Subject: Re: [PATCH] trivial, document that div64_u64() is not precise on
 32bit platforms
From: Ben Woodard <woodard@redhat.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>,
        "Behlendorf, Brian D." <behlendorf1@llnl.gov>,
        Jeremy Fitzhardinge <jeremy@goop.org>,
        Mark Grondona <mgrondona@llnl.gov>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
In-Reply-To: <20100803152812.210439dc.akpm@linux-foundation.org>
References: <20100802160951.GA13385@redhat.com>
	 <20100803152812.210439dc.akpm@linux-foundation.org>
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Date: Tue, 03 Aug 2010 19:12:41 -0500
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Andrew,

On Tue, 2010-08-03 at 15:28 -0700, Andrew Morton wrote:
> Well that was a bit lazy of us - I wonder how hard it is to fix.
> 
> At present people will test their code on 64-bit only to find out
> later
> that it doesn't work correctly on 32-bit.  Bad.  Perhaps we should
> similarly break the 64-bit version :)

I just talked to Brian. He's got an implementation that does the
division correctly. He's going to clean it up and send you a patch so
that it will work without losing precision on both 32b and 64b. It is
based upon public domain code from  http://www.hackersdelight.org and
has been proven to be correct in all cases.

-ben


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