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 // This file is part of Eigen, a lightweight C++ template library
 // for linear algebra.
 //
 // Copyright (C) 2016 Dmitry Vyukov <dvyukov@google.com>
 //
 // This Source Code Form is subject to the terms of the Mozilla
 // Public License v. 2.0. If a copy of the MPL was not distributed
 // with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 #ifndef EIGEN_CXX11_THREADPOOL_EVENTCOUNT_H_
 #define EIGEN_CXX11_THREADPOOL_EVENTCOUNT_H_
 
 namespace Eigen {
 
 // EventCount allows to wait for arbitrary predicates in non-blocking
 // algorithms. Think of condition variable, but wait predicate does not need to
 // be protected by a mutex. Usage:
 // Waiting thread does:
 //
 //   if (predicate)
 //     return act();
 //   EventCount::Waiter& w = waiters[my_index];
 //   ec.Prewait(&w);
 //   if (predicate) {
 //     ec.CancelWait(&w);
 //     return act();
 //   }
 //   ec.CommitWait(&w);
 //
 // Notifying thread does:
 //
 //   predicate = true;
 //   ec.Notify(true);
 //
 // Notify is cheap if there are no waiting threads. Prewait/CommitWait are not
 // cheap, but they are executed only if the preceding predicate check has
 // failed.
 //
 // Algorithm outline:
 // There are two main variables: predicate (managed by user) and state_.
 // Operation closely resembles Dekker mutual algorithm:
 // https://en.wikipedia.org/wiki/Dekker%27s_algorithm
 // Waiting thread sets state_ then checks predicate, Notifying thread sets
 // predicate then checks state_. Due to seq_cst fences in between these
 // operations it is guaranteed than either waiter will see predicate change
 // and won't block, or notifying thread will see state_ change and will unblock
 // the waiter, or both. But it can't happen that both threads don't see each
 // other changes, which would lead to deadlock.
 class EventCount {
  public:
   class Waiter;
 
   EventCount(MaxSizeVector<Waiter>& waiters)
       : state_(kStackMask), waiters_(waiters) {
     eigen_plain_assert(waiters.size() < (1 << kWaiterBits) - 1);
   }
 
   ~EventCount() {
     // Ensure there are no waiters.
     eigen_plain_assert(state_.load() == kStackMask);
   }
 
   // Prewait prepares for waiting.
   // After calling Prewait, the thread must re-check the wait predicate
   // and then call either CancelWait or CommitWait.
   void Prewait() {
     uint64_t state = state_.load(std::memory_order_relaxed);
     for (;;) {
       CheckState(state);
       uint64_t newstate = state + kWaiterInc;
       CheckState(newstate);
       if (state_.compare_exchange_weak(state, newstate,
                                        std::memory_order_seq_cst))
         return;
     }
   }
 
   // CommitWait commits waiting after Prewait.
   void CommitWait(Waiter* w) {
     eigen_plain_assert((w->epoch & ~kEpochMask) == 0);
     w->state = Waiter::kNotSignaled;
     const uint64_t me = (w - &waiters_[0]) | w->epoch;
     uint64_t state = state_.load(std::memory_order_seq_cst);
     for (;;) {
       CheckState(state, true);
       uint64_t newstate;
       if ((state & kSignalMask) != 0) {
         // Consume the signal and return immidiately.
         newstate = state - kWaiterInc - kSignalInc;
       } else {
         // Remove this thread from pre-wait counter and add to the waiter stack.
         newstate = ((state & kWaiterMask) - kWaiterInc) | me;
         w->next.store(state & (kStackMask | kEpochMask),
                       std::memory_order_relaxed);
       }
       CheckState(newstate);
       if (state_.compare_exchange_weak(state, newstate,
                                        std::memory_order_acq_rel)) {
         if ((state & kSignalMask) == 0) {
           w->epoch += kEpochInc;
           Park(w);
         }
         return;
       }
     }
   }
 
   // CancelWait cancels effects of the previous Prewait call.
   void CancelWait() {
     uint64_t state = state_.load(std::memory_order_relaxed);
     for (;;) {
       CheckState(state, true);
       uint64_t newstate = state - kWaiterInc;
       // We don't know if the thread was also notified or not,
       // so we should not consume a signal unconditionaly.
       // Only if number of waiters is equal to number of signals,
       // we know that the thread was notified and we must take away the signal.
       if (((state & kWaiterMask) >> kWaiterShift) ==
           ((state & kSignalMask) >> kSignalShift))
         newstate -= kSignalInc;
       CheckState(newstate);
       if (state_.compare_exchange_weak(state, newstate,
                                        std::memory_order_acq_rel))
         return;
     }
   }
 
