In the code review for:
    JDK-8288139 JavaThread touches oop after GC barrier is detached

this topic came up:
    https://github.com/openjdk/jdk19/pull/21#discussion_r898482607

Here's [~dholmes]' comment:

> Additional RFE: can we not declare this to take JavaThread and so avoid the checks and casts below?

Here's the relevant code in JDK20:

src/hotspot/share/runtime/sharedRuntime.cpp

jlong SharedRuntime::get_java_tid(Thread* thread) {
  if (thread != NULL && thread->is_Java_thread()) {
    Thread* current = Thread::current();
    guarantee(current != thread || JavaThread::cast(thread)->is_oop_safe(),
              "current cannot touch oops after its GC barrier is detached.");
    oop obj = JavaThread::cast(thread)->threadObj();
    return (obj == NULL) ? 0 : java_lang_Thread::thread_id(obj);
  }
  return 0;
}

In the current implementation, if 'thread' is NULL or not a JavaThread,
then the API returns a 0. However, if this API is only ever called on a
JavaThread, then we could change the parameter from 'Thread*' to
'JavaThread*' and do some cleanup.