GNU bug report logs - #42597
27.1; (+ -0.0) returns +0.0 when compiled

Previous Next

Full log

View this message in rfc822 format

From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Mattias Engdegård <mattiase <at> acm.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#42597: closed (27.1; (+ -0.0) returns +0.0 when compiled)
Date: Fri, 07 Aug 2020 08:53:02 +0000

[Message part 1 (text/plain, inline)]

Your message dated Fri, 7 Aug 2020 10:50:19 +0200
with message-id <8E812D2B-DAFD-433F-8A77-44946CD51F4F <at> acm.org>
and subject line Re: bug#42597: 27.1; (+ -0.0) returns +0.0 when compiled
has caused the debbugs.gnu.org bug report #42597,
regarding 27.1; (+ -0.0) returns +0.0 when compiled
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


-- 
42597: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=42597
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: Mattias Engdegård <mattiase <at> acm.org>
To: bug-gnu-emacs <at> gnu.org
Subject: 27.1; (+ -0.0) returns +0.0 when compiled
Date: Wed, 29 Jul 2020 14:07:32 +0200

Unary +, *, min and max, all of which should be identity for numbers, convert -0.0 to +0.0 when byte-compiled:

(defun f (x) (+ x))
(f -0.0)
=> -0.0
(byte-compile 'f)
(f -0.0)
=> 0.0

The reason is that byte-compile-associative transforms (+ x), (* x), (min x) and (max x) into (+ x 0).

No patch yet (sorry!) but I'm not sure what would be the best way to go about it. Some possibilities:

A. Use a full 1-argument call, like (+ x). This is more expensive (about 1.8×) since the general function call mechanism has to be used.
B. Use (* x 1) instead; this appears to work. This is also more expensive (1.6×); not sure why.
C. Add a new byte-op. Fast but probably overkill.

Better suggestions welcome!

[Message part 3 (message/rfc822, inline)]

From: Mattias Engdegård <mattiase <at> acm.org>
To: Alan Mackenzie <acm <at> muc.de>
Cc: 42597-done <at> debbugs.co.uk
Subject: Re: bug#42597: 27.1; (+ -0.0) returns +0.0 when compiled
Date: Fri, 7 Aug 2020 10:50:19 +0200

No immediate objections at least; patches pushed to master.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.