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Date: Thu, 28 Feb 2013 11:46:14 -0500
Message-ID: <CAFMmRNyCB1PC5zQ1448PyxcoXRUwqLKfooOoQ9dKJ4RDFgJ6sQ@mail.gmail.com>
Subject: Re: svn commit: r247460 - head/sys/dev/acpica
From: Ryan Stone <rysto32@gmail.com>
To: Alexey Dokuchaev <danfe@freebsd.org>
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Cc: Davide Italiano <davide@freebsd.org>, svn-src-head@freebsd.org,
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On Thu, Feb 28, 2013 at 11:25 AM, Alexey Dokuchaev <danfe@freebsd.org>wrote:

> On Thu, Feb 28, 2013 at 11:27:02AM +0000, Davide Italiano wrote:
> > New Revision: 247460
> > URL: http://svnweb.freebsd.org/changeset/base/247460
> >
> > Log:
> >   MFcalloutng (r247427 by mav):
> >   We don't need any precision here. Let it be fast and dirty shift then
> >   slow and excessively precise 64-bit division.
> >
> > -    if (sbt >= 0 && us > sbt / SBT_1US)
> > -     us = sbt / SBT_1US;
> > +    if (sbt >= 0 && us > (sbt >> 12))
> > +     us = (sbt >> 12);
>
> Does this really buy us anything?  Modern compilers should be smart enough
> to
> generate correct code.  Do you have evidence that this is not the case
> here?
> Not to mention that it obfuscates the code by using some magic constant.
>
> ./danfe
>

SBT_1US is 4294, not 4096 (1 << 12).  This is an approximation, which the
compiler obviously can't automatically apply.