ad-read-advised-function is a compiled Lisp function in `advice.el'.

(ad-read-advised-function &optional PROMPT PREDICATE DEFAULT)

Read name of advised function with completion from the minibuffer.
An optional PROMPT will be used to prompt for the function. PREDICATE
plays the same role as for `try-completion' (which see). DEFAULT will
be returned on empty input (defaults to the first advised function or
function at point for which PREDICATE returns non-nil).