faux-folly/folly/MPMCQueue.h

/*
 * Copyright 2015 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 <algorithm>
#include <atomic>
#include <assert.h>
#include <boost/noncopyable.hpp>
#include <limits>
#include <string.h>
#include <type_traits>
#include <unistd.h>

#include <folly/Traits.h>
#include <folly/detail/CacheLocality.h>
#include <folly/detail/TurnSequencer.h>

namespace folly {

namespace detail {

template<typename T, template<typename> class Atom>
struct SingleElementQueue;

template <typename T> class MPMCPipelineStageImpl;

} // namespace detail

/// MPMCQueue<T> is a high-performance bounded concurrent queue that
/// supports multiple producers, multiple consumers, and optional blocking.
/// The queue has a fixed capacity, for which all memory will be allocated
/// up front.  The bulk of the work of enqueuing and dequeuing can be
/// performed in parallel.
///
/// MPMCQueue is linearizable.  That means that if a call to write(A)
/// returns before a call to write(B) begins, then A will definitely end up
/// in the queue before B, and if a call to read(X) returns before a call
/// to read(Y) is started, that X will be something from earlier in the
/// queue than Y.  This also means that if a read call returns a value, you
/// can be sure that all previous elements of the queue have been assigned
/// a reader (that reader might not yet have returned, but it exists).
///
/// The underlying implementation uses a ticket dispenser for the head and
/// the tail, spreading accesses across N single-element queues to produce
/// a queue with capacity N.  The ticket dispensers use atomic increment,
/// which is more robust to contention than a CAS loop.  Each of the
/// single-element queues uses its own CAS to serialize access, with an
/// adaptive spin cutoff.  When spinning fails on a single-element queue
/// it uses futex()'s _BITSET operations to reduce unnecessary wakeups
/// even if multiple waiters are present on an individual queue (such as
/// when the MPMCQueue's capacity is smaller than the number of enqueuers
/// or dequeuers).
///
/// In benchmarks (contained in tao/queues/ConcurrentQueueTests)
/// it handles 1 to 1, 1 to N, N to 1, and N to M thread counts better
/// than any of the alternatives present in fbcode, for both small (~10)
/// and large capacities.  In these benchmarks it is also faster than
/// tbb::concurrent_bounded_queue for all configurations.  When there are
/// many more threads than cores, MPMCQueue is _much_ faster than the tbb
/// queue because it uses futex() to block and unblock waiting threads,
/// rather than spinning with sched_yield.
///
/// NOEXCEPT INTERACTION: tl;dr; If it compiles you're fine.  Ticket-based
/// queues separate the assignment of queue positions from the actual
/// construction of the in-queue elements, which means that the T
/// constructor used during enqueue must not throw an exception.  This is
/// enforced at compile time using type traits, which requires that T be
/// adorned with accurate noexcept information.  If your type does not
/// use noexcept, you will have to wrap it in something that provides
/// the guarantee.  We provide an alternate safe implementation for types
/// that don't use noexcept but that are marked folly::IsRelocatable
/// and boost::has_nothrow_constructor, which is common for folly types.
/// In particular, if you can declare FOLLY_ASSUME_FBVECTOR_COMPATIBLE
/// then your type can be put in MPMCQueue.
///
/// If you have a pool of N queue consumers that you want to shut down
/// after the queue has drained, one way is to enqueue N sentinel values
/// to the queue.  If the producer doesn't know how many consumers there
/// are you can enqueue one sentinel and then have each consumer requeue
/// two sentinels after it receives it (by requeuing 2 the shutdown can
/// complete in O(log P) time instead of O(P)).
template<typename T,
         template<typename> class Atom = std::atomic>
class MPMCQueue : boost::noncopyable {

  static_assert(std::is_nothrow_constructible<T,T&&>::value ||
                folly::IsRelocatable<T>::value,
      "T must be relocatable or have a noexcept move constructor");

  friend class detail::MPMCPipelineStageImpl<T>;
 public:
  typedef T value_type;

  explicit MPMCQueue(size_t queueCapacity)
    : capacity_(queueCapacity)
    , pushTicket_(0)
    , popTicket_(0)
    , pushSpinCutoff_(0)
    , popSpinCutoff_(0)
  {
    if (queueCapacity == 0)
      throw std::invalid_argument(
        "MPMCQueue with explicit capacity 0 is impossible"
      );

