text: // Copyright 2017 The Abseil Authors.
  //
  // 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
  //
  //      https://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.
  
  // This file is a no-op if the required LowLevelAlloc support is missing.
  #include "absl/base/internal/low_level_alloc.h"
  #ifndef ABSL_LOW_LEVEL_ALLOC_MISSING
  
  #include "absl/synchronization/internal/per_thread_sem.h"
  
  #include <atomic>
  
  #include "absl/base/attributes.h"
  #include "absl/base/internal/thread_identity.h"
  #include "absl/synchronization/internal/waiter.h"
  
  namespace absl {
  namespace synchronization_internal {
  
  void PerThreadSem::SetThreadBlockedCounter(std::atomic<int> *counter) {
    base_internal::ThreadIdentity *identity;
    identity = GetOrCreateCurrentThreadIdentity();
    identity->blocked_count_ptr = counter;
  }
  
  std::atomic<int> *PerThreadSem::GetThreadBlockedCounter() {
    base_internal::ThreadIdentity *identity;
    identity = GetOrCreateCurrentThreadIdentity();
    return identity->blocked_count_ptr;
  }
  
  void PerThreadSem::Init(base_internal::ThreadIdentity *identity) {
    Waiter::GetWaiter(identity)->Init();
    identity->ticker.store(0, std::memory_order_relaxed);
    identity->wait_start.store(0, std::memory_order_relaxed);
    identity->is_idle.store(false, std::memory_order_relaxed);
  }
  
  void PerThreadSem::Tick(base_internal::ThreadIdentity *identity) {
    const int ticker =
        identity->ticker.fetch_add(1, std::memory_order_relaxed) + 1;
    const int wait_start = identity->wait_start.load(std::memory_order_relaxed);
    const bool is_idle = identity->is_idle.load(std::memory_order_relaxed);
    if (wait_start && (ticker - wait_start > Waiter::kIdlePeriods) && !is_idle) {
      // Wakeup the waiting thread since it is time for it to become idle.
      Waiter::GetWaiter(identity)->Poke();
    }
  }
  
  }  // namespace synchronization_internal
  }  // namespace absl
  
  extern "C" {
  
  ABSL_ATTRIBUTE_WEAK void AbslInternalPerThreadSemPost(
      absl::base_internal::ThreadIdentity *identity) {
    absl::synchronization_internal::Waiter::GetWaiter(identity)->Post();
  }
  
  ABSL_ATTRIBUTE_WEAK bool AbslInternalPerThreadSemWait(
      absl::synchronization_internal::KernelTimeout t) {
    bool timeout = false;
    absl::base_internal::ThreadIdentity *identity;
    identity = absl::synchronization_internal::GetOrCreateCurrentThreadIdentity();
  
    // Ensure wait_start != 0.
    int ticker = identity->ticker.load(std::memory_order_relaxed);
    identity->wait_start.store(ticker ? ticker : 1, std::memory_order_relaxed);
    identity->is_idle.store(false, std::memory_order_relaxed);
  
    if (identity->blocked_count_ptr != nullptr) {
      // Increment count of threads blocked in a given thread pool.
      identity->blocked_count_ptr->fetch_add(1, std::memory_order_relaxed);
    }
  
    timeout =
        !absl::synchronization_internal::Waiter::GetWaiter(identity)->Wait(t);
  
    if (identity->blocked_count_ptr != nullptr) {
      identity->blocked_count_ptr->fetch_sub(1, std::memory_order_relaxed);
    }
    identity->is_idle.store(false, std::memory_order_relaxed);
    identity->wait_start.store(0, std::memory_order_relaxed);
    return !timeout;
  }
  
  }  // extern "C"
  
  #endif  // ABSL_LOW_LEVEL_ALLOC_MISSING
  