llvm/lib/Support/ThreadPool.cpp

   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 ThreadPool::ThreadPool(ThreadPoolStrategy S)
  24     : ThreadCount(S.compute_thread_count()) {
  25   // Create ThreadCount threads that will loop forever, wait on QueueCondition
  26   // for tasks to be queued or the Pool to be destroyed.
  27   Threads.reserve(ThreadCount);
  28   for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
  29     Threads.emplace_back([S, ThreadID, this] {
  30       S.apply_thread_strategy(ThreadID);
  31       while (true) {
  32         PackagedTaskTy Task;
  33         {
  34           std::unique_lock<std::mutex> LockGuard(QueueLock);
  35           // Wait for tasks to be pushed in the queue
  36           QueueCondition.wait(LockGuard,
  37                               [&] { return !EnableFlag || !Tasks.empty(); });
  38           // Exit condition
  39           if (!EnableFlag && Tasks.empty())
  40             return;
  41           // Yeah, we have a task, grab it and release the lock on the queue
  42
  43           // We first need to signal that we are active before popping the queue
  44           // in order for wait() to properly detect that even if the queue is
  45           // empty, there is still a task in flight.
  46           ++ActiveThreads;
  47           Task = std::move(Tasks.front());
  48           Tasks.pop();
  49         }
  50         // Run the task we just grabbed
  51         Task();
  52
  53         bool Notify;
  54         {
  55           // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
  56           std::lock_guard<std::mutex> LockGuard(QueueLock);
  57           --ActiveThreads;
  58           Notify = workCompletedUnlocked();
  59         }
  60         // Notify task completion if this is the last active thread, in case
  61         // someone waits on ThreadPool::wait().
  62         if (Notify)
  63           CompletionCondition.notify_all();
  64       }
  65     });
  66   }
  67 }
  68
  69 void ThreadPool::wait() {
  70   // Wait for all threads to complete and the queue to be empty
  71   std::unique_lock<std::mutex> LockGuard(QueueLock);
  72   CompletionCondition.wait(LockGuard, [&] { return workCompletedUnlocked(); });
  73 }
  74
  75 bool ThreadPool::isWorkerThread() const {
  76   llvm::thread::id CurrentThreadId = llvm::this_thread::get_id();
  77   for (const llvm::thread &Thread : Threads)
  78     if (CurrentThreadId == Thread.get_id())
  79       return true;
  80   return false;
  81 }
  82
  83 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
  84   /// Wrap the Task in a packaged_task to return a future object.
  85   PackagedTaskTy PackagedTask(std::move(Task));
  86   auto Future = PackagedTask.get_future();
  87   {
  88     // Lock the queue and push the new task
  89     std::unique_lock<std::mutex> LockGuard(QueueLock);
  90
  91     // Don't allow enqueueing after disabling the pool
  92     assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
  93
  94     Tasks.push(std::move(PackagedTask));
  95   }
  96   QueueCondition.notify_one();
  97   return Future.share();
  98 }
  99
 100 // The destructor joins all threads, waiting for completion.
 101 ThreadPool::~ThreadPool() {
 102   {
 103     std::unique_lock<std::mutex> LockGuard(QueueLock);
 104     EnableFlag = false;
 105   }
 106   QueueCondition.notify_all();
 107   for (auto &Worker : Threads)
 108     Worker.join();
 109 }
 110
 111 #else // LLVM_ENABLE_THREADS Disabled
 112
 113 // No threads are launched, issue a warning if ThreadCount is not 0
 114 ThreadPool::ThreadPool(ThreadPoolStrategy S)
 115     : ThreadCount(S.compute_thread_count()) {
 116   if (ThreadCount != 1) {
 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() { wait(); }
 142
 143 #endif