fib.scm

(define (fib n)
  (if (< n 2)
    n
    (+ (fib (- n 1)) (fib (- n 2)))))

;; becomes

(controller
    (assign continue (label fib-done))
  fib-loop
    (test (op <) (reg n) (const 2))
    (branch (label immediate-answer))
    ;; set up to compute fib(n-1)
    (save continue)
    (assign continue (label (after-fib-n-1)))
    (save n)                                  ; save old n
    (assign n (op -) (reg n) (const 1))       ; clobber n to n - 1
    (goto (label fib-loop))                   ; perform recursive call
    after-fib-n-1                             ; on return, val contains fib(n-1)
    (restore n)
    (restore continue)
    (assign n (op -) (reg n) (const 2))
    (save continue)
    (assign continue (label after-fib-n-2))
    (save val)                                ; save fib(n-1)
    (goto (label fib-loop))
    after-fib-n-2                             ; on return val contains fib(n-2)
    (assign n (reg val))                      ; n has fib(n-2)
    (restore val)                             ; val now fib(n-1)
    (restore continue)
    (assign val (op +) (reg val) (reg n))     ; fib(n-1) + fib(n-2)
    (goto (reg continue))                     ; return to caller, answer in val
    immediate-answer
    (assign val (reg n))
    (goto (reg continue))
  fib-done)