xref: /llvm-project-15.0.7/llvm/lib/Support/ThreadPool.cpp (revision b28f317c)

//==-- llvm/Support/ThreadPool.cpp - A ThreadPool implementation -*- C++ -*-==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements a crude C++11 based thread pool.
//
//===----------------------------------------------------------------------===//

#include "llvm/Support/ThreadPool.h"

#include "llvm/Config/llvm-config.h"
#include "llvm/Support/Threading.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;

#if LLVM_ENABLE_THREADS

ThreadPool::ThreadPool(ThreadPoolStrategy S)
    : Strategy(S), MaxThreadCount(S.compute_thread_count()) {}

void ThreadPool::grow() {
  if (Threads.size() >= MaxThreadCount)
    return; // Already hit the max thread pool size.
  if (ActiveThreads + Tasks.size() <= Threads.size())
    return; // We have enough threads for now.
  int ThreadID = Threads.size();
  Threads.emplace_back([this, ThreadID] {
    Strategy.apply_thread_strategy(ThreadID);
    while (true) {
      std::function<void()> Task;
      {
        std::unique_lock<std::mutex> LockGuard(QueueLock);
        // Wait for tasks to be pushed in the queue
        QueueCondition.wait(LockGuard,
                            [&] { return !EnableFlag || !Tasks.empty(); });
        // Exit condition
        if (!EnableFlag && Tasks.empty())
          return;
        // Yeah, we have a task, grab it and release the lock on the queue

        // We first need to signal that we are active before popping the queue
        // in order for wait() to properly detect that even if the queue is
        // empty, there is still a task in flight.
        ++ActiveThreads;
        Task = std::move(Tasks.front());
        Tasks.pop();
      }
      // Run the task we just grabbed
      Task();

      bool Notify;
      {
        // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
        std::lock_guard<std::mutex> LockGuard(QueueLock);
        --ActiveThreads;
        Notify = workCompletedUnlocked();
      }
      // Notify task completion if this is the last active thread, in case
      // someone waits on ThreadPool::wait().
      if (Notify)
        CompletionCondition.notify_all();
    }
  });
}

void ThreadPool::wait() {
  // Wait for all threads to complete and the queue to be empty
  std::unique_lock<std::mutex> LockGuard(QueueLock);
  CompletionCondition.wait(LockGuard, [&] { return workCompletedUnlocked(); });
}

bool ThreadPool::isWorkerThread() const {
  std::unique_lock<std::mutex> LockGuard(QueueLock);
  llvm::thread::id CurrentThreadId = llvm::this_thread::get_id();
  for (const llvm::thread &Thread : Threads)
    if (CurrentThreadId == Thread.get_id())
      return true;
  return false;
}

// The destructor joins all threads, waiting for completion.
ThreadPool::~ThreadPool() {
  {
    std::unique_lock<std::mutex> LockGuard(QueueLock);
    EnableFlag = false;
  }
  QueueCondition.notify_all();
  for (auto &Worker : Threads)
    Worker.join();
}

#else // LLVM_ENABLE_THREADS Disabled

// No threads are launched, issue a warning if ThreadCount is not 0
ThreadPool::ThreadPool(ThreadPoolStrategy S) : MaxThreadCount(1) {
  int ThreadCount = S.compute_thread_count();
  if (ThreadCount != 1) {
    errs() << "Warning: request a ThreadPool with " << ThreadCount
           << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
  }
}

void ThreadPool::wait() {
  // Sequential implementation running the tasks
  while (!Tasks.empty()) {
    auto Task = std::move(Tasks.front());
    Tasks.pop();
    Task();
  }
}

ThreadPool::~ThreadPool() { wait(); }

#endif