    // would sigfpe if capacity is 0
    stride_ = computeStride(queueCapacity);
    slots_ = new detail::SingleElementQueue<T,Atom>[queueCapacity +
                                                    2 * kSlotPadding];

    // ideally this would be a static assert, but g++ doesn't allow it
    assert(alignof(MPMCQueue<T,Atom>)
           >= detail::CacheLocality::kFalseSharingRange);
    assert(static_cast<uint8_t*>(static_cast<void*>(&popTicket_))
           - static_cast<uint8_t*>(static_cast<void*>(&pushTicket_))
           >= detail::CacheLocality::kFalseSharingRange);
  }

  /// A default-constructed queue is useful because a usable (non-zero
  /// capacity) queue can be moved onto it or swapped with it
  MPMCQueue() noexcept
    : capacity_(0)
    , slots_(nullptr)
    , stride_(0)
    , pushTicket_(0)
    , popTicket_(0)
    , pushSpinCutoff_(0)
    , popSpinCutoff_(0)
  {}

  /// IMPORTANT: The move constructor is here to make it easier to perform
  /// the initialization phase, it is not safe to use when there are any
  /// concurrent accesses (this is not checked).
  MPMCQueue(MPMCQueue<T,Atom>&& rhs) noexcept
    : capacity_(rhs.capacity_)
    , slots_(rhs.slots_)
    , stride_(rhs.stride_)
    , pushTicket_(rhs.pushTicket_.load(std::memory_order_relaxed))
    , popTicket_(rhs.popTicket_.load(std::memory_order_relaxed))
    , pushSpinCutoff_(rhs.pushSpinCutoff_.load(std::memory_order_relaxed))
    , popSpinCutoff_(rhs.popSpinCutoff_.load(std::memory_order_relaxed))
  {
    // relaxed ops are okay for the previous reads, since rhs queue can't
    // be in concurrent use

    // zero out rhs
    rhs.capacity_ = 0;
    rhs.slots_ = nullptr;
    rhs.stride_ = 0;
    rhs.pushTicket_.store(0, std::memory_order_relaxed);
    rhs.popTicket_.store(0, std::memory_order_relaxed);
    rhs.pushSpinCutoff_.store(0, std::memory_order_relaxed);
    rhs.popSpinCutoff_.store(0, std::memory_order_relaxed);
  }

  /// IMPORTANT: The move operator is here to make it easier to perform
  /// the initialization phase, it is not safe to use when there are any
  /// concurrent accesses (this is not checked).
  MPMCQueue<T,Atom> const& operator= (MPMCQueue<T,Atom>&& rhs) {
    if (this != &rhs) {
      this->~MPMCQueue();
      new (this) MPMCQueue(std::move(rhs));
    }
    return *this;
  }

  /// MPMCQueue can only be safely destroyed when there are no
  /// pending enqueuers or dequeuers (this is not checked).
  ~MPMCQueue() {
    delete[] slots_;
  }

