From: Andy Wingo <wingo@pobox.com>
To: Zefram <zefram@fysh.org>
Cc: 21901@debbugs.gnu.org
Subject: bug#21901: bit shift wrong on maximal right shift
Date: Mon, 20 Jun 2016 18:02:50 +0200 [thread overview]
Message-ID: <87porbg85x.fsf@pobox.com> (raw)
In-Reply-To: <20151113102203.GL13455@fysh.org> (zefram@fysh.org's message of "Fri, 13 Nov 2015 10:22:03 +0000")
On Fri 13 Nov 2015 11:22, Zefram <zefram@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.
> 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.
Fascinating, thank you! Also occurs on 2.1.3.
Andy
next prev parent reply other threads:[~2016-06-20 16:02 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-13 10:22 bug#21901: bit shift wrong on maximal right shift Zefram
2016-06-20 16:02 ` Andy Wingo [this message]
2018-10-14 9:40 ` Mark H Weaver
2018-10-20 22:27 ` Mark H Weaver
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