Subject: Re: bug#21899: let/ec continuations not distinct under compiler

From: Andy Wingo <[email-masked]>
Date: Fri, 24 Jun 2016 18:48:26 +0200

On Fri 24 Jun 2016 18:29, Andy Wingo <[email-masked]> writes:

> On Fri 13 Nov 2015 08:57, Zefram <[email-masked]> writes:
>
>> With guile 2.0.11:
>>
>> scheme@(guile-user)> (use-modules (ice-9 control))
>> scheme@(guile-user)> (list 'a (let/ec ae (list 'b (let/ec be (be 2)))))
>> $1 = (a (b 2))
>> scheme@(guile-user)> (list 'a (let/ec ae (list 'b (let/ec be (ae 2)))))
>> $2 = (a (b 2))
>> scheme@(guile-user)> (list 'a (let/ec ae (list 'b (ae 2))))
>> $3 = (a 2)
>>
>> The middle of these three cases is wrong: it attempts to invoke the outer
>> escape continuation, but only goes as far as the target of the inner one,
>> which it isn't using.  It therefore produces the same result as the first
>> case, which invokes the inner escape continuation.  It ought to behave
>> like the third case, which shows that the outer escape continuation can
>> be successfully invoked when the unused inner continuation is not present.
>
> This is a compiler bug in 2.0:
>
> scheme@(guile-user)> ,optimize (list 'a (let/ec ae (list 'b (let/ec be (ae 2)))))
> $2 = (list 'a
>       (let ((tag (list 'let/ec)))
>         (call-with-prompt
>           tag
>           (lambda ()
>             (list 'b
>                   (let ((tag-1 tag))  ;; <<<< here is the bug
>                     (call-with-prompt
>                       tag-1
>                       (lambda () (abort-to-prompt tag 2))
>                       (lambda (_ . results) (@apply values results))))))
>           (lambda (_ . results) (@apply values results)))))

Narrowed down:

  ,opt (let* ((x (list 'a))
              (y (list 'a)))
         (list x y))
  ;; ->
  (let* ((x (list 'a)) (y x)) (list x y))

It's a bug in 2.0's CSE pass somehow.

Andy