static Map<Thread, Bool> _registerThread_threads;
static O _onRegisterThread; // voidfunc(Thread)

static Thread _registerThread(Thread t) {
  if (_registerThread_threads == null)
    _registerThread_threads = newWeakHashMap();
  _registerThread_threads.put(t, true);
  vm_generalWeakSubMap("thread2mc").put(t, weakRef(mc()));
  callF(_onRegisterThread, t);
  ret t;
}

static void _registerThread() {
  _registerThread(Thread.currentThread());
}