GNU bug report logs - #5916
XFLOAT_DATA and signed zeroes

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Vincent Lefevre <vincent <at> vinc17.net>
Subject: bug#5916: closed (Re: bug#5916: XFLOAT_DATA and signed zeroes)
Date: Sun, 16 May 2010 22:36:03 +0000

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Your bug report

#5916: XFLOAT_DATA and signed zeroes

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 5916 <at> debbugs.gnu.org.

-- 
5916: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=5916
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Contact help-debbugs <at> gnu.org with problems

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From: Ken Raeburn <raeburn <at> raeburn.org>
To: 5916-done <at> debbugs.gnu.org
Subject: Re: bug#5916: XFLOAT_DATA and signed zeroes
Date: Sun, 16 May 2010 16:12:31 -0400

Should be fixed on trunk by yesterday's checkin:

>> revno: 100306 [merge]
>> committer: Ken Raeburn <raeburn <at> raeburn.org>
>> branch nick: trunk
>> timestamp: Sat 2010-05-15 17:21:30 -0400
> [...]
>>  * src/lisp.h (XFLOAT_DATA): Use "0?x:x" to generate an rvalue. (bug#5916)
>>  (LISP_MAKE_RVALUE) [!USE_LISP_UNION_TYPE && !__GNUC__]: Likewise.

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From: Vincent Lefevre <vincent <at> vinc17.net>
To: bug-gnu-emacs <at> gnu.org
Subject: XFLOAT_DATA and signed zeroes
Date: Sat, 10 Apr 2010 01:39:23 +0200

Hi,

I don't know whether there's really a bug (I haven't tested the
trunk), but someone pointed a suspicious macro[*]. The change
(on 2009-08-17) is here:

http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=6f0c16bc45815337d520a9393f8de5f9c14a06d4

It is said:

  lisp.h (XFLOAT_DATA): Produce an rvalue by adding 0 to the value.

However, adding 0 has another side effect on floats: the 0 is
converted into +0.0 (due to the addition with a double), so that
a -0.0 is transformed into a +0.0.

Could someone try the following?

  (/ 1.0 (- 0.0))

The correct result should be -1.0e+INF (negative infinity).

[*] http://groupes.codes-sources.com/article-emacs-lisp-sera-iee754-fut-0x-p0-toujours-470516.aspx

-- 
Vincent Lefèvre <vincent <at> vinc17.net> - Web: <http://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / Arénaire project (LIP, ENS-Lyon)

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