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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             : 
      14             : #include "llvm/Support/ThreadPool.h"
      15             : 
      16             : #include "llvm/Config/llvm-config.h"
      17             : #include "llvm/Support/Threading.h"
      18             : #include "llvm/Support/raw_ostream.h"
      19             : 
      20             : using namespace llvm;
      21             : 
      22             : #if LLVM_ENABLE_THREADS
      23             : 
      24             : // Default to hardware_concurrency
      25           8 : ThreadPool::ThreadPool() : ThreadPool(hardware_concurrency()) {}
      26             : 
      27         471 : ThreadPool::ThreadPool(unsigned ThreadCount)
      28         471 :     : ActiveThreads(0), EnableFlag(true) {
      29             :   // Create ThreadCount threads that will loop forever, wait on QueueCondition
      30             :   // for tasks to be queued or the Pool to be destroyed.
      31         471 :   Threads.reserve(ThreadCount);
      32        2408 :   for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
      33        1937 :     Threads.emplace_back([&] {
      34             :       while (true) {
      35             :         PackagedTaskTy Task;
      36             :         {
      37             :           std::unique_lock<std::mutex> LockGuard(QueueLock);
      38             :           // Wait for tasks to be pushed in the queue
      39             :           QueueCondition.wait(LockGuard,
      40             :                               [&] { return !EnableFlag || !Tasks.empty(); });
      41             :           // Exit condition
      42             :           if (!EnableFlag && Tasks.empty())
      43             :             return;
      44             :           // Yeah, we have a task, grab it and release the lock on the queue
      45             : 
      46             :           // We first need to signal that we are active before popping the queue
      47             :           // in order for wait() to properly detect that even if the queue is
      48             :           // empty, there is still a task in flight.
      49             :           {
      50             :             std::unique_lock<std::mutex> LockGuard(CompletionLock);
      51             :             ++ActiveThreads;
      52             :           }
      53             :           Task = std::move(Tasks.front());
      54             :           Tasks.pop();
      55             :         }
      56             :         // Run the task we just grabbed
      57             :         Task();
      58             : 
      59             :         {
      60             :           // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
      61             :           std::unique_lock<std::mutex> LockGuard(CompletionLock);
      62             :           --ActiveThreads;
      63             :         }
      64             : 
      65             :         // Notify task completion, in case someone waits on ThreadPool::wait()
      66             :         CompletionCondition.notify_all();
      67             :       }
      68             :     });
      69             :   }
      70         471 : }
      71             : 
      72         448 : void ThreadPool::wait() {
      73             :   // Wait for all threads to complete and the queue to be empty
      74         448 :   std::unique_lock<std::mutex> LockGuard(CompletionLock);
      75             :   // The order of the checks for ActiveThreads and Tasks.empty() matters because
      76             :   // any active threads might be modifying the Tasks queue, and this would be a
      77             :   // race.
      78         896 :   CompletionCondition.wait(LockGuard,
      79             :                            [&] { return !ActiveThreads && Tasks.empty(); });
      80         448 : }
      81             : 
      82         832 : std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
      83             :   /// Wrap the Task in a packaged_task to return a future object.
      84         832 :   PackagedTaskTy PackagedTask(std::move(Task));
      85         832 :   auto Future = PackagedTask.get_future();
      86             :   {
      87             :     // Lock the queue and push the new task
      88         832 :     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         832 :   QueueCondition.notify_one();
      96         832 :   return Future.share();
      97             : }
      98             : 
      99             : // The destructor joins all threads, waiting for completion.
     100         942 : ThreadPool::~ThreadPool() {
     101             :   {
     102         471 :     std::unique_lock<std::mutex> LockGuard(QueueLock);
     103         471 :     EnableFlag = false;
     104             :   }
     105         471 :   QueueCondition.notify_all();
     106        2408 :   for (auto &Worker : Threads)
     107        1937 :     Worker.join();
     108         471 : }
     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             :     Task();
     129             :   }
     130             : }
     131             : 
     132             : std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
     133             :   // Get a Future with launch::deferred execution using std::async
     134             :   auto Future = std::async(std::launch::deferred, std::move(Task)).share();
     135             :   // Wrap the future so that both ThreadPool::wait() can operate and the
     136             :   // returned future can be sync'ed on.
     137             :   PackagedTaskTy PackagedTask([Future]() { Future.get(); });
     138             :   Tasks.push(std::move(PackagedTask));
     139             :   return Future;
     140             : }
     141             : 
     142             : ThreadPool::~ThreadPool() {
     143             :   wait();
     144             : }
     145             : 
     146             : #endif