From: Richard Henderson <rth@twiddle.net>
To: Stefan Weil <sw@weilnetz.de>
Cc: Peter Maydell <peter.maydell@linaro.org>,
qemu-devel <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH 0/3] Add and use bit rotate functions
Date: Thu, 12 Sep 2013 12:23:51 -0700 [thread overview]
Message-ID: <523214C7.4090303@twiddle.net> (raw)
In-Reply-To: <1379013193-20691-1-git-send-email-sw@weilnetz.de>
On 09/12/2013 12:13 PM, Stefan Weil wrote:
> The first patch was already sent to qemu-devel and is only included
> here because patch 3 is based on it. Only patch 1 is needed for qemu-stable.
>
> It looks like shift values of 0 or 32/64 work as expected because
> the compiler "knows" the pattern used to implement the rotate operation,
> so the code does not need special handling of some shift values.
Thinking about this closer, the only two behaviors I know of for
x >> n, n >= w, w = width of x, are: shift modulo w, or zero.
Both cases work for this usage:
x << 0 | x >> 32
= x | (x >> 0) modulo
= x | x
= x
= x | 0 zero
= x
AFAIK we never actually observe missile launch as a side effect
of an out of range shift. And with that in mind, I think the
existing implementation of rotate is fine.
r~
prev parent reply other threads:[~2013-09-12 19:24 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-12 19:13 [Qemu-devel] [PATCH 0/3] Add and use bit rotate functions Stefan Weil
2013-09-12 19:13 ` [Qemu-devel] [PATCH 1/3] tci: Add implementation of rotl_i64, rotr_i64 Stefan Weil
2013-09-12 19:13 ` [Qemu-devel] [PATCH 2/3] bitops: Add rotate functions (rol8, ror8, ...) Stefan Weil
2013-09-12 19:24 ` Richard Henderson
2013-09-12 19:13 ` [Qemu-devel] [PATCH 3/3] misc: Use new rotate functions Stefan Weil
2013-09-12 19:25 ` Richard Henderson
2013-09-12 19:23 ` Richard Henderson [this message]
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