comment-padright is a compiled Lisp function in `newcomment.el'.

(comment-padright STR &optional N)

Construct a string composed of STR plus `comment-padding'.
It also adds N copies of the last non-whitespace chars of STR.
If STR already contains padding, the corresponding amount is
ignored from `comment-padding'.
N defaults to 0.
If N is `re', a regexp is returned instead, that would match
the string for any N.