refcount_policy.hpp

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//
// immer: immutable data structures for C++
// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
//
// This software is distributed under the Boost Software License, Version 1.0.
// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
//

#pragma once

#include <immer/refcount/no_refcount_policy.hpp>

#include <atomic>
#include <utility>
#include <cassert>
#include <thread>

// This has been shamelessly copied from boost...
#if defined(_MSC_VER) && _MSC_VER >= 1310 && (defined(_M_IX86) || defined(_M_X64)) && !defined(__c2__)
extern "C" void _mm_pause();
#  define IMMER_SMT_PAUSE _mm_pause()
#elif defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
#  define IMMER_SMT_PAUSE __asm__ __volatile__( "rep; nop" : : : "memory" )
#endif

namespace immer {

// This is an atomic spinlock similar to the one used by boost to provide
// "atomic" shared_ptr operations.  It also does not differ much from the one
// from libc++ or libstdc++...
struct spinlock
{
    std::atomic_flag v_{};

    bool try_lock()
    {
        return !v_.test_and_set(std::memory_order_acquire);
    }

    void lock()
    {
        for (auto k = 0u; !try_lock(); ++k) {
            if (k < 4)
                continue;
#ifdef IMMER_SMT_PAUSE
            else if (k < 16)
                IMMER_SMT_PAUSE;
#endif
            else
                std::this_thread::yield();
        }
    }

    void unlock()
    {
        v_.clear(std::memory_order_release);
    }

    struct scoped_lock
    {
        scoped_lock(const scoped_lock&) = delete;
        scoped_lock& operator=(const scoped_lock& ) = delete;

        explicit scoped_lock(spinlock& sp)
            : sp_{sp}
        { sp.lock(); }

        ~scoped_lock()
        { sp_.unlock();}

    private:
        spinlock& sp_;
    };
};

struct refcount_policy
{
    using spinlock_type = spinlock;

    mutable std::atomic<int> refcount;

    refcount_policy() : refcount{1} {};
    refcount_policy(disowned) : refcount{0} {}

    void inc()
    {
        refcount.fetch_add(1, std::memory_order_relaxed);
    }

    bool dec()
    {
        return 1 == refcount.fetch_sub(1, std::memory_order_acq_rel);
    }

    void dec_unsafe()
    {
        assert(refcount.load() > 1);
        refcount.fetch_sub(1, std::memory_order_relaxed);
    }

    bool unique()
    {
        return refcount == 1;
    }
};

} // namespace immer