  /// Returns the number of successful reads minus the number of successful
  /// writes.  Waiting blockingRead and blockingWrite calls are included,
  /// so this value can be negative.
  ssize_t size() const noexcept {
    // since both pushes and pops increase monotonically, we can get a
    // consistent snapshot either by bracketing a read of popTicket_ with
    // two reads of pushTicket_ that return the same value, or the other
    // way around.  We maximize our chances by alternately attempting
    // both bracketings.
    uint64_t pushes = pushTicket_.load(std::memory_order_acquire); // A
    uint64_t pops = popTicket_.load(std::memory_order_acquire); // B
    while (true) {
      uint64_t nextPushes = pushTicket_.load(std::memory_order_acquire); // C
      if (pushes == nextPushes) {
        // pushTicket_ didn't change from A (or the previous C) to C,
        // so we can linearize at B (or D)
        return pushes - pops;
      }
      pushes = nextPushes;
      uint64_t nextPops = popTicket_.load(std::memory_order_acquire); // D
      if (pops == nextPops) {
        // popTicket_ didn't chance from B (or the previous D), so we
        // can linearize at C
        return pushes - pops;
      }
      pops = nextPops;
    }
  }

  /// Returns true if there are no items available for dequeue
  bool isEmpty() const noexcept {
    return size() <= 0;
  }

  /// Returns true if there is currently no empty space to enqueue
  bool isFull() const noexcept {
    // careful with signed -> unsigned promotion, since size can be negative
    return size() >= static_cast<ssize_t>(capacity_);
  }

  /// Returns is a guess at size() for contexts that don't need a precise
  /// value, such as stats.
  ssize_t sizeGuess() const noexcept {
    return writeCount() - readCount();
  }

  /// Doesn't change
  size_t capacity() const noexcept {
    return capacity_;
  }

  /// Returns the total number of calls to blockingWrite or successful
  /// calls to write, including those blockingWrite calls that are
  /// currently blocking
  uint64_t writeCount() const noexcept {
    return pushTicket_.load(std::memory_order_acquire);
  }

  /// Returns the total number of calls to blockingRead or successful
  /// calls to read, including those blockingRead calls that are currently
  /// blocking
  uint64_t readCount() const noexcept {
    return popTicket_.load(std::memory_order_acquire);
  }

  /// Enqueues a T constructed from args, blocking until space is
  /// available.  Note that this method signature allows enqueue via
  /// move, if args is a T rvalue, via copy, if args is a T lvalue, or
  /// via emplacement if args is an initializer list that can be passed
  /// to a T constructor.
  template <typename ...Args>
  void blockingWrite(Args&&... args) noexcept {
    enqueueWithTicket(pushTicket_++, std::forward<Args>(args)...);
  }

  /// If an item can be enqueued with no blocking, does so and returns
  /// true, otherwise returns false.  This method is similar to
  /// writeIfNotFull, but if you don't have a specific need for that
  /// method you should use this one.
  ///
  /// One of the common usages of this method is to enqueue via the
  /// move constructor, something like q.write(std::move(x)).  If write
  /// returns false because the queue is full then x has not actually been
  /// consumed, which looks strange.  To understand why it is actually okay
  /// to use x afterward, remember that std::move is just a typecast that
  /// provides an rvalue reference that enables use of a move constructor
  /// or operator.  std::move doesn't actually move anything.  It could
  /// more accurately be called std::rvalue_cast or std::move_permission.
  template <typename ...Args>
  bool write(Args&&... args) noexcept {
    uint64_t ticket;
    if (tryObtainReadyPushTicket(ticket)) {
      // we have pre-validated that the ticket won't block
      enqueueWithTicket(ticket, std::forward<Args>(args)...);
      return true;
    } else {
      return false;
    }
  }

  /// If the queue is not full, enqueues and returns true, otherwise
  /// returns false.  Unlike write this method can be blocked by another
  /// thread, specifically a read that has linearized (been assigned
  /// a ticket) but not yet completed.  If you don't really need this
  /// function you should probably use write.
  ///
  /// MPMCQueue isn't lock-free, so just because a read operation has
  /// linearized (and isFull is false) doesn't mean that space has been
  /// made available for another write.  In this situation write will
  /// return false, but writeIfNotFull will wait for the dequeue to finish.
  /// This method is required if you are composing queues and managing
  /// your own wakeup, because it guarantees that after every successful
  /// write a readIfNotEmpty will succeed.
  template <typename ...Args>
  bool writeIfNotFull(Args&&... args) noexcept {
    uint64_t ticket;
    if (tryObtainPromisedPushTicket(ticket)) {
      // some other thread is already dequeuing the slot into which we
      // are going to enqueue, but we might have to wait for them to finish
      enqueueWithTicket(ticket, std::forward<Args>(args)...);
      return true;
    } else {
      return false;
    }
  }

