LLVM  4.0.0
ThreadPool.cpp
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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 
15 
16 #include "llvm/Config/llvm-config.h"
18 
19 using namespace llvm;
20 
21 #if LLVM_ENABLE_THREADS
22 
23 // Default to std::thread::hardware_concurrency
24 ThreadPool::ThreadPool() : ThreadPool(std::thread::hardware_concurrency()) {}
25 
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 
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 
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.
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 
116 
117 // No threads are launched, issue a warning if ThreadCount is not 0
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 
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 
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 
155  wait();
156 }
157 
158 #endif
raw_ostream & errs()
This returns a reference to a raw_ostream for standard error.
std::packaged_task< void()> PackagedTaskTy
Definition: ThreadPool.h:56
~ThreadPool()
Blocking destructor: the pool will wait for all the threads to complete.
Definition: ThreadPool.cpp:103
A ThreadPool for asynchronous parallel execution on a defined number of threads.
Definition: ThreadPool.h:51
ThreadPool()
Construct a pool with the number of core available on the system (or whatever the value returned by s...
Definition: ThreadPool.cpp:24
Basic Alias true
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
void wait()
Blocking wait for all the threads to complete and the queue to be empty.
Definition: ThreadPool.cpp:75
std::thread thread
Definition: thread.h:41