GNU bug report logs - #69872
29.2; Async native compilation of seq.el test uses up resources and hangs

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Andrea Corallo <acorallo <at> gnu.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#67883: closed (29.1.90; Native compiler hangs when compiling
 code with circular objects)
Date: Mon, 01 Apr 2024 20:48:02 +0000

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Your message dated Mon, 01 Apr 2024 16:46:47 -0400
with message-id <yp1frw4oloo.fsf <at> fencepost.gnu.org>
and subject line Re: bug#57957: bug#69872: 29.2; Async native compilation of seq.el test uses up resources and hangs
has caused the debbugs.gnu.org bug report #69872,
regarding 29.1.90; Native compiler hangs when compiling code with circular objects
to be marked as done.

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From: hokomo <hokomo <at> disroot.org>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.1.90; Native compiler hangs when compiling code with circular
 objects
Date: Mon, 18 Dec 2023 18:22:41 +0100

Hello,

The Emacs Lisp native compiler goes into an infinite loop when 
compiling code that contains circular objects.

For example, put the following into a file and use M-x 
emacs-lisp-native-compile; it should hang:

 (defun test1 ()
   '#1=(1 2 3 . #1#))

It seems like this only happens for certain top-level forms, such 
as `defun' (which may be expected behavior, due to the way the 
file compiler processes top-level forms). For example, this 
doesn't hang:

 (lambda ()
   '#1=(1 2 3 . #1#))

Here's a more interesting example, which is how I stumbled upon 
this issue in the first place:

 (defun cycle-pure (list)
   (declare (pure t) (side-effect-free t))
   (let ((newlist (append list ())))
     (nconc newlist newlist)))

 (defun test2 ()
   (cycle-pure '(1 2 3)))

(The definition of `cycle-pure' is copied from the `-cycle' 
function from the dash.el package.)

If `cycle-pure' is not yet defined when compiling, the compiler 
doesn't hang.

However, if `cycle-pure' is defined (e.g. via M-x eval-defun), the 
compiler hangs. This is weird because, unlike `test1' above, 
`test2' doesn't contain a circular list itself; it only builds it 
at run-time.

In contrast, this example doesn't hang, regardless of whether 
`cycle' is defined or not (note the removed declarations):

 (defun cycle (list)
   (let ((newlist (append list ())))
     (nconc newlist newlist)))

 (defun test3 ()
   (cycle '(1 2 3)))

My conjecture is that the `pure' and `side-effect-free' 
declarations within `cycle-pure' (which I assume are picked up 
only once the function definition is loaded) allow the compiler to 
do some constant folding when it sees the expression `(cycle-pure 
'(1 2 3))'. This results in the compiler manipulating a circular 
list at compile-time, just like in `test1', and leads to a hang.

Kind regards,
hokomo

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From: Andrea Corallo <acorallo <at> gnu.org>
To: Andrea Corallo <akrl <at> sdf.org>
Cc: Lars Tveito <larstvei <at> ifi.uio.no>, hokomo <hokomo <at> disroot.org>,
 69872-done <at> debbugs.gnu.org,
 Gerd Möllmann <gerd.moellmann <at> gmail.com>,
 57957 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>,
 Lars Ingebrigtsen <larsi <at> gnus.org>
Subject: Re: bug#57957: bug#69872: 29.2; Async native compilation of seq.el
 test uses up resources and hangs
Date: Mon, 01 Apr 2024 16:46:47 -0400

Andrea Corallo <acorallo <at> gnu.org> writes:

> Andrea Corallo <akrl <at> sdf.org> writes:
>
>> Stefan Monnier <monnier <at> iro.umontreal.ca> writes:
>>
>>>> The question is of course also how much effort circular lists deserve in
>>>> the compiler.  From my point of view, not too much because they aren't
>>>> very useful, in code at least.  And dealing with them costs runtime.
>>>
>>> Circular data structures are perfectly normal in source code under
>>> a `quote` but the code itself should never be cyclic (so I think it
>>> would be OK to signal an error (and/or to inf-loop) when asked to
>>> compile a chunk of code that has a cycle).  So we *should* handle
>>> circular data structures correctly.
>>
>> I agree, Saturday I should have some time to look into this.
>
> A little time after... I finally managed to get to it, sorry for the
> delay.
>
> I've pushed into master c5de73a95a6, it fixes my reprodurer here and
> adds a test for this.
>
> If anyone could confirm this is fixed i'll be (extremely) happy to close
> this :)
>
> Thanks
>
>   Andrea

Right I'm closing this as I believe it's fixed, happy to re-open if it's
not the case.

Thanks!

  Andrea

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