7f19d64164
git-subtree-dir: third_party/immer git-subtree-split: ad3e3556d38bb75966dd24c61a774970a7c7957e
101 lines
2.4 KiB
C++
101 lines
2.4 KiB
C++
//
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// immer: immutable data structures for C++
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// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
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//
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// This software is distributed under the Boost Software License, Version 1.0.
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// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
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//
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#pragma once
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#include <immer/refcount/no_refcount_policy.hpp>
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#include <atomic>
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#include <cassert>
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#include <thread>
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#include <utility>
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// This has been shamelessly copied from boost...
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#if defined(_MSC_VER) && _MSC_VER >= 1310 && \
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(defined(_M_IX86) || defined(_M_X64)) && !defined(__c2__)
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extern "C" void _mm_pause();
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#define IMMER_SMT_PAUSE _mm_pause()
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#elif defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
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#define IMMER_SMT_PAUSE __asm__ __volatile__("rep; nop" : : : "memory")
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#endif
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namespace immer {
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// This is an atomic spinlock similar to the one used by boost to provide
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// "atomic" shared_ptr operations. It also does not differ much from the one
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// from libc++ or libstdc++...
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struct spinlock
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{
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std::atomic_flag v_{};
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bool try_lock() { return !v_.test_and_set(std::memory_order_acquire); }
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void lock()
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{
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for (auto k = 0u; !try_lock(); ++k) {
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if (k < 4)
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continue;
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#ifdef IMMER_SMT_PAUSE
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else if (k < 16)
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IMMER_SMT_PAUSE;
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#endif
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else
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std::this_thread::yield();
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}
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}
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void unlock() { v_.clear(std::memory_order_release); }
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struct scoped_lock
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{
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scoped_lock(const scoped_lock&) = delete;
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scoped_lock& operator=(const scoped_lock&) = delete;
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explicit scoped_lock(spinlock& sp)
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: sp_{sp}
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{
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sp.lock();
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}
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~scoped_lock() { sp_.unlock(); }
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private:
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spinlock& sp_;
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};
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};
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/*!
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* A reference counting policy implemented using an *atomic* `int`
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* count. It is **thread-safe**.
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*/
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struct refcount_policy
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{
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using spinlock_type = spinlock;
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mutable std::atomic<int> refcount;
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refcount_policy()
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: refcount{1} {};
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refcount_policy(disowned)
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: refcount{0}
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{}
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void inc() { refcount.fetch_add(1, std::memory_order_relaxed); }
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bool dec() { return 1 == refcount.fetch_sub(1, std::memory_order_acq_rel); }
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void dec_unsafe()
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{
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assert(refcount.load() > 1);
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refcount.fetch_sub(1, std::memory_order_relaxed);
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}
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bool unique() { return refcount == 1; }
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};
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} // namespace immer
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