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
17#if LLVM_ENABLE_THREADS
18#include "llvm/Support/FormatVariadic.h"
19#include "llvm/Support/Threading.h"
20#else
21#include "llvm/Support/raw_ostream.h"
22#endif
23
24using namespace llvm;
25
26#if LLVM_ENABLE_THREADS
27
28// A note on thread groups: Tasks are by default in no group (represented
29// by nullptr ThreadPoolTaskGroup pointer in the Tasks queue) and functionality
30// here normally works on all tasks regardless of their group (functions
31// in that case receive nullptr ThreadPoolTaskGroup pointer as argument).
32// A task in a group has a pointer to that ThreadPoolTaskGroup in the Tasks
33// queue, and functions called to work only on tasks from one group take that
34// pointer.
35
36ThreadPool::ThreadPool(ThreadPoolStrategy S)
37 : Strategy(S), MaxThreadCount(S.compute_thread_count()) {}
38
39void ThreadPool::grow(int requested) {
40 llvm::sys::ScopedWriter LockGuard(ThreadsLock);
41 if (Threads.size() >= MaxThreadCount)
42 return; // Already hit the max thread pool size.
43 int newThreadCount = std::min<int>(a: requested, b: MaxThreadCount);
44 while (static_cast<int>(Threads.size()) < newThreadCount) {
45 int ThreadID = Threads.size();
46 Threads.emplace_back(args: [this, ThreadID] {
47 set_thread_name(formatv(Fmt: "llvm-worker-{0}", Vals: ThreadID));
48 Strategy.apply_thread_strategy(ThreadPoolNum: ThreadID);
49 processTasks(WaitingForGroup: nullptr);
50 });
51 }
52}
53
54#ifndef NDEBUG
55// The group of the tasks run by the current thread.
56static LLVM_THREAD_LOCAL std::vector<ThreadPoolTaskGroup *>
57 *CurrentThreadTaskGroups = nullptr;
58#endif
59
60// WaitingForGroup == nullptr means all tasks regardless of their group.
61void ThreadPool::processTasks(ThreadPoolTaskGroup *WaitingForGroup) {
62 while (true) {
63 std::function<void()> Task;
64 ThreadPoolTaskGroup *GroupOfTask;
65 {
66 std::unique_lock<std::mutex> LockGuard(QueueLock);
67 bool workCompletedForGroup = false; // Result of workCompletedUnlocked()
68 // Wait for tasks to be pushed in the queue
69 QueueCondition.wait(lock&: LockGuard, p: [&] {
70 return !EnableFlag || !Tasks.empty() ||
71 (WaitingForGroup != nullptr &&
72 (workCompletedForGroup =
73 workCompletedUnlocked(Group: WaitingForGroup)));
74 });
75 // Exit condition
76 if (!EnableFlag && Tasks.empty())
77 return;
78 if (WaitingForGroup != nullptr && workCompletedForGroup)
79 return;
80 // Yeah, we have a task, grab it and release the lock on the queue
81
82 // We first need to signal that we are active before popping the queue
83 // in order for wait() to properly detect that even if the queue is
84 // empty, there is still a task in flight.
85 ++ActiveThreads;
86 Task = std::move(Tasks.front().first);
87 GroupOfTask = Tasks.front().second;
88 // Need to count active threads in each group separately, ActiveThreads
89 // would never be 0 if waiting for another group inside a wait.
90 if (GroupOfTask != nullptr)
91 ++ActiveGroups[GroupOfTask]; // Increment or set to 1 if new item
92 Tasks.pop_front();
93 }
94#ifndef NDEBUG
95 if (CurrentThreadTaskGroups == nullptr)
96 CurrentThreadTaskGroups = new std::vector<ThreadPoolTaskGroup *>;
97 CurrentThreadTaskGroups->push_back(x: GroupOfTask);
98#endif
99
100 // Run the task we just grabbed
101 Task();
102
103#ifndef NDEBUG
104 CurrentThreadTaskGroups->pop_back();
105 if (CurrentThreadTaskGroups->empty()) {
106 delete CurrentThreadTaskGroups;
107 CurrentThreadTaskGroups = nullptr;
108 }
109#endif
110
111 bool Notify;
112 bool NotifyGroup;
113 {
114 // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
115 std::lock_guard<std::mutex> LockGuard(QueueLock);
116 --ActiveThreads;
117 if (GroupOfTask != nullptr) {
118 auto A = ActiveGroups.find(Val: GroupOfTask);
119 if (--(A->second) == 0)
120 ActiveGroups.erase(I: A);
121 }
122 Notify = workCompletedUnlocked(Group: GroupOfTask);
123 NotifyGroup = GroupOfTask != nullptr && Notify;
124 }
125 // Notify task completion if this is the last active thread, in case
126 // someone waits on ThreadPool::wait().
