Subject: bug#21901: closed (Re: bug#21901: bit shift wrong on maximal right shift)

From: [email-masked] (GNU bug Tracking System)
Date: Sun, 14 Oct 2018 09:42:02 +0000

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 [email-masked].

-- 
21901: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=21901
GNU Bug Tracking System
Contact [email-masked] with problems

Subject: Re: bug#21901: bit shift wrong on maximal right shift
From: Mark H Weaver <[email-masked]>
To: Zefram <[email-masked]>
Cc: [email-masked]
Date: Sun, 14 Oct 2018 05:40:51 -0400
List-Id: <debbugs-submit.debbugs.gnu.org>

Zefram <[email-masked]> 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

Subject: bit shift wrong on maximal right shift
From: Zefram <[email-masked]>
To: [email-masked]
Date: Fri, 13 Nov 2015 10:22:03 +0000
List-Id: <debbugs-submit.debbugs.gnu.org>

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