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//===-- llvm/Support/TaskQueue.h - A TaskQueue implementation ---*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file defines a crude C++11 based task queue.
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/ThreadPool.h"
#include "llvm/Support/thread.h"
#include <atomic>
#include <cassert>
#include <condition_variable>
#include <deque>
#include <functional>
#include <future>
#include <memory>
#include <mutex>
#include <utility>
namespace llvm {
/// TaskQueue executes serialized work on a user-defined Thread Pool. It
/// guarantees that if task B is enqueued after task A, task B begins after
/// task A completes and there is no overlap between the two.
class TaskQueue {
// Because we don't have init capture to use move-only local variables that
// are captured into a lambda, we create the promise inside an explicit
// callable struct. We want to do as much of the wrapping in the
// type-specialized domain (before type erasure) and then erase this into a
// std::function.
template <typename Callable> struct Task {
using ResultTy = typename std::result_of<Callable()>::type;
explicit Task(Callable C, TaskQueue &Parent)
: C(std::move(C)), P(std::make_shared<std::promise<ResultTy>>()),
Parent(&Parent) {}
template<typename T>
void invokeCallbackAndSetPromise(T*) {
void invokeCallbackAndSetPromise(void*) {
void operator()() noexcept {
ResultTy *Dummy = nullptr;
Callable C;
std::shared_ptr<std::promise<ResultTy>> P;
TaskQueue *Parent;
/// Construct a task queue with no work.
TaskQueue(ThreadPool &Scheduler) : Scheduler(Scheduler) { (void)Scheduler; }
/// Blocking destructor: the queue will wait for all work to complete.
~TaskQueue() {
/// Asynchronous submission of a task to the queue. The returned future can be
/// used to wait for the task (and all previous tasks that have not yet
/// completed) to finish.
template <typename Callable>
std::future<typename std::result_of<Callable()>::type> async(Callable &&C) {
"TaskQueue requires building with LLVM_ENABLE_THREADS!");
Task<Callable> T{std::move(C), *this};
using ResultTy = typename std::result_of<Callable()>::type;
std::future<ResultTy> F = T.P->get_future();
std::lock_guard<std::mutex> Lock(QueueLock);
// If there's already a task in flight, just queue this one up. If
// there is not a task in flight, bypass the queue and schedule this
// task immediately.
if (IsTaskInFlight)
else {
IsTaskInFlight = true;
return std::move(F);
void completeTask() {
// We just completed a task. If there are no more tasks in the queue,
// update IsTaskInFlight to false and stop doing work. Otherwise
// schedule the next task (while not holding the lock).
std::function<void()> Continuation;
std::lock_guard<std::mutex> Lock(QueueLock);
if (Tasks.empty()) {
IsTaskInFlight = false;
Continuation = std::move(Tasks.front());
/// The thread pool on which to run the work.
ThreadPool &Scheduler;
/// State which indicates whether the queue currently is currently processing
/// any work.
bool IsTaskInFlight = false;
/// Mutex for synchronizing access to the Tasks array.
std::mutex QueueLock;
/// Tasks waiting for execution in the queue.
std::deque<std::function<void()>> Tasks;
} // namespace llvm