1 //==-- llvm/Support/ThreadPool.cpp - 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 implements a crude C++11 based thread pool.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/Support/ThreadPool.h"
14
15 #include "llvm/Config/llvm-config.h"
16 #include "llvm/Support/Threading.h"
17 #include "llvm/Support/raw_ostream.h"
18
19 using namespace llvm;
20
21 #if LLVM_ENABLE_THREADS
22
ThreadPool(ThreadPoolStrategy S)23 ThreadPool::ThreadPool(ThreadPoolStrategy S)
24 : ThreadCount(S.compute_thread_count()) {
25 // Create ThreadCount threads that will loop forever, wait on QueueCondition
26 // for tasks to be queued or the Pool to be destroyed.
27 Threads.reserve(ThreadCount);
28 for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
29 Threads.emplace_back([S, ThreadID, this] {
30 S.apply_thread_strategy(ThreadID);
31 while (true) {
32 PackagedTaskTy Task;
33 {
34 std::unique_lock<std::mutex> LockGuard(QueueLock);
35 // Wait for tasks to be pushed in the queue
36 QueueCondition.wait(LockGuard,
37 [&] { return !EnableFlag || !Tasks.empty(); });
38 // Exit condition
39 if (!EnableFlag && Tasks.empty())
40 return;
41 // Yeah, we have a task, grab it and release the lock on the queue
42
43 // We first need to signal that we are active before popping the queue
44 // in order for wait() to properly detect that even if the queue is
45 // empty, there is still a task in flight.
46 ++ActiveThreads;
47 Task = std::move(Tasks.front());
48 Tasks.pop();
49 }
50 // Run the task we just grabbed
51 Task();
52
53 bool Notify;
54 {
55 // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
56 std::lock_guard<std::mutex> LockGuard(QueueLock);
57 --ActiveThreads;
58 Notify = workCompletedUnlocked();
59 }
60 // Notify task completion if this is the last active thread, in case
61 // someone waits on ThreadPool::wait().
62 if (Notify)
63 CompletionCondition.notify_all();
64 }
65 });
66 }
67 }
68
wait()69 void ThreadPool::wait() {
70 // Wait for all threads to complete and the queue to be empty
71 std::unique_lock<std::mutex> LockGuard(QueueLock);
72 CompletionCondition.wait(LockGuard, [&] { return workCompletedUnlocked(); });
73 }
74
asyncImpl(TaskTy Task)75 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
76 /// Wrap the Task in a packaged_task to return a future object.
77 PackagedTaskTy PackagedTask(std::move(Task));
78 auto Future = PackagedTask.get_future();
79 {
80 // Lock the queue and push the new task
81 std::unique_lock<std::mutex> LockGuard(QueueLock);
82
83 // Don't allow enqueueing after disabling the pool
84 assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
85
86 Tasks.push(std::move(PackagedTask));
87 }
88 QueueCondition.notify_one();
89 return Future.share();
90 }
91
92 // The destructor joins all threads, waiting for completion.
~ThreadPool()93 ThreadPool::~ThreadPool() {
94 {
95 std::unique_lock<std::mutex> LockGuard(QueueLock);
96 EnableFlag = false;
97 }
98 QueueCondition.notify_all();
99 for (auto &Worker : Threads)
100 Worker.join();
101 }
102
103 #else // LLVM_ENABLE_THREADS Disabled
104
105 // No threads are launched, issue a warning if ThreadCount is not 0
ThreadPool(ThreadPoolStrategy S)106 ThreadPool::ThreadPool(ThreadPoolStrategy S)
107 : ThreadCount(S.compute_thread_count()) {
108 if (ThreadCount != 1) {
109 errs() << "Warning: request a ThreadPool with " << ThreadCount
110 << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
111 }
112 }
113
wait()114 void ThreadPool::wait() {
115 // Sequential implementation running the tasks
116 while (!Tasks.empty()) {
117 auto Task = std::move(Tasks.front());
118 Tasks.pop();
119 Task();
120 }
121 }
122
asyncImpl(TaskTy Task)123 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
124 // Get a Future with launch::deferred execution using std::async
125 auto Future = std::async(std::launch::deferred, std::move(Task)).share();
126 // Wrap the future so that both ThreadPool::wait() can operate and the
127 // returned future can be sync'ed on.
128 PackagedTaskTy PackagedTask([Future]() { Future.get(); });
129 Tasks.push(std::move(PackagedTask));
130 return Future;
131 }
132
~ThreadPool()133 ThreadPool::~ThreadPool() { wait(); }
134
135 #endif
136