(let ((foo 'undefined))
  (setq foo
        (lambda (n)
          (declare (optimize (space 3)))
          (if (zerop n)
              :end
              (funcall foo (1- n)))))
  (funcall foo 100000000))


(labels ((foo (n)
           (declare (optimize (space 3)))
           (if (zerop n)
               :end
               (foo (1- n)))))
  (foo 100000000))