------------------------------------------------
class LightweightSemaphore {
  public:
    typedef std::make_signed<std::size_t>::type ssize_t;
 
  private:
    std::atomic<ssize_t> m_count;
    details::Semaphore m_sema;
    int m_maxSpins;
 
    bool waitWithPartialSpinning(std::int64_t timeout_usecs = -1)
    {
        ssize_t oldCount;
        int spin = m_maxSpins;
        while (--spin >= 0) {
            oldCount = m_count.load(std::memory_order_relaxed);
            if ((oldCount > 0) && m_count.compare_exchange_strong(
                                      oldCount, oldCount - 1, std::memory_order_acquire, std::memory_order_relaxed))
                return true;
            std::atomic_signal_fence(std::memory_order_acquire); // Prevent the compiler from collapsing the loop.
        }
        oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
        if (oldCount > 0)
            return true;
        if (timeout_usecs < 0) {
            if (m_sema.wait())
                return true;
        }
        if (timeout_usecs > 0 && m_sema.timed_wait((std::uint64_t)timeout_usecs))
            return true;
        // At this point, we've timed out waiting for the semaphore, but the
        // count is still decremented indicating we may still be waiting on
        // it. So we have to re-adjust the count, but only if the semaphore
        // wasn't signaled enough times for us too since then. If it was, we
        // need to release the semaphore too.
        while (true) {
            oldCount = m_count.load(std::memory_order_acquire);
            if (oldCount >= 0 && m_sema.try_wait())
                return true;
            if (oldCount < 0 && m_count.compare_exchange_strong(
                                    oldCount, oldCount + 1, std::memory_order_relaxed, std::memory_order_relaxed))
                return false;
        }
    }
 
    ssize_t waitManyWithPartialSpinning(ssize_t max, std::int64_t timeout_usecs = -1)
    {
        assert(max > 0);
        ssize_t oldCount;
        int spin = m_maxSpins;
        while (--spin >= 0) {
            oldCount = m_count.load(std::memory_order_relaxed);
            if (oldCount > 0) {
                ssize_t newCount = oldCount > max ? oldCount - max : 0;
                if (m_count.compare_exchange_strong(
                        oldCount, newCount, std::memory_order_acquire, std::memory_order_relaxed))
                    return oldCount - newCount;
            }
            std::atomic_signal_fence(std::memory_order_acquire);
        }
        oldCount = m_count.fetch_sub(1, std::memory_order_acquire);
        if (oldCount <= 0) {
            if ((timeout_usecs == 0) || (timeout_usecs < 0 && !m_sema.wait()) ||
                (timeout_usecs > 0 && !m_sema.timed_wait((std::uint64_t)timeout_usecs))) {
                while (true) {
                    oldCount = m_count.load(std::memory_order_acquire);
                    if (oldCount >= 0 && m_sema.try_wait())
                        break;
                    if (oldCount < 0 &&
                        m_count.compare_exchange_strong(
                            oldCount, oldCount + 1, std::memory_order_relaxed, std::memory_order_relaxed))
                        return 0;
                }
            }
        }
        if (max > 1)
            return 1 + tryWaitMany(max - 1);
        return 1;
    }
 
  public:
    LightweightSemaphore(ssize_t initialCount = 0, int maxSpins = 10000)
        : m_count(initialCount)
        , m_maxSpins(maxSpins)
    {
        assert(initialCount >= 0);
        assert(maxSpins >= 0);
    }
 
    bool tryWait()
    {
        ssize_t oldCount = m_count.load(std::memory_order_relaxed);
        while (oldCount > 0) {
            if (m_count.compare_exchange_weak(
                    oldCount, oldCount - 1, std::memory_order_acquire, std::memory_order_relaxed))
                return true;
        }
        return false;
    }
 
    bool wait() { return tryWait() || waitWithPartialSpinning(); }
 
    bool wait(std::int64_t timeout_usecs) { return tryWait() || waitWithPartialSpinning(timeout_usecs); }
 
    // Acquires between 0 and (greedily) max, inclusive
    ssize_t tryWaitMany(ssize_t max)
    {
        assert(max >= 0);
        ssize_t oldCount = m_count.load(std::memory_order_relaxed);
        while (oldCount > 0) {
            ssize_t newCount = oldCount > max ? oldCount - max : 0;
            if (m_count.compare_exchange_weak(oldCount, newCount, std::memory_order_acquire, std::memory_order_relaxed))
                return oldCount - newCount;
        }
        return 0;
    }
 
    // Acquires at least one, and (greedily) at most max
    ssize_t waitMany(ssize_t max, std::int64_t timeout_usecs)
    {
        assert(max >= 0);
        ssize_t result = tryWaitMany(max);
        if (result == 0 && max > 0)
            result = waitManyWithPartialSpinning(max, timeout_usecs);
        return result;
    }
 
    ssize_t waitMany(ssize_t max)
    {
        ssize_t result = waitMany(max, -1);
        assert(result > 0);
        return result;
    }
 
    void signal(ssize_t count = 1)
    {
        assert(count >= 0);
        ssize_t oldCount = m_count.fetch_add(count, std::memory_order_release);
        ssize_t toRelease = -oldCount < count ? -oldCount : count;
        if (toRelease > 0) {
            m_sema.signal((int)toRelease);
        }
    }
 
    std::size_t availableApprox() const
    {
        ssize_t count = m_count.load(std::memory_order_relaxed);
        return count > 0 ? static_cast<std::size_t>(count) : 0;
    }
};
 
} // end namespace moodycamel