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From: Andrea Corallo <acorallo@gnu.org>
To: 67883@debbugs.gnu.org
Cc: hokomo@disroot.org
Subject: bug#67883: 29.1.90; Native compiler hangs when compiling code with circular objects
Date: Tue, 19 Dec 2023 08:29:44 -0500	[thread overview]
Message-ID: <yp1r0jiz5fb.fsf@fencepost.gnu.org> (raw)
In-Reply-To: <yp1v88uz7bh.fsf@fencepost.gnu.org> (Andrea Corallo's message of "Tue, 19 Dec 2023 07:48:50 -0500")

Andrea Corallo <acorallo@gnu.org> writes:

> hokomo via "Bug reports for GNU Emacs, the Swiss army knife of text
> editors" <bug-gnu-emacs@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.





  reply	other threads:[~2023-12-19 13:29 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-12-18 17:22 bug#67883: 29.1.90; Native compiler hangs when compiling code with circular objects hokomo via Bug reports for GNU Emacs, the Swiss army knife of text editors
2023-12-19 12:48 ` Andrea Corallo
2023-12-19 13:29   ` Andrea Corallo [this message]
2023-12-24 19:41     ` Stefan Kangas
2023-12-26  8:23       ` Andrea Corallo
2023-12-26 15:36         ` Stefan Kangas
2023-12-27 18:48           ` hokomo via Bug reports for GNU Emacs, the Swiss army knife of text editors

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