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 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 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 CompletionCondition.wait(LockGuard, 79 [&] { return Tasks.empty() && !ActiveThreads; }); 80 } 81 82 std::shared_future<ThreadPool::VoidTy> ThreadPool::asyncImpl(TaskTy Task) { 83 /// Wrap the Task in a packaged_task to return a future object. 84 PackagedTaskTy PackagedTask(std::move(Task)); 85 auto Future = PackagedTask.get_future(); 86 { 87 // Lock the queue and push the new task 88 std::unique_lock<std::mutex> LockGuard(QueueLock); 89 90 // Don't allow enqueueing after disabling the pool 91 assert(EnableFlag && "Queuing a thread during ThreadPool destruction"); 92 93 Tasks.push(std::move(PackagedTask)); 94 } 95 QueueCondition.notify_one(); 96 return Future.share(); 97 } 98 99 // The destructor joins all threads, waiting for completion. 100 ThreadPool::~ThreadPool() { 101 { 102 std::unique_lock<std::mutex> LockGuard(QueueLock); 103 EnableFlag = false; 104 } 105 QueueCondition.notify_all(); 106 for (auto &Worker : Threads) 107 Worker.join(); 108 } 109 110 #else // LLVM_ENABLE_THREADS Disabled 111 112 ThreadPool::ThreadPool() : ThreadPool(0) {} 113 114 // No threads are launched, issue a warning if ThreadCount is not 0 115 ThreadPool::ThreadPool(unsigned ThreadCount) 116 : ActiveThreads(0) { 117 if (ThreadCount) { 118 errs() << "Warning: request a ThreadPool with " << ThreadCount 119 << " threads, but LLVM_ENABLE_THREADS has been turned off\n"; 120 } 121 } 122 123 void ThreadPool::wait() { 124 // Sequential implementation running the tasks 125 while (!Tasks.empty()) { 126 auto Task = std::move(Tasks.front()); 127 Tasks.pop(); 128 #ifndef _MSC_VER 129 Task(); 130 #else 131 Task(/* unused */ false); 132 #endif 133 } 134 } 135 136 std::shared_future<ThreadPool::VoidTy> ThreadPool::asyncImpl(TaskTy Task) { 137 #ifndef _MSC_VER 138 // Get a Future with launch::deferred execution using std::async 139 auto Future = std::async(std::launch::deferred, std::move(Task)).share(); 140 // Wrap the future so that both ThreadPool::wait() can operate and the 141 // returned future can be sync'ed on. 142 PackagedTaskTy PackagedTask([Future]() { Future.get(); }); 143 #else 144 auto Future = std::async(std::launch::deferred, std::move(Task), false).share(); 145 PackagedTaskTy PackagedTask([Future](bool) -> bool { Future.get(); return false; }); 146 #endif 147 Tasks.push(std::move(PackagedTask)); 148 return Future; 149 } 150 151 ThreadPool::~ThreadPool() { 152 wait(); 153 } 154 155 #endif 156
