1 //==-- llvm/Support/ThreadPool.cpp - A ThreadPool implementation -*- C++ -*-==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements a crude C++11 based thread pool.
12 //===----------------------------------------------------------------------===//
14 #include "llvm/Support/ThreadPool.h"
16 #include "llvm/Config/llvm-config.h"
17 #include "llvm/Support/raw_ostream.h"
21 #if LLVM_ENABLE_THREADS
23 // Default to std::thread::hardware_concurrency
24 ThreadPool::ThreadPool() : ThreadPool(std::thread::hardware_concurrency()) {}
26 ThreadPool::ThreadPool(unsigned ThreadCount)
27 : ActiveThreads(0), EnableFlag(true) {
28 // Create ThreadCount threads that will loop forever, wait on QueueCondition
29 // for tasks to be queued or the Pool to be destroyed.
30 Threads.reserve(ThreadCount);
31 for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
32 Threads.emplace_back([&] {
36 std::unique_lock<std::mutex> LockGuard(QueueLock);
37 // Wait for tasks to be pushed in the queue
38 QueueCondition.wait(LockGuard,
39 [&] { return !EnableFlag || !Tasks.empty(); });
41 if (!EnableFlag && Tasks.empty())
43 // Yeah, we have a task, grab it and release the lock on the queue
45 // We first need to signal that we are active before popping the queue
46 // in order for wait() to properly detect that even if the queue is
47 // empty, there is still a task in flight.
50 std::unique_lock<std::mutex> LockGuard(CompletionLock);
52 Task = std::move(Tasks.front());
55 // Run the task we just grabbed
59 Task(/* unused */ false);
63 // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
64 std::unique_lock<std::mutex> LockGuard(CompletionLock);
68 // Notify task completion, in case someone waits on ThreadPool::wait()
69 CompletionCondition.notify_all();
75 void ThreadPool::wait() {
76 // Wait for all threads to complete and the queue to be empty
77 std::unique_lock<std::mutex> LockGuard(CompletionLock);
78 // The order of the checks for ActiveThreads and Tasks.empty() matters because
79 // any active threads might be modifying the Tasks queue, and this would be a
81 CompletionCondition.wait(LockGuard,
82 [&] { return !ActiveThreads && Tasks.empty(); });
85 std::shared_future<ThreadPool::VoidTy> ThreadPool::asyncImpl(TaskTy Task) {
86 /// Wrap the Task in a packaged_task to return a future object.
87 PackagedTaskTy PackagedTask(std::move(Task));
88 auto Future = PackagedTask.get_future();
90 // Lock the queue and push the new task
91 std::unique_lock<std::mutex> LockGuard(QueueLock);
93 // Don't allow enqueueing after disabling the pool
94 assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
96 Tasks.push(std::move(PackagedTask));
98 QueueCondition.notify_one();
99 return Future.share();
102 // The destructor joins all threads, waiting for completion.
103 ThreadPool::~ThreadPool() {
105 std::unique_lock<std::mutex> LockGuard(QueueLock);
108 QueueCondition.notify_all();
109 for (auto &Worker : Threads)
113 #else // LLVM_ENABLE_THREADS Disabled
115 ThreadPool::ThreadPool() : ThreadPool(0) {}
117 // No threads are launched, issue a warning if ThreadCount is not 0
118 ThreadPool::ThreadPool(unsigned ThreadCount)
121 errs() << "Warning: request a ThreadPool with " << ThreadCount
122 << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
126 void ThreadPool::wait() {
127 // Sequential implementation running the tasks
128 while (!Tasks.empty()) {
129 auto Task = std::move(Tasks.front());
134 Task(/* unused */ false);
139 std::shared_future<ThreadPool::VoidTy> ThreadPool::asyncImpl(TaskTy Task) {
141 // Get a Future with launch::deferred execution using std::async
142 auto Future = std::async(std::launch::deferred, std::move(Task)).share();
143 // Wrap the future so that both ThreadPool::wait() can operate and the
144 // returned future can be sync'ed on.
145 PackagedTaskTy PackagedTask([Future]() { Future.get(); });
147 auto Future = std::async(std::launch::deferred, std::move(Task), false).share();
148 PackagedTaskTy PackagedTask([Future](bool) -> bool { Future.get(); return false; });
150 Tasks.push(std::move(PackagedTask));
154 ThreadPool::~ThreadPool() {