comment-region is an interactive compiled Lisp function in `newcomment.el'.

(comment-region BEG END &optional ARG)

Comment or uncomment each line in the region.
With just C-u prefix arg, uncomment each line in region BEG .. END.
Numeric prefix ARG means use ARG comment characters.
If ARG is negative, delete that many comment characters instead.

The strings used as comment starts are built from `comment-start'
and `comment-padding'; the strings used as comment ends are built
from `comment-end' and `comment-padding'.

By default, the `comment-start' markers are inserted at the
current indentation of the region, and comments are terminated on
each line (even for syntaxes in which newline does not end the
comment and blank lines do not get comments). This can be
changed with `comment-style'.