1//===-- llvm/Support/ThreadPool.h - A ThreadPool implementation -*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file defines a crude C++11 based thread pool.
10//
11//===----------------------------------------------------------------------===//
12
13#ifndef LLVM_SUPPORT_THREAD_POOL_H
14#define LLVM_SUPPORT_THREAD_POOL_H
15
16#include "llvm/Config/llvm-config.h"
17#include "llvm/Support/thread.h"
18
19#include <future>
20
21#include <atomic>
22#include <condition_variable>
23#include <functional>
24#include <memory>
25#include <mutex>
26#include <queue>
27#include <utility>
28
29namespace llvm {
30
31/// A ThreadPool for asynchronous parallel execution on a defined number of
32/// threads.
33///
34/// The pool keeps a vector of threads alive, waiting on a condition variable
35/// for some work to become available.
36class ThreadPool {
37public:
38  using TaskTy = std::function<void()>;
39  using PackagedTaskTy = std::packaged_task<void()>;
40
41  /// Construct a pool with the number of threads found by
42  /// hardware_concurrency().
43  ThreadPool();
44
45  /// Construct a pool of \p ThreadCount threads
46  ThreadPool(unsigned ThreadCount);
47
48  /// Blocking destructor: the pool will wait for all the threads to complete.
49  ~ThreadPool();
50
51  /// Asynchronous submission of a task to the pool. The returned future can be
52  /// used to wait for the task to finish and is *non-blocking* on destruction.
53  template <typename Function, typename... Args>
54  inline std::shared_future<void> async(Function &&F, Args &&... ArgList) {
55    auto Task =
56        std::bind(std::forward<Function>(F), std::forward<Args>(ArgList)...);
57    return asyncImpl(std::move(Task));
58  }
59
60  /// Asynchronous submission of a task to the pool. The returned future can be
61  /// used to wait for the task to finish and is *non-blocking* on destruction.
62  template <typename Function>
63  inline std::shared_future<void> async(Function &&F) {
64    return asyncImpl(std::forward<Function>(F));
65  }
66
67  /// Blocking wait for all the threads to complete and the queue to be empty.
68  /// It is an error to try to add new tasks while blocking on this call.
69  void wait();
70
71private:
72  /// Asynchronous submission of a task to the pool. The returned future can be
73  /// used to wait for the task to finish and is *non-blocking* on destruction.
74  std::shared_future<void> asyncImpl(TaskTy F);
75
76  /// Threads in flight
77  std::vector<llvm::thread> Threads;
78
79  /// Tasks waiting for execution in the pool.
80  std::queue<PackagedTaskTy> Tasks;
81
82  /// Locking and signaling for accessing the Tasks queue.
83  std::mutex QueueLock;
84  std::condition_variable QueueCondition;
85
86  /// Locking and signaling for job completion
87  std::mutex CompletionLock;
88  std::condition_variable CompletionCondition;
89
90  /// Keep track of the number of thread actually busy
91  std::atomic<unsigned> ActiveThreads;
92
93#if LLVM_ENABLE_THREADS // avoids warning for unused variable
94  /// Signal for the destruction of the pool, asking thread to exit.
95  bool EnableFlag;
96#endif
97};
98}
99
100#endif // LLVM_SUPPORT_THREAD_POOL_H
101