Implement if 💃

?!

Like, does this actually actually work?!  It seems like it does, as in
the almost most-trivial example works, but it is pretty exciting!  Is it
turing complete yet?  (I don't think so, but... maybe?)

Yaaaay!

scm/inc/compiler.scm

@@ -5,6 +5,13 @@
  (display (apply format instr args))
  (display "\n"))
(define label-counter 0)
(define (unique-label)
  (let ((c label-counter))
    (set! label-counter (+ c 1))
    (format "label_~d" c)))
(define wordsize 4)
(define fixnum-shift 2)
@@ -48,6 +55,9 @@
(define (let? x)
  (and (list? x) (eq? (car x) 'let) (list? (cadr x)) (list? (caadr x))))
(define (if? x)
  (and (list? x) (eq? (car x) 'if)))
(define (emit-expr x si env)
  (cond
    ((immediate? x)
@@ -56,6 +66,8 @@
     (emit "movl ~a(%rsp), %eax" (lookup x env)))
    ((let? x)
     (emit-let (bindings x) (body x) si env))
    ((if? x)
     (emit-if (test x) (conseq x) (altern x) si env))
    ((primcall? x) (emit-primitive-call x si env))))
(define (lookup x env)
@@ -90,6 +102,24 @@
(define (lhs binding) (car binding))
(define (rhs binding) (cadr binding))
(define (emit-if test conseq altern si env)
  (let ((L0 (unique-label)) (L1 (unique-label)))
    (emit-expr test si env)
    (emit "cmpl $~a, %eax" (immediate-rep #f))
    (emit "je ~a" L0)
    (emit-expr conseq si env)
    (emit "jmp ~a" L1)
    (emit-label L0)
    (emit-expr altern si env)
    (emit-label L1)))
(define (emit-label L)
  (display (format "~a:\n" L)))
(define (test if-expr)   (cadr if-expr))
(define (conseq if-expr) (caddr if-expr))
(define (altern if-expr) (cadddr if-expr))
(define (emit-primitive-call x si env)
  (case (primcall-op x)
    ((add1)

scm/inc/program.scm

@@ -1,3 +1,3 @@
(load "compiler.scm")
(compile-program '(let ((x 3) (y 4)) (+ x y)))
(compile-program '(let ((x 3) (y 4)) (if (integer? (+ x y)) 42 #f)))