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* bug#67883: 29.1.90; Native compiler hangs when compiling code with circular objects
@ 2023-12-18 17:22 hokomo via Bug reports for GNU Emacs, the Swiss army knife of text editors
  2023-12-19 12:48 ` Andrea Corallo
  0 siblings, 1 reply; 7+ messages in thread
From: hokomo via Bug reports for GNU Emacs, the Swiss army knife of text editors @ 2023-12-18 17:22 UTC (permalink / raw)
  To: 67883

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





^ permalink raw reply	[flat|nested] 7+ messages in thread

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Thread overview: 7+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
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
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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