LLVM  14.0.0git
ThreadPool.h
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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_THREADPOOL_H
14 #define LLVM_SUPPORT_THREADPOOL_H
15 
16 #include "llvm/Config/llvm-config.h"
17 #include "llvm/Support/Threading.h"
18 #include "llvm/Support/thread.h"
19 
20 #include <future>
21 
22 #include <atomic>
23 #include <condition_variable>
24 #include <functional>
25 #include <memory>
26 #include <mutex>
27 #include <queue>
28 #include <utility>
29 
30 namespace llvm {
31 
32 /// A ThreadPool for asynchronous parallel execution on a defined number of
33 /// threads.
34 ///
35 /// The pool keeps a vector of threads alive, waiting on a condition variable
36 /// for some work to become available.
37 class ThreadPool {
38 public:
39  /// Construct a pool using the hardware strategy \p S for mapping hardware
40  /// execution resources (threads, cores, CPUs)
41  /// Defaults to using the maximum execution resources in the system, but
42  /// accounting for the affinity mask.
44 
45  /// Blocking destructor: the pool will wait for all the threads to complete.
46  ~ThreadPool();
47 
48  /// Asynchronous submission of a task to the pool. The returned future can be
49  /// used to wait for the task to finish and is *non-blocking* on destruction.
50  template <typename Function, typename... Args>
51  inline auto async(Function &&F, Args &&...ArgList) {
52  auto Task =
53  std::bind(std::forward<Function>(F), std::forward<Args>(ArgList)...);
54  return async(std::move(Task));
55  }
56 
57  /// Asynchronous submission of a task to the pool. The returned future can be
58  /// used to wait for the task to finish and is *non-blocking* on destruction.
59  template <typename Func>
60  auto async(Func &&F) -> std::shared_future<decltype(F())> {
61  return asyncImpl(std::function<decltype(F())()>(std::forward<Func>(F)));
62  }
63 
64  /// Blocking wait for all the threads to complete and the queue to be empty.
65  /// It is an error to try to add new tasks while blocking on this call.
66  void wait();
67 
68  unsigned getThreadCount() const { return ThreadCount; }
69 
70  /// Returns true if the current thread is a worker thread of this thread pool.
71  bool isWorkerThread() const;
72 
73 private:
74  /// Helpers to create a promise and a callable wrapper of \p Task that sets
75  /// the result of the promise. Returns the callable and a future to access the
76  /// result.
77  template <typename ResTy>
78  static std::pair<std::function<void()>, std::future<ResTy>>
79  createTaskAndFuture(std::function<ResTy()> Task) {
80  std::shared_ptr<std::promise<ResTy>> Promise =
81  std::make_shared<std::promise<ResTy>>();
82  auto F = Promise->get_future();
83  return {
84  [Promise = std::move(Promise), Task]() { Promise->set_value(Task()); },
85  std::move(F)};
86  }
87  static std::pair<std::function<void()>, std::future<void>>
88  createTaskAndFuture(std::function<void()> Task) {
89  std::shared_ptr<std::promise<void>> Promise =
90  std::make_shared<std::promise<void>>();
91  auto F = Promise->get_future();
92  return {[Promise = std::move(Promise), Task]() {
93  Task();
94  Promise->set_value();
95  },
96  std::move(F)};
97  }
98 
99  bool workCompletedUnlocked() { return !ActiveThreads && Tasks.empty(); }
100 
101  /// Asynchronous submission of a task to the pool. The returned future can be
102  /// used to wait for the task to finish and is *non-blocking* on destruction.
103  template <typename ResTy>
104  std::shared_future<ResTy> asyncImpl(std::function<ResTy()> Task) {
105 
106 #if LLVM_ENABLE_THREADS
107  /// Wrap the Task in a std::function<void()> that sets the result of the
108  /// corresponding future.
