byte-opt.el is one of those files Jamie Zawinski laid his golden hands on. It seems that back in the days, there wasn’t much of a concern about Emacs Lisp execution speed until he got annoyed enough to bolt on an optimizer. Its sources start with a wonderful quote:
“No matter how hard you try, you can’t make a racehorse out of a pig. You can, however, make a faster pig.”
I recommend reading it to get an idea what compiler jargon like “peephole optimizer” could possibly mean. During my last study, I found this curious piece of code:
(defun byte-optimize-approx-equal (x y) (<= (* (abs (- x y)) 100) (abs (+ x y))))
So, according to this 99 and 100 are equal. Awesome!