  /// Moves a dequeued element onto elem, blocking until an element
  /// is available
  void blockingRead(T& elem) noexcept {
    dequeueWithTicket(popTicket_++, elem);
  }

  /// If an item can be dequeued with no blocking, does so and returns
  /// true, otherwise returns false.
  bool read(T& elem) noexcept {
    uint64_t ticket;
    if (tryObtainReadyPopTicket(ticket)) {
      // the ticket has been pre-validated to not block
      dequeueWithTicket(ticket, elem);
      return true;
    } else {
      return false;
    }
  }

  /// If the queue is not empty, dequeues and returns true, otherwise
  /// returns false.  If the matching write is still in progress then this
  /// method may block waiting for it.  If you don't rely on being able
  /// to dequeue (such as by counting completed write) then you should
  /// prefer read.
  bool readIfNotEmpty(T& elem) noexcept {
    uint64_t ticket;
    if (tryObtainPromisedPopTicket(ticket)) {
      // the matching enqueue already has a ticket, but might not be done
      dequeueWithTicket(ticket, elem);
      return true;
    } else {
      return false;
    }
  }

 private:
  enum {
    /// Once every kAdaptationFreq we will spin longer, to try to estimate
    /// the proper spin backoff
    kAdaptationFreq = 128,

    /// To avoid false sharing in slots_ with neighboring memory
    /// allocations, we pad it with this many SingleElementQueue-s at
    /// each end
    kSlotPadding = (detail::CacheLocality::kFalseSharingRange - 1)
        / sizeof(detail::SingleElementQueue<T,Atom>) + 1
  };

  /// The maximum number of items in the queue at once
  size_t FOLLY_ALIGN_TO_AVOID_FALSE_SHARING capacity_;

  /// An array of capacity_ SingleElementQueue-s, each of which holds
  /// either 0 or 1 item.  We over-allocate by 2 * kSlotPadding and don't
  /// touch the slots at either end, to avoid false sharing
  detail::SingleElementQueue<T,Atom>* slots_;

  /// The number of slots_ indices that we advance for each ticket, to
  /// avoid false sharing.  Ideally slots_[i] and slots_[i + stride_]
  /// aren't on the same cache line
  int stride_;

  /// Enqueuers get tickets from here
  Atom<uint64_t> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING pushTicket_;

  /// Dequeuers get tickets from here
  Atom<uint64_t> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING popTicket_;

  /// This is how many times we will spin before using FUTEX_WAIT when
  /// the queue is full on enqueue, adaptively computed by occasionally
  /// spinning for longer and smoothing with an exponential moving average
  Atom<uint32_t> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING pushSpinCutoff_;

  /// The adaptive spin cutoff when the queue is empty on dequeue
  Atom<uint32_t> FOLLY_ALIGN_TO_AVOID_FALSE_SHARING popSpinCutoff_;

  /// Alignment doesn't prevent false sharing at the end of the struct,
  /// so fill out the last cache line
  char padding_[detail::CacheLocality::kFalseSharingRange -
                sizeof(Atom<uint32_t>)];


