GNU bug report logs - #21901
bit shift wrong on maximal right shift

Previous Next

Full log

View this message in rfc822 format

From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Zefram <zefram <at> fysh.org>
Subject: bug#21901: closed (Re: bug#21901: bit shift wrong on maximal
 right shift)
Date: Sun, 14 Oct 2018 09:42:02 +0000

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

Your bug report

#21901: bit shift wrong on maximal right shift

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

The explanation is attached below, along with your original report.
If you require more details, please reply to 21901 <at> debbugs.gnu.org.

-- 
21901: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=21901
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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

From: Mark H Weaver <mhw <at> netris.org>
To: Zefram <zefram <at> fysh.org>
Cc: 21901-done <at> debbugs.gnu.org
Subject: Re: bug#21901: bit shift wrong on maximal right shift
Date: Sun, 14 Oct 2018 05:40:51 -0400

Zefram <zefram <at> fysh.org> writes:

> With Guile 2.0.11:
>
> scheme@(guile-user)> (ash 123 (ash -1 63))
> $1 = 123
>
> Correct result would of course be zero.  Problem only occurs for
> exactly this shift distance: one bit less produces the right answer.

Nice catch!

It's finally fixed in commit 1990aa916382d0afcebd5315a6d6f555949ff654 on
the stable-2.2 branch.  The fix will be in Guile 2.2.5.

Thanks for the report.

      Mark

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

From: Zefram <zefram <at> fysh.org>
To: bug-guile <at> gnu.org
Subject: bit shift wrong on maximal right shift
Date: Fri, 13 Nov 2015 10:22:03 +0000

With Guile 2.0.11:

scheme@(guile-user)> (ash 123 (ash -1 63))
$1 = 123

Correct result would of course be zero.  Problem only occurs for
exactly this shift distance: one bit less produces the right answer.
Problem also occurs on Guile 1.8.8.  Looking at the implementation,
the problem is attributable to the negation of the shift distance,
which in twos-complement fails to produce the expected positive result.

Note the resemblance to bug #14864, fixed in 2.0.10.  This bug is of
very similar form, but is distinct.  The test cases of #14864 pass for
me on the 2.0.11 that shows the problem with a 2^63 bit shift.  My bug
does occur with the rnrs bitwise-arithmetic-shift-right, which was used
in #14864, as well as with ash.

-zefram

Previous Next

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