Here's something I did for myself a long time ago: take some of the fundamental code like alloc.c, alloc_re.c, fmt_str.c and str_*.c, and start commenting it yourself. I found that it was so expressive and concise that after a short time the code is very clear and the comments get in the way.

The only real noise is that no-one uses SunOS cc anymore and compilers will optimize loops (expanding in place, or doing other magic optimizations when appropriate) so loop-unrolling doesn't apparently buy an increase in performance anymore.