atomic_backoff.hpp

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// SPDX-License-Identifier: BSD-3-Clause
/* Copyright 2019-2020, Intel Corporation */
 
#ifndef LIBPMEMOBJ_ATOMIC_BACKOFF_HPP
#define LIBPMEMOBJ_ATOMIC_BACKOFF_HPP
 
#include <thread>
 
#if _MSC_VER
#include <intrin.h>
#include <windows.h>
#endif
 
namespace pmem
{
namespace detail
{
 
class atomic_backoff {
    static const int32_t LOOPS_BEFORE_YIELD = 16;
    int32_t count;
 
    static inline void
    __pause(int32_t delay)
    {
        for (; delay > 0; --delay) {
#if _MSC_VER
            YieldProcessor();
#elif __GNUC__ && (__i386__ || __x86_64__)
            // Only i386 and x86-64 have pause instruction
            __builtin_ia32_pause();
#endif
        }
    }
 
public:
    atomic_backoff(const atomic_backoff &) = delete;
    atomic_backoff &operator=(const atomic_backoff &) = delete;
 
    /* In many cases, an object of this type is initialized eagerly on hot
     * path, as in for(atomic_backoff b; ; b.pause()) {...} For this reason,
     * the construction cost must be very small! */
    atomic_backoff() : count(1)
    {
    }
 
    atomic_backoff(bool) : count(1)
    {
        pause();
    }
 
    void
    pause()
    {
        if (count <= LOOPS_BEFORE_YIELD) {
            __pause(count);
            /* Pause twice as long the next time. */
            count *= 2;
        } else {
            /* Pause is so long that we might as well yield CPU to
             * scheduler. */
            std::this_thread::yield();
        }
    }
 
    bool
    bounded_pause()
    {
        __pause(count);
        if (count < LOOPS_BEFORE_YIELD) {
            /* Pause twice as long the next time. */
            count *= 2;
            return true;
        } else {
            return false;
        }
    }
 
    void
    reset()
    {
        count = 1;
    }
}; /* class atomic_backoff */
 
} /* namespace detail */
 
} /* namespace pmem */
 
#endif