  /// We assign tickets in increasing order, but we don't want to
  /// access neighboring elements of slots_ because that will lead to
  /// false sharing (multiple cores accessing the same cache line even
  /// though they aren't accessing the same bytes in that cache line).
  /// To avoid this we advance by stride slots per ticket.
  ///
  /// We need gcd(capacity, stride) to be 1 so that we will use all
  /// of the slots.  We ensure this by only considering prime strides,
  /// which either have no common divisors with capacity or else have
  /// a zero remainder after dividing by capacity.  That is sufficient
  /// to guarantee correctness, but we also want to actually spread the
  /// accesses away from each other to avoid false sharing (consider a
  /// stride of 7 with a capacity of 8).  To that end we try a few taking
  /// care to observe that advancing by -1 is as bad as advancing by 1
  /// when in comes to false sharing.
  ///
  /// The simple way to avoid false sharing would be to pad each
  /// SingleElementQueue, but since we have capacity_ of them that could
  /// waste a lot of space.
  static int computeStride(size_t capacity) noexcept {
    static const int smallPrimes[] = { 2, 3, 5, 7, 11, 13, 17, 19, 23 };

    int bestStride = 1;
    size_t bestSep = 1;
    for (int stride : smallPrimes) {
      if ((stride % capacity) == 0 || (capacity % stride) == 0) {
        continue;
      }
      size_t sep = stride % capacity;
      sep = std::min(sep, capacity - sep);
      if (sep > bestSep) {
        bestStride = stride;
        bestSep = sep;
      }
    }
    return bestStride;
  }

  /// Returns the index into slots_ that should be used when enqueuing or
  /// dequeuing with the specified ticket
  size_t idx(uint64_t ticket) noexcept {
    return ((ticket * stride_) % capacity_) + kSlotPadding;
  }

  /// Maps an enqueue or dequeue ticket to the turn should be used at the
  /// corresponding SingleElementQueue
  uint32_t turn(uint64_t ticket) noexcept {
    return ticket / capacity_;
  }

  /// Tries to obtain a push ticket for which SingleElementQueue::enqueue
  /// won't block.  Returns true on immediate success, false on immediate
  /// failure.
  bool tryObtainReadyPushTicket(uint64_t& rv) noexcept {
    auto ticket = pushTicket_.load(std::memory_order_acquire); // A
    while (true) {
      if (!slots_[idx(ticket)].mayEnqueue(turn(ticket))) {
        // if we call enqueue(ticket, ...) on the SingleElementQueue
        // right now it would block, but this might no longer be the next
        // ticket.  We can increase the chance of tryEnqueue success under
        // contention (without blocking) by rechecking the ticket dispenser
        auto prev = ticket;
        ticket = pushTicket_.load(std::memory_order_acquire); // B
        if (prev == ticket) {
          // mayEnqueue was bracketed by two reads (A or prev B or prev
          // failing CAS to B), so we are definitely unable to enqueue
          return false;
        }
      } else {
        // we will bracket the mayEnqueue check with a read (A or prev B
        // or prev failing CAS) and the following CAS.  If the CAS fails
        // it will effect a load of pushTicket_
        if (pushTicket_.compare_exchange_strong(ticket, ticket + 1)) {
          rv = ticket;
          return true;
        }
      }
    }
  }

  /// Tries to obtain a push ticket which can be satisfied if all
  /// in-progress pops complete.  This function does not block, but
  /// blocking may be required when using the returned ticket if some
  /// other thread's pop is still in progress (ticket has been granted but
  /// pop has not yet completed).
  bool tryObtainPromisedPushTicket(uint64_t& rv) noexcept {
    auto numPushes = pushTicket_.load(std::memory_order_acquire); // A
    while (true) {
      auto numPops = popTicket_.load(std::memory_order_acquire); // B
      // n will be negative if pops are pending
      int64_t n = numPushes - numPops;
      if (n >= static_cast<ssize_t>(capacity_)) {
        // Full, linearize at B.  We don't need to recheck the read we
        // performed at A, because if numPushes was stale at B then the
        // real numPushes value is even worse
        return false;
      }
      if (pushTicket_.compare_exchange_strong(numPushes, numPushes + 1)) {
        rv = numPushes;
        return true;
      }
    }
  }

