Subject: 29.1.90; Native compiler hangs when compiling code with circular objects

From: hokomo <[email-masked]>
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