I found the answer myself. V6 itself has a nifty cref binary and its
source! It is quite impressive how things were neat at that time.
In message "[TUHS] cross reference tool for pre-K&R C"
on 03/04/13, nao <nao(a)tom-yam.or.jp> writes:
I really enjoy hacking ancient unix rather seriously
and want a cross
reference tools for them. Since some of them are written in pre-K&R C,
modern tools are not very useful. Is there any cross reference tool
for pre-K&R C source codes?