  /// Tries to obtain a pop ticket for which SingleElementQueue::dequeue
  /// won't block.  Returns true on immediate success, false on immediate
  /// failure.
  bool tryObtainReadyPopTicket(uint64_t& rv) noexcept {
    auto ticket = popTicket_.load(std::memory_order_acquire);
    while (true) {
      if (!slots_[idx(ticket)].mayDequeue(turn(ticket))) {
        auto prev = ticket;
        ticket = popTicket_.load(std::memory_order_acquire);
        if (prev == ticket) {
          return false;
        }
      } else {
        if (popTicket_.compare_exchange_strong(ticket, ticket + 1)) {
          rv = ticket;
          return true;
        }
      }
    }
  }

  /// Similar to tryObtainReadyPopTicket, but returns a pop ticket whose
  /// corresponding push ticket has already been handed out, rather than
  /// returning one whose corresponding push ticket has already been
  /// completed.  This means that there is a possibility that the caller
  /// will block when using the ticket, but it allows the user to rely on
  /// the fact that if enqueue has succeeded, tryObtainPromisedPopTicket
  /// will return true.  The "try" part of this is that we won't have
  /// to block waiting for someone to call enqueue, although we might
  /// have to block waiting for them to finish executing code inside the
  /// MPMCQueue itself.
  bool tryObtainPromisedPopTicket(uint64_t& rv) noexcept {
    auto numPops = popTicket_.load(std::memory_order_acquire); // A
    while (true) {
      auto numPushes = pushTicket_.load(std::memory_order_acquire); // B
      if (numPops >= numPushes) {
        // Empty, or empty with pending pops.  Linearize at B.  We don't
        // need to recheck the read we performed at A, because if numPops
        // is stale then the fresh value is larger and the >= is still true
        return false;
      }
      if (popTicket_.compare_exchange_strong(numPops, numPops + 1)) {
        rv = numPops;
        return true;
      }
    }
  }

  // Given a ticket, constructs an enqueued item using args
  template <typename ...Args>
  void enqueueWithTicket(uint64_t ticket, Args&&... args) noexcept {
    slots_[idx(ticket)].enqueue(turn(ticket),
                                pushSpinCutoff_,
                                (ticket % kAdaptationFreq) == 0,
                                std::forward<Args>(args)...);
  }

  // Given a ticket, dequeues the corresponding element
  void dequeueWithTicket(uint64_t ticket, T& elem) noexcept {
    slots_[idx(ticket)].dequeue(turn(ticket),
                                popSpinCutoff_,
                                (ticket % kAdaptationFreq) == 0,
                                elem);
  }
};


namespace detail {

/// SingleElementQueue implements a blocking queue that holds at most one
/// item, and that requires its users to assign incrementing identifiers
/// (turns) to each enqueue and dequeue operation.  Note that the turns
/// used by SingleElementQueue are doubled inside the TurnSequencer
template <typename T, template <typename> class Atom>
struct SingleElementQueue {

  ~SingleElementQueue() noexcept {
    if ((sequencer_.uncompletedTurnLSB() & 1) == 1) {
      // we are pending a dequeue, so we have a constructed item
      destroyContents();
    }
  }

  /// enqueue using in-place noexcept construction
  template <typename ...Args,
            typename = typename std::enable_if<
              std::is_nothrow_constructible<T,Args...>::value>::type>
  void enqueue(const uint32_t turn,
               Atom<uint32_t>& spinCutoff,
               const bool updateSpinCutoff,
               Args&&... args) noexcept {
    sequencer_.waitForTurn(turn * 2, spinCutoff, updateSpinCutoff);
    new (&contents_) T(std::forward<Args>(args)...);
    sequencer_.completeTurn(turn * 2);
  }

