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Subject: |
29.1.90; Native compiler hangs when compiling code with circular objects |
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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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--- Begin Message ---
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Subject: |
Re: bug#57957: bug#69872: 29.2; Async native compilation of seq.el test uses up resources and hangs |
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Date: |
Mon, 01 Apr 2024 16:46:47 -0400 |
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User-agent: |
Gnus/5.13 (Gnus v5.13) |
Andrea Corallo <acorallo@gnu.org> writes:
> Andrea Corallo <akrl@sdf.org> writes:
>
>> Stefan Monnier <monnier@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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