1 //==-- llvm/Support/ThreadPool.cpp - 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 implements a crude C++11 based thread pool.
11 //
12 //===----------------------------------------------------------------------===//
13
14 #include "llvm/Support/ThreadPool.h"
15
16 #include "llvm/Config/llvm-config.h"
17 #include "llvm/Support/raw_ostream.h"
18
19 using namespace llvm;
20
21 #if LLVM_ENABLE_THREADS
22
23 // Default to std::thread::hardware_concurrency
ThreadPool()24 ThreadPool::ThreadPool() : ThreadPool(std::thread::hardware_concurrency()) {}
25
ThreadPool(unsigned ThreadCount)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([&] {
33 while (true) {
34 PackagedTaskTy Task;
35 {
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(); });
40 // Exit condition
41 if (!EnableFlag && Tasks.empty())
42 return;
43 // Yeah, we have a task, grab it and release the lock on the queue
44
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.
48 {
49 ++ActiveThreads;
50 std::unique_lock<std::mutex> LockGuard(CompletionLock);
51 }
52 Task = std::move(Tasks.front());
53 Tasks.pop();
54 }
55 // Run the task we just grabbed
56 #ifndef _MSC_VER
57 Task();
58 #else
59 Task(/* unused */ false);
60 #endif
61
62 {
63 // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
64 std::unique_lock<std::mutex> LockGuard(CompletionLock);
65 --ActiveThreads;
66 }
67
68 // Notify task completion, in case someone waits on ThreadPool::wait()
69 CompletionCondition.notify_all();
70 }
71 });
72 }
73 }
74
wait()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
80 // race.
81 CompletionCondition.wait(LockGuard,
82 [&] { return !ActiveThreads && Tasks.empty(); });
83 }
84
asyncImpl(TaskTy Task)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();
89 {
90 // Lock the queue and push the new task
91 std::unique_lock<std::mutex> LockGuard(QueueLock);
92
93 // Don't allow enqueueing after disabling the pool
94 assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
95
96 Tasks.push(std::move(PackagedTask));
97 }
98 QueueCondition.notify_one();
99 return Future.share();
100 }
101
102 // The destructor joins all threads, waiting for completion.
~ThreadPool()103 ThreadPool::~ThreadPool() {
104 {
105 std::unique_lock<std::mutex> LockGuard(QueueLock);
106 EnableFlag = false;
107 }
108 QueueCondition.notify_all();
109 for (auto &Worker : Threads)
110 Worker.join();
111 }
112
113 #else // LLVM_ENABLE_THREADS Disabled
114
ThreadPool()115 ThreadPool::ThreadPool() : ThreadPool(0) {}
116
117 // No threads are launched, issue a warning if ThreadCount is not 0
ThreadPool(unsigned ThreadCount)118 ThreadPool::ThreadPool(unsigned ThreadCount)
119 : ActiveThreads(0) {
120 if (ThreadCount) {
121 errs() << "Warning: request a ThreadPool with " << ThreadCount
122 << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
123 }
124 }
125
wait()126 void ThreadPool::wait() {
127 // Sequential implementation running the tasks
128 while (!Tasks.empty()) {
129 auto Task = std::move(Tasks.front());
130 Tasks.pop();
131 #ifndef _MSC_VER
132 Task();
133 #else
134 Task(/* unused */ false);
135 #endif
136 }
137 }
138
asyncImpl(TaskTy Task)139 std::shared_future<ThreadPool::VoidTy> ThreadPool::asyncImpl(TaskTy Task) {
140 #ifndef _MSC_VER
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(); });
146 #else
147 auto Future = std::async(std::launch::deferred, std::move(Task), false).share();
148 PackagedTaskTy PackagedTask([Future](bool) -> bool { Future.get(); return false; });
149 #endif
150 Tasks.push(std::move(PackagedTask));
151 return Future;
152 }
153
~ThreadPool()154 ThreadPool::~ThreadPool() {
155 wait();
156 }
157
158 #endif
159