109  auto R = createTaskAndFuture(Task);
110 
111  {
112  // Lock the queue and push the new task
113  std::unique_lock<std::mutex> LockGuard(QueueLock);
114 
115  // Don't allow enqueueing after disabling the pool
116  assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
117  Tasks.push(std::move(R.first));
118  }
119  QueueCondition.notify_one();
120  return R.second.share();
121 
122 #else // LLVM_ENABLE_THREADS Disabled
123 
124  // Get a Future with launch::deferred execution using std::async
125  auto Future = std::async(std::launch::deferred, std::move(Task)).share();
126  // Wrap the future so that both ThreadPool::wait() can operate and the
127  // returned future can be sync'ed on.
128  Tasks.push([Future]() { Future.get(); });
129  return Future;
130 #endif
131  }
132 
133  /// Threads in flight
134  std::vector<llvm::thread> Threads;
135 
136  /// Tasks waiting for execution in the pool.
137  std::queue<std::function<void()>> Tasks;
138 
139  /// Locking and signaling for accessing the Tasks queue.
140  std::mutex QueueLock;
141  std::condition_variable QueueCondition;
142 
143  /// Signaling for job completion
144  std::condition_variable CompletionCondition;
145 
146  /// Keep track of the number of thread actually busy
147  unsigned ActiveThreads = 0;
148 
149 #if LLVM_ENABLE_THREADS // avoids warning for unused variable
150  /// Signal for the destruction of the pool, asking thread to exit.
151  bool EnableFlag = true;
152 #endif
153 
154  unsigned ThreadCount;
155 };
156 }
157 
158 #endif // LLVM_SUPPORT_THREADPOOL_H
llvm::hardware_concurrency
ThreadPoolStrategy hardware_concurrency(unsigned ThreadCount=0)
Returns a default thread strategy where all available hardware resources are to be used,...
Definition: Threading.h:188
llvm
This is an optimization pass for GlobalISel generic memory operations.
Definition: AllocatorList.h:23
llvm::ThreadPool
A ThreadPool for asynchronous parallel execution on a defined number of threads.
Definition: ThreadPool.h:37
llvm::Function
Definition: Function.h:62
llvm::ThreadPoolStrategy
This tells how a thread pool will be used.
Definition: Threading.h:119
llvm::ThreadPool::ThreadPool
ThreadPool(ThreadPoolStrategy S=hardware_concurrency())
Construct a pool using the hardware strategy S for mapping hardware execution resources (threads,...
Definition: ThreadPool.cpp:23
F
#define F(x, y, z)
Definition: MD5.cpp:56
llvm::RISCVFenceField::R
@ R
Definition: RISCVBaseInfo.h:207
llvm::ThreadPool::getThreadCount
unsigned getThreadCount() const
Definition: ThreadPool.h:68
Threading.h
move
compiles ldr LCPI1_0 ldr ldr mov lsr tst moveq r1 ldr LCPI1_1 and r0 bx lr It would be better to do something like to fold the shift into the conditional move
Definition: README.txt:546
llvm::ThreadPool::isWorkerThread
bool isWorkerThread() const
Returns true if the current thread is a worker thread of this thread pool.
Definition: ThreadPool.cpp:75
llvm::ThreadPool::~ThreadPool
~ThreadPool()
Blocking destructor: the pool will wait for all the threads to complete.
Definition: ThreadPool.cpp:84
assert
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
function
print Print MemDeps of function
Definition: MemDepPrinter.cpp:83
llvm::ThreadPool::async
auto async(Func &&F) -> std::shared_future< decltype(F())>
Asynchronous submission of a task to the pool.
Definition: ThreadPool.h:60
llvm::ThreadPool::async
auto async(Function &&F, Args &&...ArgList)
Asynchronous submission of a task to the pool.
Definition: ThreadPool.h:51
S
add sub stmia L5 ldr r0 bl L_printf $stub Instead of a and a wouldn t it be better to do three moves *Return an aggregate type is even return S
Definition: README.txt:210
llvm::ThreadPool::wait
void wait()
Blocking wait for all the threads to complete and the queue to be empty.
Definition: ThreadPool.cpp:69
thread.h
llvm::AMDGPU::HSAMD::Kernel::Key::Args
constexpr char Args[]
Key for Kernel::Metadata::mArgs.
Definition: AMDGPUMetadata.h:389