Closes #138.

Author: dvanhorn

langs/a86/ast.rkt

@@ -318,6 +318,7 @@
     (check-unique-label-decls p)
     (check-label-targets-declared p)
     (check-has-initial-label p)
+    (check-initial-label-global p)
     ;; anything else?
     p))
 
@@ -383,3 +384,11 @@
 (define (check-has-initial-label asm)
   (unless (findf Label? asm)
     (error 'prog "no initial label found")))
+
+;; Asm -> Void
+(define (check-initial-label-global asm)
+  (match (findf Label? asm)
+    [(Label init)
+     (unless (member init (map (lambda (i) (match i [(Global l) l]))
+                               (filter Global? asm)))
+       (error 'prog "initial label undeclared as global: ~v" init))]))

langs/a86/test/errors.rkt

@@ -20,3 +20,12 @@
 (check-not-exn       (thunk (Add 'rax (sub1 (expt 2 32)))))
 (check-exn exn:fail? (thunk (Sub 'rax (expt 2 32))))
 (check-not-exn       (thunk (Sub 'rax (sub1 (expt 2 32)))))
+
+;; Check prog
+(check-exn exn:fail? (thunk (prog (Ret))))
+(check-exn exn:fail? (thunk (prog (Label 'start) (Ret))))
+(check-exn exn:fail? (thunk (prog (Global 'foo) (Label 'start) (Label 'foo) (Ret))))
+(check-not-exn       (thunk (prog (Global 'start) (Label 'start) (Ret))))
+(check-not-exn       (thunk (prog (Label 'start) (Ret) (Global 'start))))
+
+