GNU bug report logs - #67883
29.1.90; Native compiler hangs when compiling code with circular objects

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Message #14 received at 67883 <at> debbugs.gnu.org (full text, mbox):

From: Andrea Corallo <acorallo <at> gnu.org>
To: 67883 <at> debbugs.gnu.org
Cc: hokomo <at> disroot.org
Subject: Re: bug#67883: 29.1.90; Native compiler hangs when compiling code
 with circular objects
Date: Tue, 19 Dec 2023 08:29:44 -0500

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

> hokomo via "Bug reports for GNU Emacs, the Swiss army knife of text
> editors" <bug-gnu-emacs <at> gnu.org> writes:
>
>> 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
>
> I'm pretty sure I've investigated this in the past.  IIRC this is due
> some cl- function hanging on a circular list.  Can't find the bug number
> ATM.
>
>   Andrea

Okay I believe this most likely is a duplicate of bug#57957.

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