completion-pcm--find-all-completions is a compiled Lisp function in `minibuffer.el'.

(completion-pcm--find-all-completions STRING TABLE PRED POINT &optional FILTER)

Find all completions for STRING at POINT in TABLE, satisfying PRED.
POINT is a position inside STRING.
FILTER is a function applied to the return value, that can be used, e.g. to
filter out additional entries (because TABLE might not obey PRED).