Step 6: Conditional Expressions

1 files changed, 49 insertions, 3 deletions

diff --git a/compiler.scm b/compiler.scm
index b373ecc..4305f2c 100644
--- a/compiler.scm
+++ b/compiler.scm
@@ -56,6 +56,10 @@
                = < <= > >=
                eq? char=?))))
 
+(define (if? x)
+  (and (pair? x)
+       (eq? (first x) 'if)))
+
 (define (primcall-op x)
   (first x))
 
@@ -78,6 +82,22 @@
 (define (body x)
   (third x))
 
+(define (test x)
+  (second x))
+
+(define (consequent x)
+  (third x))
+
+(define (alternate x)
+  (fourth x))
+
+(define unique-counter (make-parameter 0))
+
+(define (unique-label)
+  (let ((n (unique-counter)))
+    (unique-counter (+ n 1))
+    (format #f "L~a" n)))
+
 (define (register name)
   (format #f "%~a" name))
 
@@ -138,6 +158,15 @@
 (define (emit-sete dest)
   (emit "sete ~a" dest))
 
+(define (emit-je label)
+  (emit "je ~a" label))
+
+(define (emit-jmp label)
+  (emit "jmp ~a" label))
+
+(define (emit-label label)
+  (format #t "~a:\n" label))
+
 (define (emit-tag-check x mask tag si env)
   (emit-expr x si env)
   (emit-and (immediate mask) rax)
@@ -246,6 +275,18 @@
                 (extend-env (lhs b) si new-env)
                 (- si wordsize))))))
 
+(define (emit-if test conseq altern si env)
+  (let ((L0 (unique-label))
+        (L1 (unique-label)))
+    (emit-expr test si env)
+    (emit-cmp (immediate (immediate-rep #f)) rax)
+    (emit-je L0)
+    (emit-expr conseq si env)
+    (emit-jmp L1)
+    (emit-label L0)
+    (emit-expr altern si env)
+    (emit-label L1)))
+
 (define (emit-expr x si env)
   (cond
    ((immediate? x)
@@ -256,12 +297,15 @@
     (emit-let (bindings x) (body x) si env))
    ((primcall? x)
     (emit-primitive-call x si env))
+   ((if? x)
+    (emit-if (test x) (consequent x) (alternate x) si env))
    (else
     (error "unknown expression" x))))
 
 (define (compile-program x)
-  (emit-expr x (- wordsize) '())
-  (emit "ret"))
+  (parameterize ((unique-counter 0))
+    (emit-expr x (- wordsize) '())
+    (emit "ret")))
 
 (define (compile-and-run x)
   (with-output-to-file "scheme_entry.S"
@@ -337,4 +381,6 @@ scheme_entry:
   (test-case '(char=? #\a #\b) "#f")
   (test-case '(char=? #\b #\b) "#t")
   (test-case '(let ((x 1)) x) "1")
-  (test-case '(let ((x 1) (y 2)) (+ x y)) "3"))
+  (test-case '(let ((x 1) (y 2)) (+ x y)) "3")
+  (test-case '(if #t 1 2) "1")
+  (test-case '(if #f 1 2) "2"))