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 void ThreadPool::grow() {
24   if (Threads.size() >= MaxThreadCount)
25     return; // Already hit the max thread pool size.
26   if (ActiveThreads + Tasks.size() <= Threads.size())
27     return; // We have enough threads for now.
28   int ThreadID = Threads.size();
29   Threads.emplace_back([this, ThreadID] {
30     Strategy.apply_thread_strategy(ThreadID);
31     while (true) {
32       std::function<void()> 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 void ThreadPool::wait() {
69   // Wait for all threads to complete and the queue to be empty
70   std::unique_lock<std::mutex> LockGuard(QueueLock);
71   CompletionCondition.wait(LockGuard, [&] { return workCompletedUnlocked(); });
72 }
73 
74 bool ThreadPool::isWorkerThread() const {
75   std::unique_lock<std::mutex> LockGuard(QueueLock);
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 // The destructor joins all threads, waiting for completion.
84 ThreadPool::~ThreadPool() {
85   {
86     std::unique_lock<std::mutex> LockGuard(QueueLock);
87     EnableFlag = false;
88   }
89   QueueCondition.notify_all();
90   for (auto &Worker : Threads)
91     Worker.join();
92 }
93 
94 #else // LLVM_ENABLE_THREADS Disabled
95 
96 // No threads are launched, issue a warning if ThreadCount is not 0
97 ThreadPool::ThreadPool(ThreadPoolStrategy S)
98     : ThreadCount(S.compute_thread_count()) {
99   if (ThreadCount != 1) {
100     errs() << "Warning: request a ThreadPool with " << ThreadCount
101            << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
102   }
103 }
104 
105 void ThreadPool::wait() {
106   // Sequential implementation running the tasks
107   while (!Tasks.empty()) {
108     auto Task = std::move(Tasks.front());
109     Tasks.pop();
110     Task();
111   }
112 }
113 
114 ThreadPool::~ThreadPool() { wait(); }
115 
116 #endif
117