   // Notify wakes one or all waiting threads.
   // Must be called after changing the associated wait predicate.
   void Notify(bool notifyAll) {
     std::atomic_thread_fence(std::memory_order_seq_cst);
     uint64_t state = state_.load(std::memory_order_acquire);
     for (;;) {
       CheckState(state);
       const uint64_t waiters = (state & kWaiterMask) >> kWaiterShift;
       const uint64_t signals = (state & kSignalMask) >> kSignalShift;
       // Easy case: no waiters.
       if ((state & kStackMask) == kStackMask && waiters == signals) return;
       uint64_t newstate;
       if (notifyAll) {
         // Empty wait stack and set signal to number of pre-wait threads.
         newstate =
             (state & kWaiterMask) | (waiters << kSignalShift) | kStackMask;
       } else if (signals < waiters) {
         // There is a thread in pre-wait state, unblock it.
         newstate = state + kSignalInc;
       } else {
         // Pop a waiter from list and unpark it.
         Waiter* w = &waiters_[state & kStackMask];
         uint64_t next = w->next.load(std::memory_order_relaxed);
         newstate = (state & (kWaiterMask | kSignalMask)) | next;
       }
       CheckState(newstate);
       if (state_.compare_exchange_weak(state, newstate,
                                        std::memory_order_acq_rel)) {
         if (!notifyAll && (signals < waiters))
           return;  // unblocked pre-wait thread
         if ((state & kStackMask) == kStackMask) return;
         Waiter* w = &waiters_[state & kStackMask];
         if (!notifyAll) w->next.store(kStackMask, std::memory_order_relaxed);
         Unpark(w);
         return;
       }
     }
   }
 
   class Waiter {
     friend class EventCount;
     // Align to 128 byte boundary to prevent false sharing with other Waiter
     // objects in the same vector.
     EIGEN_ALIGN_TO_BOUNDARY(128) std::atomic<uint64_t> next;
     std::mutex mu;
     std::condition_variable cv;
     uint64_t epoch = 0;
     unsigned state = kNotSignaled;
     enum {
       kNotSignaled,
       kWaiting,
       kSignaled,
     };
   };
 
  private:
   // State_ layout:
   // - low kWaiterBits is a stack of waiters committed wait
   //   (indexes in waiters_ array are used as stack elements,
   //   kStackMask means empty stack).
   // - next kWaiterBits is count of waiters in prewait state.
   // - next kWaiterBits is count of pending signals.
   // - remaining bits are ABA counter for the stack.
   //   (stored in Waiter node and incremented on push).
   static const uint64_t kWaiterBits = 14;
   static const uint64_t kStackMask = (1ull << kWaiterBits) - 1;
   static const uint64_t kWaiterShift = kWaiterBits;
   static const uint64_t kWaiterMask = ((1ull << kWaiterBits) - 1)
                                       << kWaiterShift;
   static const uint64_t kWaiterInc = 1ull << kWaiterShift;
   static const uint64_t kSignalShift = 2 * kWaiterBits;
   static const uint64_t kSignalMask = ((1ull << kWaiterBits) - 1)
                                       << kSignalShift;
   static const uint64_t kSignalInc = 1ull << kSignalShift;
   static const uint64_t kEpochShift = 3 * kWaiterBits;
   static const uint64_t kEpochBits = 64 - kEpochShift;
   static const uint64_t kEpochMask = ((1ull << kEpochBits) - 1) << kEpochShift;
   static const uint64_t kEpochInc = 1ull << kEpochShift;
   std::atomic<uint64_t> state_;
   MaxSizeVector<Waiter>& waiters_;
 
   static void CheckState(uint64_t state, bool waiter = false) {
     static_assert(kEpochBits >= 20, "not enough bits to prevent ABA problem");
     const uint64_t waiters = (state & kWaiterMask) >> kWaiterShift;
     const uint64_t signals = (state & kSignalMask) >> kSignalShift;
     eigen_plain_assert(waiters >= signals);
     eigen_plain_assert(waiters < (1 << kWaiterBits) - 1);
     eigen_plain_assert(!waiter || waiters > 0);
     (void)waiters;
     (void)signals;
   }
 
   void Park(Waiter* w) {
     std::unique_lock<std::mutex> lock(w->mu);
     while (w->state != Waiter::kSignaled) {
       w->state = Waiter::kWaiting;
       w->cv.wait(lock);
     }
   }
 
   void Unpark(Waiter* w) {
     for (Waiter* next; w; w = next) {
       uint64_t wnext = w->next.load(std::memory_order_relaxed) & kStackMask;
       next = wnext == kStackMask ? nullptr : &waiters_[wnext];
       unsigned state;
       {
         std::unique_lock<std::mutex> lock(w->mu);
         state = w->state;
         w->state = Waiter::kSignaled;
       }
       // Avoid notifying if it wasn't waiting.
       if (state == Waiter::kWaiting) w->cv.notify_one();
     }
   }
 
   EventCount(const EventCount&) = delete;
   void operator=(const EventCount&) = delete;
 };
 
 }  // namespace Eigen
 
 #endif  // EIGEN_CXX11_THREADPOOL_EVENTCOUNT_H_

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