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_THREADPOOL_H
14#define LLVM_SUPPORT_THREADPOOL_H
15
16#include "llvm/Config/llvm-config.h"
17#include "llvm/Support/Threading.h"
18#include "llvm/Support/thread.h"
19
20#include <future>
21
22#include <atomic>
23#include <condition_variable>
24#include <functional>
25#include <memory>
26#include <mutex>
27#include <queue>
28#include <utility>
29
30namespace llvm {
31
32/// A ThreadPool for asynchronous parallel execution on a defined number of
33/// threads.
34///
35/// The pool keeps a vector of threads alive, waiting on a condition variable
36/// for some work to become available.
37class ThreadPool {
38public:
39 using TaskTy = std::function<void()>;
40 using PackagedTaskTy = std::packaged_task<void()>;
41
42 /// Construct a pool using the hardware strategy \p S for mapping hardware
43 /// execution resources (threads, cores, CPUs)
44 /// Defaults to using the maximum execution resources in the system, but
45 /// accounting for the affinity mask.
46 ThreadPool(ThreadPoolStrategy S = hardware_concurrency());
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
71 unsigned getThreadCount() const { return ThreadCount; }
72
73private:
74 bool workCompletedUnlocked() { return !ActiveThreads && Tasks.empty(); }
75
76 /// Asynchronous submission of a task to the pool. The returned future can be
77 /// used to wait for the task to finish and is *non-blocking* on destruction.
78 std::shared_future<void> asyncImpl(TaskTy F);
79
80 /// Threads in flight
81 std::vector<llvm::thread> Threads;
82
83 /// Tasks waiting for execution in the pool.
84 std::queue<PackagedTaskTy> Tasks;
85
86 /// Locking and signaling for accessing the Tasks queue.
87 std::mutex QueueLock;
88 std::condition_variable QueueCondition;
89
90 /// Signaling for job completion
91 std::condition_variable CompletionCondition;
92
93 /// Keep track of the number of thread actually busy
94 unsigned ActiveThreads = 0;
95
96#if LLVM_ENABLE_THREADS // avoids warning for unused variable
97 /// Signal for the destruction of the pool, asking thread to exit.
98 bool EnableFlag = true;
99#endif
100
101 unsigned ThreadCount;
102};
103}
104
105#endif // LLVM_SUPPORT_THREADPOOL_H
106