https://github.com/sbcl/sbcl/tree/master/src/compilerDoesn't look simple to me.
This one is in one file, but more than half a megabyte of code spanning close to 12000 lines:
https://github.com/clisp-lang/clisp/blob/master/src/compiler...
The Emacs Lisp byte compiler also doesn't look like a good small example for neophytes:
http://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/emacs-l...