SimpleLock.h

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/*
 * This file belongs to the Galois project, a C++ library for exploiting
 * parallelism. The code is being released under the terms of the 3-Clause BSD
 * License (a copy is located in LICENSE.txt at the top-level directory).
 *
 * Copyright (C) 2018, The University of Texas at Austin. All rights reserved.
 * UNIVERSITY EXPRESSLY DISCLAIMS ANY AND ALL WARRANTIES CONCERNING THIS
 * SOFTWARE AND DOCUMENTATION, INCLUDING ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR ANY PARTICULAR PURPOSE, NON-INFRINGEMENT AND WARRANTIES OF
 * PERFORMANCE, AND ANY WARRANTY THAT MIGHT OTHERWISE ARISE FROM COURSE OF
 * DEALING OR USAGE OF TRADE.  NO WARRANTY IS EITHER EXPRESS OR IMPLIED WITH
 * RESPECT TO THE USE OF THE SOFTWARE OR DOCUMENTATION. Under no circumstances
 * shall University be liable for incidental, special, indirect, direct or
 * consequential damages or loss of profits, interruption of business, or
 * related expenses which may arise from use of Software or Documentation,
 * including but not limited to those resulting from defects in Software and/or
 * Documentation, or loss or inaccuracy of data of any kind.
 */

#ifndef GALOIS_SUBSTRATE_SIMPLELOCK_H
#define GALOIS_SUBSTRATE_SIMPLELOCK_H

#include <atomic>
#include <cassert>
#include <mutex>

#include "galois/config.h"
#include "galois/substrate/CompilerSpecific.h"

namespace galois {
namespace substrate {


class SimpleLock {
  mutable std::atomic<int> _lock;
  void slow_lock() const;

public:
  constexpr SimpleLock() : _lock(0) {}
  // relaxed order for copy
  SimpleLock(const SimpleLock& p)
      : _lock(p._lock.load(std::memory_order_relaxed)) {}

  SimpleLock& operator=(const SimpleLock& p) {
    if (&p == this)
      return *this;
    // relaxed order for initialization
    _lock.store(p._lock.load(std::memory_order_relaxed),
                std::memory_order_relaxed);
    return *this;
  }

  inline void lock() const {
    int oldval = 0;
    if (_lock.load(std::memory_order_relaxed))
      goto slow_path;
    if (!_lock.compare_exchange_weak(oldval, 1, std::memory_order_acq_rel,
                                     std::memory_order_relaxed))
      goto slow_path;
    assert(is_locked());
    return;
  slow_path:
    slow_lock();
  }

  inline void unlock() const {
    assert(is_locked());
    // HMMMM
    _lock.store(0, std::memory_order_release);
    //_lock = 0;
  }

  inline bool try_lock() const {
    int oldval = 0;
    if (_lock.load(std::memory_order_relaxed))
      return false;
    if (!_lock.compare_exchange_weak(oldval, 1, std::memory_order_acq_rel))
      return false;
    assert(is_locked());
    return true;
  }

  inline bool is_locked() const {
    return _lock.load(std::memory_order_acquire) & 1;
  }
};


class DummyLock {
public:
  inline void lock() const {}
  inline void unlock() const {}
  inline bool try_lock() const { return true; }
  inline bool is_locked() const { return false; }
};

template <bool Enabled>
using CondLock =
    typename std::conditional<Enabled, SimpleLock, DummyLock>::type;

using lock_guard_galois = std::lock_guard<SimpleLock>;

#define MAKE_LOCK_GUARD(__x)                                                   \
  galois::substrate::lock_guard_galois locker##___COUNTER__(__x)

} // end namespace substrate
} // end namespace galois

#endif