127 if (Notify)
128 CompletionCondition.notify_all();
129 // If this was a task in a group, notify also threads waiting for tasks
130 // in this function on QueueCondition, to make a recursive wait() return
131 // after the group it's been waiting for has finished.
132 if (NotifyGroup)
133 QueueCondition.notify_all();
134 }
135}
136
137bool ThreadPool::workCompletedUnlocked(ThreadPoolTaskGroup *Group) const {
138 if (Group == nullptr)
139 return !ActiveThreads && Tasks.empty();
140 return ActiveGroups.count(Val: Group) == 0 &&
141 !llvm::any_of(Range: Tasks,
142 P: [Group](const auto &T) { return T.second == Group; });
143}
144
145void ThreadPool::wait() {
146 assert(!isWorkerThread()); // Would deadlock waiting for itself.
147 // Wait for all threads to complete and the queue to be empty
148 std::unique_lock<std::mutex> LockGuard(QueueLock);
149 CompletionCondition.wait(lock&: LockGuard,
150 p: [&] { return workCompletedUnlocked(Group: nullptr); });
151}
152
153void ThreadPool::wait(ThreadPoolTaskGroup &Group) {
154 // Wait for all threads in the group to complete.
155 if (!isWorkerThread()) {
156 std::unique_lock<std::mutex> LockGuard(QueueLock);
157 CompletionCondition.wait(lock&: LockGuard,
158 p: [&] { return workCompletedUnlocked(Group: &Group); });
159 return;
160 }
161 // Make sure to not deadlock waiting for oneself.
162 assert(CurrentThreadTaskGroups == nullptr ||
163 !llvm::is_contained(*CurrentThreadTaskGroups, &Group));
164 // Handle the case of recursive call from another task in a different group,
165 // in which case process tasks while waiting to keep the thread busy and avoid
166 // possible deadlock.
167 processTasks(WaitingForGroup: &Group);
168}
169
170bool ThreadPool::isWorkerThread() const {
171 llvm::sys::ScopedReader LockGuard(ThreadsLock);
172 llvm::thread::id CurrentThreadId = llvm::this_thread::get_id();
173 for (const llvm::thread &Thread : Threads)
174 if (CurrentThreadId == Thread.get_id())
175 return true;
176 return false;
177}
178
179// The destructor joins all threads, waiting for completion.
180ThreadPool::~ThreadPool() {
181 {
182 std::unique_lock<std::mutex> LockGuard(QueueLock);
183 EnableFlag = false;
184 }
185 QueueCondition.notify_all();
186 llvm::sys::ScopedReader LockGuard(ThreadsLock);
187 for (auto &Worker : Threads)
188 Worker.join();
189}
190
191#else // LLVM_ENABLE_THREADS Disabled
192
193// No threads are launched, issue a warning if ThreadCount is not 0
194ThreadPool::ThreadPool(ThreadPoolStrategy S) : MaxThreadCount(1) {
195 int ThreadCount = S.compute_thread_count();
196 if (ThreadCount != 1) {
197 errs() << "Warning: request a ThreadPool with " << ThreadCount
198 << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
199 }
200}
201
202void ThreadPool::wait() {
203 // Sequential implementation running the tasks
204 while (!Tasks.empty()) {
205 auto Task = std::move(Tasks.front().first);
206 Tasks.pop_front();
207 Task();
208 }
209}
210
211void ThreadPool::wait(ThreadPoolTaskGroup &) {
212 // Simply wait for all, this works even if recursive (the running task
213 // is already removed from the queue).
214 wait();
215}
216
217bool ThreadPool::isWorkerThread() const {
218 report_fatal_error("LLVM compiled without multithreading");
219}
220
221ThreadPool::~ThreadPool() { wait(); }
222
223#endif
224

source code of llvm/lib/Support/ThreadPool.cpp