linux-kernel - Re: [PATCH] Optimize is_power_of

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Date:	Fri, 15 Jun 2007 14:28:16 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Vegard Nossum <vegard.nossum@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Optimize is_power_of_2().

On Fri, 15 Jun 2007, H. Peter Anvin wrote:

> Vegard Nossum wrote:
> > From: Vegard Nossum <vegard@...tkore.net>
> > Date: Fri, 15 Jun 2007 18:35:49 +0200
> > Subject: [PATCH] Optimize is_power_of_2().
> >
> > Rationale: Removes one conditional branch and reduces icache
> > footprint. Proof: If n is false, the product of n and any value is
> > false. If n is true, the truth of (n * x) is the truth of x.
>
> You realize that on a lot of platforms, multiplication is done by an
> out-of-line subroutine, and that even on those that aren't, it's
> generally a long-pipeline operation, right?

i was *just* about to mention somthing of the sort, but probably not
as succinctly.

rday
-- 
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Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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