init.el

(defun comment-or-uncomment-region-or-line (&optional lines)
  "If the line or region is not a comment, comments region
if mark is active, line otherwise. If the line or region
is a comment, uncomment."
  (interactive "P")
  (if mark-active
      (if (< (mark) (point))
          (comment-or-uncomment-region (mark) (point))
	(comment-or-uncomment-region (point) (mark)))
    (comment-or-uncomment-line lines)))
(global-set-key "\M-\\" 'comment-or-uncomment-region-or-line)