  /// enqueue using move construction, either real (if
  /// is_nothrow_move_constructible) or simulated using relocation and
  /// default construction (if IsRelocatable and has_nothrow_constructor)
  template <typename = typename std::enable_if<
                (folly::IsRelocatable<T>::value &&
                 boost::has_nothrow_constructor<T>::value) ||
                std::is_nothrow_constructible<T,T&&>::value>::type>
  void enqueue(const uint32_t turn,
               Atom<uint32_t>& spinCutoff,
               const bool updateSpinCutoff,
               T&& goner) noexcept {
    enqueueImpl(
        turn,
        spinCutoff,
        updateSpinCutoff,
        std::move(goner),
        typename std::conditional<std::is_nothrow_constructible<T,T&&>::value,
                                  ImplByMove, ImplByRelocation>::type());
  }

  bool mayEnqueue(const uint32_t turn) const noexcept {
    return sequencer_.isTurn(turn * 2);
  }

  void dequeue(uint32_t turn,
               Atom<uint32_t>& spinCutoff,
               const bool updateSpinCutoff,
               T& elem) noexcept {
    dequeueImpl(turn,
                spinCutoff,
                updateSpinCutoff,
                elem,
                typename std::conditional<folly::IsRelocatable<T>::value,
                                          ImplByRelocation,
                                          ImplByMove>::type());
  }

  bool mayDequeue(const uint32_t turn) const noexcept {
    return sequencer_.isTurn(turn * 2 + 1);
  }

 private:
  /// Storage for a T constructed with placement new
  typename std::aligned_storage<sizeof(T),alignof(T)>::type contents_;

  /// Even turns are pushes, odd turns are pops
  TurnSequencer<Atom> sequencer_;

  T* ptr() noexcept {
    return static_cast<T*>(static_cast<void*>(&contents_));
  }

  void destroyContents() noexcept {
    try {
      ptr()->~T();
    } catch (...) {
      // g++ doesn't seem to have std::is_nothrow_destructible yet
    }
#ifndef NDEBUG
    memset(&contents_, 'Q', sizeof(T));
#endif
  }

  /// Tag classes for dispatching to enqueue/dequeue implementation.
  struct ImplByRelocation {};
  struct ImplByMove {};

  /// enqueue using nothrow move construction.
  void enqueueImpl(const uint32_t turn,
                   Atom<uint32_t>& spinCutoff,
                   const bool updateSpinCutoff,
                   T&& goner,
                   ImplByMove) noexcept {
    sequencer_.waitForTurn(turn * 2, spinCutoff, updateSpinCutoff);
    new (&contents_) T(std::move(goner));
    sequencer_.completeTurn(turn * 2);
  }

  /// enqueue by simulating nothrow move with relocation, followed by
  /// default construction to a noexcept relocation.
  void enqueueImpl(const uint32_t turn,
                   Atom<uint32_t>& spinCutoff,
                   const bool updateSpinCutoff,
                   T&& goner,
                   ImplByRelocation) noexcept {
    sequencer_.waitForTurn(turn * 2, spinCutoff, updateSpinCutoff);
    memcpy(&contents_, &goner, sizeof(T));
    sequencer_.completeTurn(turn * 2);
    new (&goner) T();
  }

  /// dequeue by destructing followed by relocation.  This version is preferred,
  /// because as much work as possible can be done before waiting.
  void dequeueImpl(uint32_t turn,
                   Atom<uint32_t>& spinCutoff,
                   const bool updateSpinCutoff,
                   T& elem,
                   ImplByRelocation) noexcept {
    try {
      elem.~T();
    } catch (...) {
      // unlikely, but if we don't complete our turn the queue will die
    }
    sequencer_.waitForTurn(turn * 2 + 1, spinCutoff, updateSpinCutoff);
    memcpy(&elem, &contents_, sizeof(T));
    sequencer_.completeTurn(turn * 2 + 1);
  }

  /// dequeue by nothrow move assignment.
  void dequeueImpl(uint32_t turn,
                   Atom<uint32_t>& spinCutoff,
                   const bool updateSpinCutoff,
                   T& elem,
                   ImplByMove) noexcept {
    sequencer_.waitForTurn(turn * 2 + 1, spinCutoff, updateSpinCutoff);
    elem = std::move(*ptr());
    destroyContents();
    sequencer_.completeTurn(turn * 2 + 1);
  }
};

} // namespace detail

} // namespace folly