Create threadpool for src/user and primarily compiler usage.

PiperOrigin-RevId: 864328236
Change-Id: Iffaa9b8e6eacdb038020af9d6ad82d7cb7a6c941

src/user/user_threadpool.h

@@ -0,0 +1,85 @@
+// Copyright 2026 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_USER_USER_THREADPOOL_H_
+#define MUJOCO_SRC_USER_USER_THREADPOOL_H_
+
+#include <condition_variable>
+#include <cstdint>
+#include <functional>
+#include <mutex>
+#include <queue>
+#include <thread>
+#include <vector>
+
+
+namespace mujoco::user {
+
+// ThreadPool class
+class ThreadPool {
+ public:
+  // constructor
+  explicit ThreadPool(int num_threads);
+
+  // destructor
+  ~ThreadPool();
+
+  int NumThreads() const { return threads_.size(); }
+
+  // returns an ID between 0 and NumThreads() - 1. must be called within
+  // worker thread (returns -1 if not).
+  static int WorkerId() { return worker_id_; }
+
+  // ----- methods ----- //
+  // set task for threadpool
+  void Schedule(std::function<void()> task);
+
+  // return number of tasks completed
+  std::uint64_t GetCount() {
+    std::lock_guard<std::mutex> lock(m_);
+    return ctr_;
+  }
+
+  // reset count to zero
+  void ResetCount() {
+    std::lock_guard<std::mutex> lock(m_);
+    ctr_ = 0;
+  }
+
+  // wait for count, then return
+  void WaitCount(int value) {
+    std::unique_lock<std::mutex> lock(m_);
+    cv_ext_.wait(lock, [&]() { return ctr_ >= value; });
+  }
+
+ private:
+  // ----- methods ----- //
+
+  // execute task with available thread
+  void WorkerThread(int i);
+
+  constinit static thread_local int worker_id_;
+
+  // ----- members ----- //
+  std::vector<std::thread> threads_;
+  std::mutex m_;
+  std::condition_variable cv_in_;
+  std::condition_variable cv_ext_;
+  std::queue<std::function<void()>> queue_;
+  std::uint64_t ctr_;
+};
+
+}  // namespace mujoco::user
+
+#endif  // MUJOCO_SRC_USER_USER_THREADPOOL_H_