subr doc representation

It works this way (doc.c): doc type is (char *). If (EMACS_INT) doc >= 0, it is really a pointer. Otherwise we should use (off_t) (- (EMACS_INT) doc) for seeking in `DOC' file. It used to be this way in emacs-18.55, so it worked for all known platforms for more than 10 yrs, and my ones are no exception. But what forbids it to stop working? What requires valid pointers to be non- negative when converted to integer? Once it is not the case, doc implementation has to be rewritten. Most surprising is why design its own representation rules for doc? We just have a way to store both integer and pointer in a single word distinguishably: Lisp_Object. Why not reuse it for Lisp_Subr.doc?