include/llvm/Support/ThreadPool.h

   1 //===-- llvm/Support/ThreadPool.h - 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 defines a crude C++11 based thread pool.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #ifndef LLVM_SUPPORT_THREAD_POOL_H
  14 #define LLVM_SUPPORT_THREAD_POOL_H
  15
  16 #include "llvm/Config/llvm-config.h"
  17 #include "llvm/Support/thread.h"
  18
  19 #include <future>
  20
  21 #include <atomic>
  22 #include <condition_variable>
  23 #include <functional>
  24 #include <memory>
  25 #include <mutex>
  26 #include <queue>
  27 #include <utility>
  28
  29 namespace llvm {
  30
  31 /// A ThreadPool for asynchronous parallel execution on a defined number of
  32 /// threads.
  33 ///
  34 /// The pool keeps a vector of threads alive, waiting on a condition variable
  35 /// for some work to become available.
  36 class ThreadPool {
  37 public:
  38   using TaskTy = std::function<void()>;
  39   using PackagedTaskTy = std::packaged_task<void()>;
  40
  41   /// Construct a pool with the number of threads found by
  42   /// hardware_concurrency().
  43   ThreadPool();
  44
  45   /// Construct a pool of \p ThreadCount threads
  46   ThreadPool(unsigned ThreadCount);
  47
  48   /// Blocking destructor: the pool will wait for all the threads to complete.
  49   ~ThreadPool();
  50
  51   /// Asynchronous submission of a task to the pool. The returned future can be
  52   /// used to wait for the task to finish and is *non-blocking* on destruction.
  53   template <typename Function, typename... Args>
  54   inline std::shared_future<void> async(Function &&F, Args &&... ArgList) {
  55     auto Task =
  56         std::bind(std::forward<Function>(F), std::forward<Args>(ArgList)...);
  57     return asyncImpl(std::move(Task));
  58   }
  59
  60   /// Asynchronous submission of a task to the pool. The returned future can be
  61   /// used to wait for the task to finish and is *non-blocking* on destruction.
  62   template <typename Function>
  63   inline std::shared_future<void> async(Function &&F) {
  64     return asyncImpl(std::forward<Function>(F));
  65   }
  66
  67   /// Blocking wait for all the threads to complete and the queue to be empty.
  68   /// It is an error to try to add new tasks while blocking on this call.
  69   void wait();
  70
  71 private:
  72   /// Asynchronous submission of a task to the pool. The returned future can be
  73   /// used to wait for the task to finish and is *non-blocking* on destruction.
  74   std::shared_future<void> asyncImpl(TaskTy F);
  75
  76   /// Threads in flight
  77   std::vector<llvm::thread> Threads;
  78
  79   /// Tasks waiting for execution in the pool.
  80   std::queue<PackagedTaskTy> Tasks;
  81
  82   /// Locking and signaling for accessing the Tasks queue.
  83   std::mutex QueueLock;
  84   std::condition_variable QueueCondition;
  85
  86   /// Locking and signaling for job completion
  87   std::mutex CompletionLock;
  88   std::condition_variable CompletionCondition;
  89
  90   /// Keep track of the number of thread actually busy
  91   std::atomic<unsigned> ActiveThreads;
  92
  93 #if LLVM_ENABLE_THREADS // avoids warning for unused variable
  94   /// Signal for the destruction of the pool, asking thread to exit.
  95   bool EnableFlag;
  96 #endif
  97 };
  98 }
  99
 100 #endif // LLVM_SUPPORT_THREAD_POOL_H