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

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

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Your bug report

#42597: 27.1; (+ -0.0) returns +0.0 when compiled

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 42597 <at> debbugs.gnu.org.

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

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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.

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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!

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