bug#46906: 28.0.50; byte compiler infloops trying to compile infloop

[Pip Cet <pipcet@gmail.com>]

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On Wed, Mar 3, 2021 at 11:34 PM Andy Moreton <andrewjmoreton@gmail.com> wrote:
> On Wed 03 Mar 2021, Pip Cet wrote:
> > On Wed, Mar 3, 2021 at 9:15 PM Pip Cet <pipcet@gmail.com> wrote:
> >>
> >> Recipe in emacs -Q:
> >>
> >> In *scratch*, evaluate:
> >>
> >> (byte-compile (lambda (x)
> >>                 (if x (f))
> >>                 (while t)))
> >>
> >> Expected result:
> >>
> >> A (fairly useless) bytecode object
> >>
> >> Actual result:
> >>
> >> A spinning hourglass.
> >
> > Seems to be a fairly obvious bug in byte-opt.el, patch attached.
>
> Perhaps add your reproducer as a test case ?

Absolutely, thanks for the suggestion. See attached patch.

Should there, maybe, be a tag for tests that are known to loop forever
when they fail? It feels weird to write a test without a single should
in it...

Pip

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From 2f430107f46b506dbb8dd79d9ce5a10cc2b3de9b Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@gmail.com>
Date: Wed, 3 Mar 2021 21:37:13 +0000
Subject: [PATCH] Fix bytecompiler infloop compiling infloops (bug#46906)

* lisp/emacs-lisp/byte-opt.el (byte-optimize-lapcode): Don't apply
optimization if we can't change anything.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-infloop): New test.
---
 lisp/emacs-lisp/byte-opt.el            | 6 +++---
 test/lisp/emacs-lisp/bytecomp-tests.el | 5 +++++
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/lisp/emacs-lisp/byte-opt.el b/lisp/emacs-lisp/byte-opt.el
index b51ba801552d6..aedfde6c0c6ab 100644
--- a/lisp/emacs-lisp/byte-opt.el
+++ b/lisp/emacs-lisp/byte-opt.el
@@ -2012,9 +2012,9 @@ byte-optimize-lapcode
 	 ((and (memq (car lap0) byte-goto-ops)
 	       (memq (car (setq tmp (nth 1 (memq (cdr lap0) lap))))
 		     '(byte-goto byte-return)))
-	  (cond ((and (not (eq tmp lap0))
-		      (or (eq (car lap0) 'byte-goto)
-			  (eq (car tmp) 'byte-goto)))
+	  (cond ((and (or (eq (car lap0) 'byte-goto)
+			  (eq (car tmp) 'byte-goto))
+                      (not (eq (cdr tmp) (cdr lap0))))
 		 (byte-compile-log-lap "  %s [%s]\t-->\t%s"
 				       (car lap0) tmp tmp)
 		 (if (eq (car tmp) 'byte-return)
diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index 03c267ccd0fef..eeb75c53ee2a9 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -1222,6 +1222,11 @@ bytecomp-reify-function
         (byte-compile 'counter)
         (should (equal (counter) 1))))))
 
+(ert-deftest bytecomp-infloop ()
+  "Check that optimizing an infinite loop does not loop indefinitely."
+  ;; This form should not cause the optimizer to loop (bug#46906)
+  (byte-compile (lambda (x) (if (eq x x) (f)) (while t))))
+
 ;; Local Variables:
 ;; no-byte-compile: t
 ;; End:
-- 
2.30.1