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
19using namespace llvm;
20
21#if LLVM_ENABLE_THREADS
22
23// Default to hardware_concurrency
24ThreadPool::ThreadPool() : ThreadPool(hardware_concurrency()) {}
25
26ThreadPool::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
71void 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
81std::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.
99ThreadPool::~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
111ThreadPool::ThreadPool() : ThreadPool(0) {}
112
113// No threads are launched, issue a warning if ThreadCount is not 0
114ThreadPool::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
122void 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
131std::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
141ThreadPool::~ThreadPool() {
142  wait();
143}
144
145#endif
146