_ncGuards.put(ref, guard);
_ncGuards.put(ref, guard.copyRO());
  OPT_RegisterOperand makeNullCheckGuard(OPT_Register ref) {    OPT_RegisterOperand guard = (OPT_RegisterOperand)_ncGuards.get(ref);    if (guard == null) {      guard = temps.makeTempValidation();      _ncGuards.put(ref, guard);    }    return guard;  }