bug#54478: unwanted source references showing up w/ -O0 (with patch)

[Matt Wette]

In guile-3.0.8, when I compile w/ optimiation-level 0, I am seeing references
to the beginning let-form.   The following illustrates the issue.  Consider

foo.scm:
(define *a* (make-parameter 0))
(define (foo . args)
    (let* ((b (+ (*a*) 1))
           (c (+ (*a*) b 2)))
      (simple-format #t "foo a= ~S\n" (*a*))
      (simple-format #t "foo b= ~S\n" b)
      (simple-format #t "foo c= ~S\n" c)))

and p2.scm:
(use-modules (system base compile))
(use-modules (system vm program))
(use-modules (ice-9 pretty-print))

(compile-and-load "foo.scm" #:optimization-level 0)
(define sources (program-sources-pre-retire foo))
(pretty-print sources)

With guile-3.0.8, I see the followiing output.
Note the references back to line 2.((0 "foo.scm" 1 . 0)
 (20 "foo.scm" 2 . 4)
 (20 "foo.scm" 2 . 15)
 (76 "foo.scm" 2 . 18)
 (132 "foo.scm" 2 . 17)
 (152 "foo.scm" 2 . 23)
 (156 "foo.scm" 2 . 14)
 (176 "foo.scm" 2 . 4)
 (176 "foo.scm" 3 . 15)
 (232 "foo.scm" 3 . 18)
 (288 "foo.scm" 3 . 17)
 (308 "foo.scm" 3 . 23)
 (312 "foo.scm" 3 . 25)
 (316 "foo.scm" 3 . 14)
 (336 "foo.scm" 2 . 4)
 (336 "foo.scm" 4 . 7)
 (392 "foo.scm" 4 . 21)
 (396 "foo.scm" 4 . 24)
 (404 "foo.scm" 4 . 39)
 (460 "foo.scm" 4 . 38)
 (480 "foo.scm" 4 . 6)
 (496 "foo.scm" 2 . 4)
 (496 "foo.scm" 5 . 7)
 (552 "foo.scm" 5 . 21)
 (556 "foo.scm" 5 . 24)
 (564 "foo.scm" 5 . 38)
 (568 "foo.scm" 5 . 6)
 (584 "foo.scm" 6 . 7)
 (640 "foo.scm" 6 . 21)
 (644 "foo.scm" 6 . 24)
 (652 "foo.scm" 6 . 38)
 (656 "foo.scm" 6 . 6))

With the patch shown below, this is what I get:
((0 "foo.scm" 1 . 0)
 (20 "foo.scm" 2 . 4)
 (20 "foo.scm" 2 . 15)
 (76 "foo.scm" 2 . 18)
 (132 "foo.scm" 2 . 17)
 (152 "foo.scm" 2 . 23)
 (156 "foo.scm" 2 . 14)
 (176 "foo.scm" 2 . 4)
 (176 "foo.scm" 3 . 15)
 (232 "foo.scm" 3 . 18)
 (288 "foo.scm" 3 . 17)
 (308 "foo.scm" 3 . 23)
 (312 "foo.scm" 3 . 25)
 (316 "foo.scm" 3 . 14)
 (336 "foo.scm" 4 . 7)
 (392 "foo.scm" 4 . 21)
 (396 "foo.scm" 4 . 24)
 (404 "foo.scm" 4 . 39)
 (460 "foo.scm" 4 . 38)
 (480 "foo.scm" 4 . 6)
 (496 "foo.scm" 5 . 7)
 (552 "foo.scm" 5 . 21)
 (556 "foo.scm" 5 . 24)
 (564 "foo.scm" 5 . 38)
 (568 "foo.scm" 5 . 6)
 (584 "foo.scm" 6 . 7)
 (640 "foo.scm" 6 . 21)
 (644 "foo.scm" 6 . 24)
 (652 "foo.scm" 6 . 38)
 (656 "foo.scm" 6 . 6))


Patch follows.  In a sequence, the source of the forms should be emitted,
not the parent sequence source (start).
--- guile-3.0.8-dist/module/language/tree-il/compile-bytecode.scm       
2021-09-30 05:30:38.000000000 -0700
+++ guile-3.0.8-srcfix/module/language/tree-il/compile-bytecode.scm     
2022-03-19 09:37:28.126740690 -0700
@@ -910,7 +910,7 @@
     (define (visit-seq exp env ctx)
       (match exp
         (($ <seq> src head tail)
-         (maybe-emit-source src)
+         ;;(maybe-emit-source src)
          (for-effect head env)
          (for-context tail env ctx))))