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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 <cassert>
#include <thread>
#include <utility>
// 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_;
};
};
/*!
* A reference counting policy implemented using an *atomic* `int`
* count. It is **thread-safe**.
*/
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
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