In src/os/windows/vm/os_windows.cpp, the code for os::allocate_thread_local_storage() is

  int os::allocate_thread_local_storage() {
    return TlsAlloc();
  }

It would be better to assert if Windows runs out of TLS slots, vis.

  int os::allocate_thread_local_storage() {
    int key = TlsAlloc();
    assert(key != TLS_OUT_OF_INDEXES, "cannot allocate thread local storage");
    return key;
  }