This optimization is not terribly important, but it illustrates the subtlety of interprocedural optimizations for imperative languages.
Single assignment elimination attempts to convert local variables
that are assigned once at the head of their scope into
(lambda (... I1 ... In ...) (begin D ...) (begin (set! I1 E1) ... (set! In En) E ...))
(lambda (... IGNORED ... IGNORED ...) (let* ((I1 E1) ... (In En)) (begin D ...) (begin E ...)))
provided for each k:
Ikdoes not occur in
contain no procedure calls or
Ikis assigned only once.
The third condition is probably not necessary.
The second condition is necessary because, for example,
(define f (lambda (x) (set! x (g)) (lambda () x)))
is not equivalent to
(define f (lambda (IGNORED) ((lambda (x) (lambda () x)) (g))))
in all Scheme contexts.
In particular, if
g is defined by
(define g (let ((n 0) (get-x (lambda () 0)) (set-x (lambda (y) -1))) (lambda () (case n ((0) (set! n 1) (let ((h (f 0))) (if (= n 2) (set! get-x h)) (write (get-x)) (newline) (set! n 3) (g))) ((1) (set! n 2) (call-with-current-continuation (lambda (k) (set! set-x k) 3))) ((3) (set-x (+ 1 (get-x))))))))
then the first definition of
(g) to print all the integers beginning with 3,
but the second definition of
(g) to print 3 over and over again.
Single assignment elimination is followed by assignment elimination.