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 23 // Default to hardware_concurrency 24 ThreadPool::ThreadPool() : ThreadPool(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 std::unique_lock<std::mutex> LockGuard(CompletionLock); 50 ++ActiveThreads; 51 } 52 Task = std::move(Tasks.front()); 53 Tasks.pop(); 54 } 55 // Run the task we just grabbed 56 Task(); 57 58 { 59 // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait() 60 std::unique_lock<std::mutex> LockGuard(CompletionLock); 61 --ActiveThreads; 62 } 63 64 // Notify task completion, in case someone waits on ThreadPool::wait() 65 CompletionCondition.notify_all(); 66 } 67 }); 68 } 69 } 70 71 void ThreadPool::wait() { 72 // Wait for all threads to complete and the queue to be empty 73 std::unique_lock<std::mutex> LockGuard(CompletionLock); 74 // The order of the checks for ActiveThreads and Tasks.empty() matters because 75 // any active threads might be modifying the Tasks queue, and this would be a 76 // race. 77 CompletionCondition.wait(LockGuard, 78 [&] { return !ActiveThreads && Tasks.empty(); }); 79 } 80 81 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) { 82 /// Wrap the Task in a packaged_task to return a future object. 83 PackagedTaskTy PackagedTask(std::move(Task)); 84 auto Future = PackagedTask.get_future(); 85 { 86 // Lock the queue and push the new task 87 std::unique_lock<std::mutex> LockGuard(QueueLock); 88 89 // Don't allow enqueueing after disabling the pool 90 assert(EnableFlag && "Queuing a thread during ThreadPool destruction"); 91 92 Tasks.push(std::move(PackagedTask)); 93 } 94 QueueCondition.notify_one(); 95 return Future.share(); 96 } 97 98 // The destructor joins all threads, waiting for completion. 99 ThreadPool::~ThreadPool() { 100 { 101 std::unique_lock<std::mutex> LockGuard(QueueLock); 102 EnableFlag = false; 103 } 104 QueueCondition.notify_all(); 105 for (auto &Worker : Threads) 106 Worker.join(); 107 } 108 109 #else // LLVM_ENABLE_THREADS Disabled 110 111 ThreadPool::ThreadPool() : ThreadPool(0) {} 112 113 // No threads are launched, issue a warning if ThreadCount is not 0 114 ThreadPool::ThreadPool(unsigned ThreadCount) 115 : ActiveThreads(0) { 116 if (ThreadCount) { 117 errs() << "Warning: request a ThreadPool with " << ThreadCount 118 << " threads, but LLVM_ENABLE_THREADS has been turned off\n"; 119 } 120 } 121 122 void ThreadPool::wait() { 123 // Sequential implementation running the tasks 124 while (!Tasks.empty()) { 125 auto Task = std::move(Tasks.front()); 126 Tasks.pop(); 127 Task(); 128 } 129 } 130 131 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) { 132 // Get a Future with launch::deferred execution using std::async 133 auto Future = std::async(std::launch::deferred, std::move(Task)).share(); 134 // Wrap the future so that both ThreadPool::wait() can operate and the 135 // returned future can be sync'ed on. 136 PackagedTaskTy PackagedTask([Future]() { Future.get(); }); 137 Tasks.push(std::move(PackagedTask)); 138 return Future; 139 } 140 141 ThreadPool::~ThreadPool() { 142 wait(); 143 } 144 145 #endif 146