1//===-- llvm/Support/ThreadPool.h - A ThreadPool implementation -*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file defines a crude C++11 based thread pool.
11//
12//===----------------------------------------------------------------------===//
13
14#ifndef LLVM_SUPPORT_THREAD_POOL_H
15#define LLVM_SUPPORT_THREAD_POOL_H
16
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#ifndef _MSC_VER
39  using VoidTy = void;
40  using TaskTy = std::function<void()>;
41  using PackagedTaskTy = std::packaged_task<void()>;
42#else
43  // MSVC 2013 has a bug and can't use std::packaged_task<void()>;
44  // We force it to use bool(bool) instead.
45  using VoidTy = bool;
46  using TaskTy = std::function<bool(bool)>;
47  using PackagedTaskTy = std::packaged_task<bool(bool)>;
48#endif
49
50  /// Construct a pool with the number of core available on the system (or
51  /// whatever the value returned by std::thread::hardware_concurrency() is).
52  ThreadPool();
53
54  /// Construct a pool of \p ThreadCount threads
55  ThreadPool(unsigned ThreadCount);
56
57  /// Blocking destructor: the pool will wait for all the threads to complete.
58  ~ThreadPool();
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, typename... Args>
63  inline std::shared_future<VoidTy> async(Function &&F, Args &&... ArgList) {
64    auto Task =
65        std::bind(std::forward<Function>(F), std::forward<Args>(ArgList)...);
66#ifndef _MSC_VER
67    return asyncImpl(std::move(Task));
68#else
69    // This lambda has to be marked mutable because MSVC 2013's std::bind call
70    // operator isn't const qualified.
71    return asyncImpl([Task](VoidTy) mutable -> VoidTy {
72      Task();
73      return VoidTy();
74    });
75#endif
76  }
77
78  /// Asynchronous submission of a task to the pool. The returned future can be
79  /// used to wait for the task to finish and is *non-blocking* on destruction.
80  template <typename Function>
81  inline std::shared_future<VoidTy> async(Function &&F) {
82#ifndef _MSC_VER
83    return asyncImpl(std::forward<Function>(F));
84#else
85    return asyncImpl([F] (VoidTy) -> VoidTy { F(); return VoidTy(); });
86#endif
87  }
88
89  /// Blocking wait for all the threads to complete and the queue to be empty.
90  /// It is an error to try to add new tasks while blocking on this call.
91  void wait();
92
93private:
94  /// Asynchronous submission of a task to the pool. The returned future can be
95  /// used to wait for the task to finish and is *non-blocking* on destruction.
96  std::shared_future<VoidTy> asyncImpl(TaskTy F);
97
98  /// Threads in flight
99  std::vector<llvm::thread> Threads;
100
101  /// Tasks waiting for execution in the pool.
102  std::queue<PackagedTaskTy> Tasks;
103
104  /// Locking and signaling for accessing the Tasks queue.
105  std::mutex QueueLock;
106  std::condition_variable QueueCondition;
107
108  /// Locking and signaling for job completion
109  std::mutex CompletionLock;
110  std::condition_variable CompletionCondition;
111
112  /// Keep track of the number of thread actually busy
113  std::atomic<unsigned> ActiveThreads;
114
115#if LLVM_ENABLE_THREADS // avoids warning for unused variable
116  /// Signal for the destruction of the pool, asking thread to exit.
117  bool EnableFlag;
118#endif
119};
120}
121
122#endif // LLVM_SUPPORT_THREAD_POOL_H
123