More on Con.

Author: dvanhorn

www/notes/con.scrbl

@@ -164,10 +164,66 @@ if-expressions.
 
 Suppose we want to compile @racket['(if (zero? 8) 2 3)]...
 
-We already know how to compile the @racket['8], @racket['2], and
-@racket['3] part.
+We already know how to compile the @racket[8], @racket[2], and
+@racket[3] part.
 
-What needs to happen? ...
+What needs to happen?
+
+@itemlist[
+@item{Execute the code for @racket[8] leaving the result in @racket['rax],}
+@item{check whether @racket['rax] holds zero,}
+@item{if it does, execute the code for @racket[2],}
+@item{if it doesn't, execute the code for @racket[3].}
+]
+
+We can determine whether @racket[8] evaluates to @racket[0] using a
+comparison instruction: @racket['(cmp rax 0)].  To do the conditional
+execution, we will need to jump to different parts of the code to
+either execute the code for @racket[2] or @racket[3].  There are
+several ways we could accomplish this, but we take the following
+approach: immediately after the comparison, do a conditional jump to
+the code for the else branch when non-zero. Should the jump not occur,
+the next instructions will carry out the evaluation of the then
+branch, then (unconditionally) jump over the else branch code.
+
+To accomplish this, we will need two new labels: one for the else
+branch code and one for the end of the else branch code.  The
+@racket[gensym] function can be used to generate symbols that have not
+appeared before.
+
+In total, the code for this example would look like:
+
+@racketblock[
+'((mov rax 8)
+  (cmp rax 0)
+  (jne if-else-begin)
+  (mov rax 2)
+  (jmp if-else-end)
+  if-else-begin
+  (mov rax 3)
+  if-else-end)
+]
+
+Notice that the @racket['(mov rax 8)], @racket['(mov rax 3)] and
+@racket['(mov rax 2)] parts are just the instructions generated by
+compiling @racket[8], @racket[2] and @racket[3].  Generalizing from
+this, we arrive at the following code for the compiler:
+
+@racketblock[
+(let ((c0 (compile-e e0))
+      (c1 (compile-e e1))
+      (c2 (compile-e e2))
+      (l0 (gensym "if-else-begin"))
+      (l1 (gensym "if-else-end")))
+  '(,@c0
+    (cmp rax 0)
+    (jne ,l0)
+    ,@c1
+    (jmp ,l1)
+    ,l0
+    ,@c2
+    ,l1))
+]
 
 
 @ex[

www/notes/con/compile.rkt

@@ -22,27 +22,14 @@
     [`(if (zero? ,e0) ,e1 ,e2)
      (let ((c0 (compile-e e0))
            (c1 (compile-e e1))
-           (c2 (compile-e e2)))
-       (match (gen-if-labels)
-         [(list if-f if-x)
-          `(,@c0
-            (cmp rax 0)
-            (jne ,if-f)
-            ,@c1
-            (jmp ,if-x)
-            ,if-f
-            ,@c2
-            ,if-x)]))]))
-
-;; -> [List Label Label]
-;; Guaranteed to be unique on each call
-(define gen-if-labels
-  (let ((i 0))
-    (λ ()
-      (set! i (add1 i))
-      (list (lab "f" i)
-            (lab "x" i)))))
-
-;; String Integer -> Symbol
-(define (lab s i)
-  (string->symbol (string-append "if_" s "_" (number->string i))))
+           (c2 (compile-e e2))
+           (l0 (gensym "if"))
+           (l1 (gensym "if")))
+       `(,@c0
+         (cmp rax 0)
+         (jne ,l0)
+         ,@c1
+         (jmp ,l1)
+         ,l0
+         ,@c2
+         ,l1))]))