Create threadpool for src/user and primarily compiler usage.

PiperOrigin-RevId: 864328236
Change-Id: Iffaa9b8e6eacdb038020af9d6ad82d7cb7a6c941

src/user/user_threadpool.cc

@@ -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.
+
+#include "user/user_threadpool.h"
+
+#include <condition_variable>
+#include <functional>
+#include <mutex>
+#include <thread>
+#include <utility>
+
+namespace mujoco::user {
+
+constinit thread_local int ThreadPool::worker_id_ = -1;
+
+// ThreadPool constructor
+ThreadPool::ThreadPool(int num_threads) : ctr_(0) {
+  for (int i = 0; i < num_threads; i++) {
+    threads_.push_back(std::thread(&ThreadPool::WorkerThread, this, i));
+  }
+}
+
+// ThreadPool destructor
+ThreadPool::~ThreadPool() {
+  {
+    std::unique_lock<std::mutex> lock(m_);
+    for (int i = 0; i < threads_.size(); i++) {
+      queue_.push(nullptr);
+    }
+    cv_in_.notify_all();
+  }
+  for (auto& thread : threads_) {
+    thread.join();
+  }
+}
+
+// ThreadPool scheduler
+void ThreadPool::Schedule(std::function<void()> task) {
+  std::unique_lock<std::mutex> lock(m_);
+  queue_.push(std::move(task));
+  cv_in_.notify_one();
+}
+
+// ThreadPool worker
+void ThreadPool::WorkerThread(int i) {
+  worker_id_ = i;
+  while (true) {
+    auto task = [&]() {
+      std::unique_lock<std::mutex> lock(m_);
+      cv_in_.wait(lock, [&]() { return !queue_.empty(); });
+      std::function<void()> task = std::move(queue_.front());
+      queue_.pop();
+      cv_in_.notify_one();
+      return task;
+    }();
+    if (task == nullptr) {
+      {
+        std::unique_lock<std::mutex> lock(m_);
+        ++ctr_;
+        cv_ext_.notify_one();
+      }
+      break;
+    }
+    task();
+
+    {
+      std::unique_lock<std::mutex> lock(m_);
+      ++ctr_;
+      cv_ext_.notify_one();
+    }
+  }
+}
+
+}  // namespace mujoco::user