code: def add_string_label(self, str_):
          
          if self.STRING_LABELS.get(str_, None) is None:
              self.STRING_LABELS[str_] = backend.tmp_label()
  
          return self.STRING_LABELS[str_]
docs: Maps ("folds") the given string, returning an unique label ID.
          This allows several constant labels to be initialized to the same address
          thus saving memory space.
          :param str_: the string to map
          :return: the unique label ID