GNU bug report logs - #50268
28.0.50; Assertion warning during native compilation

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Mattias Engdegård <mattiase <at> acm.org>
Subject: bug#50720: closed (Re: bug#50268: 28.0.50; Assertion warning
 during native compilation)
Date: Sat, 25 Sep 2021 18:38:02 +0000

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Your bug report

#50268: unnamed &rest broken

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 50720 <at> debbugs.gnu.org.

-- 
50268: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=50268
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Mattias Engdegård <mattiase <at> acm.org>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: Lars Ingebrigtsen <larsi <at> gnus.org>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>, 50268-done <at> debbugs.gnu.org,
 Noam Postavsky <npostavs <at> gmail.com>
Subject: Re: bug#50268: 28.0.50; Assertion warning during native compilation
Date: Sat, 25 Sep 2021 20:37:11 +0200

24 sep. 2021 kl. 08.07 skrev Eli Zaretskii <eliz <at> gnu.org>:

> Let's do it now.

Thank you, pushed to master. Closing.

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From: Mattias Engdegård <mattiase <at> acm.org>
To: bug-gnu-emacs <at> gnu.org
Subject: unnamed &rest broken
Date: Tue, 21 Sep 2021 13:15:54 +0200

The interpreter and compiler allow &rest to be used without a variable name following but the generated byte code is completely broken:

(funcall (byte-compile (lambda (&rest) 'ta)))

crashes, and

(defun boo (a &rest)
  (if a a (list 1 2 3 4)))

(boo 'hiss)
=> hiss        ; interpreted
=> (1 2 3 4)   ; compiled

The reason is that the compiler generates code from the argument variable list but the byte-code interpreter will only look at the signature code which was generated from the actual signature:

(byte-compile (lambda (&rest) 'ta))
=> #[128 "\300\207" [ta] 1 "..."]

The 128 indicates zero positional parameters and a &rest argument, and the 1 is the maximum stack size required which is wrong; 2 stack slots are needed and that's what we get if naming the argument:

(byte-compile (lambda (&rest _r) 'ta))
=> #[128 "\300\207" [ta] 2 "..."]

In the `boo` case above, it is clear that the compiler doesn't expect any &rest param to have been pushed at all so the stack offsets are wrong.

Now, either we fix this bug or we stop pretending that unnamed &rest arguments work at all and signal an error, because it's clear from the above that they can't have seen much use.

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