CoreCachedSharedPtr.h

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/*
 * Copyright 2017-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 <memory>

#include <folly/concurrency/AtomicSharedPtr.h>
#include <folly/concurrency/CacheLocality.h>
#include <folly/container/Enumerate.h>
#include <folly/synchronization/Hazptr.h>

namespace folly {

template <class T, size_t kNumSlots = 64>
class CoreCachedSharedPtr {
 public:
  explicit CoreCachedSharedPtr(const std::shared_ptr<T>& p = nullptr) {
    reset(p);
  }

  void reset(const std::shared_ptr<T>& p = nullptr) {
    // Allocate each Holder in a different CoreRawAllocator stripe to
    // prevent false sharing. Their control blocks will be adjacent
    // thanks to allocate_shared().
    for (auto slot : folly::enumerate(slots_)) {
      auto alloc = getCoreAllocator<Holder, kNumSlots>(slot.index);
      auto holder = std::allocate_shared<Holder>(alloc, p);
      *slot = std::shared_ptr<T>(holder, p.get());
    }
  }

  std::shared_ptr<T> get() const {
    return slots_[AccessSpreader<>::current(kNumSlots)];
  }

 private:
  using Holder = std::shared_ptr<T>;

  template <class, size_t>
  friend class CoreCachedWeakPtr;

  std::array<std::shared_ptr<T>, kNumSlots> slots_;
};

template <class T, size_t kNumSlots = 64>
class CoreCachedWeakPtr {
 public:
  explicit CoreCachedWeakPtr(const CoreCachedSharedPtr<T, kNumSlots>& p) {
    for (auto slot : folly::enumerate(slots_)) {
      *slot = p.slots_[slot.index];
    }
  }

  std::weak_ptr<T> get() const {
    return slots_[AccessSpreader<>::current(kNumSlots)];
  }

 private:
  std::array<std::weak_ptr<T>, kNumSlots> slots_;
};

template <class T, size_t kNumSlots = 64>
class AtomicCoreCachedSharedPtr {
 public:
  explicit AtomicCoreCachedSharedPtr(const std::shared_ptr<T>& p = nullptr) {
    reset(p);
  }

  ~AtomicCoreCachedSharedPtr() {
    auto slots = slots_.load(std::memory_order_acquire);
    // Delete of AtomicCoreCachedSharedPtr must be synchronized, no
    // need for stlots->retire().
    if (slots) {
      delete slots;
    }
  }

  void reset(const std::shared_ptr<T>& p = nullptr) {
    auto newslots = folly::make_unique<Slots>();
    // Allocate each Holder in a different CoreRawAllocator stripe to
    // prevent false sharing. Their control blocks will be adjacent
    // thanks to allocate_shared().
    for (auto slot : folly::enumerate(newslots->slots_)) {
      auto alloc = getCoreAllocator<Holder, kNumSlots>(slot.index);
      auto holder = std::allocate_shared<Holder>(alloc, p);
      *slot = std::shared_ptr<T>(holder, p.get());
    }

    auto oldslots = slots_.exchange(newslots.release());
    if (oldslots) {
      oldslots->retire();
    }
  }

  std::shared_ptr<T> get() const {
    folly::hazptr_local<1> hazptr;
    auto slots = hazptr[0].get_protected(slots_);
    if (!slots) {
      return nullptr;
    }
    return (slots->slots_)[AccessSpreader<>::current(kNumSlots)];
  }

 private:
  using Holder = std::shared_ptr<T>;
  struct Slots : folly::hazptr_obj_base<Slots> {
    std::array<std::shared_ptr<T>, kNumSlots> slots_;
  };
  std::atomic<Slots*> slots_{nullptr};
};

} // namespace folly