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 07:48:50 -0500 [thread overview]
Message-ID: <yp1v88uz7bh.fsf@fencepost.gnu.org> (raw)
In-Reply-To: <87a5q7bc2f.fsf@disroot.org> (hokomo via's message of "Mon, 18 Dec 2023 18:22:41 +0100")
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
next prev parent reply other threads:[~2023-12-19 12:48 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 [this message]
2023-12-19 13:29 ` Andrea Corallo
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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