AtomicBitSet.h

Go to the documentation of this file.

/*
 * Copyright 2013-present Facebook, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <array>
#include <atomic>
#include <cassert>
#include <cstddef>
#include <limits>

#include <boost/noncopyable.hpp>

#include <folly/Portability.h>

namespace folly {

template <size_t N>
class AtomicBitSet : private boost::noncopyable {
 public:
  AtomicBitSet();

  bool set(size_t idx, std::memory_order order = std::memory_order_seq_cst);

  bool reset(size_t idx, std::memory_order order = std::memory_order_seq_cst);

  bool set(
      size_t idx,
      bool value,
      std::memory_order order = std::memory_order_seq_cst);

  bool test(size_t idx, std::memory_order order = std::memory_order_seq_cst)
      const;

  bool operator[](size_t idx) const;

  constexpr size_t size() const {
    return N;
  }

 private:
  // Pick the largest lock-free type available
#if (ATOMIC_LLONG_LOCK_FREE == 2)
  typedef unsigned long long BlockType;
#elif (ATOMIC_LONG_LOCK_FREE == 2)
  typedef unsigned long BlockType;
#else
  // Even if not lock free, what can we do?
  typedef unsigned int BlockType;
#endif
  typedef std::atomic<BlockType> AtomicBlockType;

  static constexpr size_t kBitsPerBlock =
      std::numeric_limits<BlockType>::digits;

  static constexpr size_t blockIndex(size_t bit) {
    return bit / kBitsPerBlock;
  }

  static constexpr size_t bitOffset(size_t bit) {
    return bit % kBitsPerBlock;
  }

  // avoid casts
  static constexpr BlockType kOne = 1;

  std::array<AtomicBlockType, N> data_;
};

// value-initialize to zero
template <size_t N>
inline AtomicBitSet<N>::AtomicBitSet() : data_() {}

template <size_t N>
inline bool AtomicBitSet<N>::set(size_t idx, std::memory_order order) {
  assert(idx < N * kBitsPerBlock);
  BlockType mask = kOne << bitOffset(idx);
  return data_[blockIndex(idx)].fetch_or(mask, order) & mask;
}

template <size_t N>
inline bool AtomicBitSet<N>::reset(size_t idx, std::memory_order order) {
  assert(idx < N * kBitsPerBlock);
  BlockType mask = kOne << bitOffset(idx);
  return data_[blockIndex(idx)].fetch_and(~mask, order) & mask;
}

template <size_t N>
inline bool
AtomicBitSet<N>::set(size_t idx, bool value, std::memory_order order) {
  return value ? set(idx, order) : reset(idx, order);
}

template <size_t N>
inline bool AtomicBitSet<N>::test(size_t idx, std::memory_order order) const {
  assert(idx < N * kBitsPerBlock);
  BlockType mask = kOne << bitOffset(idx);
  return data_[blockIndex(idx)].load(order) & mask;
}

template <size_t N>
inline bool AtomicBitSet<N>::operator[](size_t idx) const {
  return test(idx);
}

} // namespace folly