linux-kernel - Re: [RFC] div64

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Date:	Mon, 5 Mar 2007 15:57:14 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [RFC] div64_64 support

On 03 Mar 2007 03:31:52 +0100
Andi Kleen <andi@...stfloor.org> wrote:

> Stephen Hemminger <shemminger@...ux-foundation.org> writes:
> 
> > Here is another way to handle the 64 bit divide case.
> > It allows full 64 bit divide by adding the support routine
> > GCC needs.
> 
> Not supplying that was intentional by Linus so that people
> think twice (or more often) before they using such expensive
> operations. A plain / looks too innocent.
> 
> Is it really needed by CUBIC anyways?  It uses it for getting
> the cubic root, but the algorithm recommended by Hacker's Delight
> (great book) doesn't use any divisions at all. Probably better 
> to use a better algorithm without divisions.
> 

I tried the code from Hacker's Delight.
It is cool, but performance is CPU (and data) dependent:

Average # of usecs per operation:

		Hacker		Newton
Pentium 3	68.6	<	90.4
T2050		98.6	>	92.0
U1400		450	>	415
Xeon		70 	<	90
Xeon (newer)	71	<	78

EM64T		21.8	<	24.6
AMD64		23.4	<	32.0

It might be worth the change for code size reduction though.


-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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