Imported Upstream version 5.18.0.225

Former-commit-id: 10196d987d5fc5564b9d3b33b1fdf13190f4d0b5

external/llvm/lib/Support/ThreadPool.cpp

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+//==-- llvm/Support/ThreadPool.cpp - A ThreadPool implementation -*- C++ -*-==//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a crude C++11 based thread pool.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Config/llvm-config.h"
+#include "llvm/Support/Threading.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+#if LLVM_ENABLE_THREADS
+
+#include "llvm/Support/ThreadPool.h"
+
+// Default to hardware_concurrency
+ThreadPool::ThreadPool() : ThreadPool(hardware_concurrency()) {}
+
+ThreadPool::ThreadPool(unsigned ThreadCount)
+    : ActiveThreads(0), EnableFlag(true) {
+  // Create ThreadCount threads that will loop forever, wait on QueueCondition
+  // for tasks to be queued or the Pool to be destroyed.
+  Threads.reserve(ThreadCount);
+  for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
+    Threads.emplace_back([&] {
+      while (true) {
+        PackagedTaskTy Task;
+        {
+          std::unique_lock<std::mutex> LockGuard(QueueLock);
+          // Wait for tasks to be pushed in the queue
+          QueueCondition.wait(LockGuard,
+                              [&] { return !EnableFlag || !Tasks.empty(); });
+          // Exit condition
+          if (!EnableFlag && Tasks.empty())
+            return;
+          // Yeah, we have a task, grab it and release the lock on the queue
+
+          // We first need to signal that we are active before popping the queue
+          // in order for wait() to properly detect that even if the queue is
+          // empty, there is still a task in flight.
+          {
+            std::unique_lock<std::mutex> LockGuard(CompletionLock);
+            ++ActiveThreads;
+          }
+          Task = std::move(Tasks.front());
+          Tasks.pop();
+        }
+        // Run the task we just grabbed
+        Task();
+
+        {
+          // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
+          std::unique_lock<std::mutex> LockGuard(CompletionLock);
+          --ActiveThreads;
+        }
+
+        // Notify task completion, in case someone waits on ThreadPool::wait()
+        CompletionCondition.notify_all();
+      }
+    });
+  }
+}
+
+void ThreadPool::wait() {
+  // Wait for all threads to complete and the queue to be empty
+  std::unique_lock<std::mutex> LockGuard(CompletionLock);
+  // The order of the checks for ActiveThreads and Tasks.empty() matters because
+  // any active threads might be modifying the Tasks queue, and this would be a
+  // race.
+  CompletionCondition.wait(LockGuard,
+                           [&] { return !ActiveThreads && Tasks.empty(); });
+}
+
+std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
+  /// Wrap the Task in a packaged_task to return a future object.
+  PackagedTaskTy PackagedTask(std::move(Task));
+  auto Future = PackagedTask.get_future();
+  {
+    // Lock the queue and push the new task
+    std::unique_lock<std::mutex> LockGuard(QueueLock);
+
+    // Don't allow enqueueing after disabling the pool
+    assert(EnableFlag && "Queuing a thread during ThreadPool destruction");
+
+    Tasks.push(std::move(PackagedTask));
+  }
+  QueueCondition.notify_one();
+  return Future.share();
+}
+
+// The destructor joins all threads, waiting for completion.
+ThreadPool::~ThreadPool() {
+  {
+    std::unique_lock<std::mutex> LockGuard(QueueLock);
+    EnableFlag = false;
+  }
+  QueueCondition.notify_all();
+  for (auto &Worker : Threads)
+    Worker.join();
+}
+
+#else // LLVM_ENABLE_THREADS Disabled
+
+#if 0
+
+ThreadPool::ThreadPool() : ThreadPool(0) {}
+
+// No threads are launched, issue a warning if ThreadCount is not 0
+ThreadPool::ThreadPool(unsigned ThreadCount)
+    : ActiveThreads(0) {
+  if (ThreadCount) {
+    errs() << "Warning: request a ThreadPool with " << ThreadCount
+           << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
+  }
+}
+
+void ThreadPool::wait() {
+  // Sequential implementation running the tasks
+  while (!Tasks.empty()) {
+    auto Task = std::move(Tasks.front());
+    Tasks.pop();
+    Task();
+  }
+}
+
+std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
+  // Get a Future with launch::deferred execution using std::async
+  auto Future = std::async(std::launch::deferred, std::move(Task)).share();
+  // Wrap the future so that both ThreadPool::wait() can operate and the
+  // returned future can be sync'ed on.
+  PackagedTaskTy PackagedTask([Future]() { Future.get(); });
+  Tasks.push(std::move(PackagedTask));
+  return Future;
+}
+
+ThreadPool::~ThreadPool() {
+  wait();
+}
+
+#endif
+
+#endif