linux-kernel - Re: [PATCH] Optimize is_power_of

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Message-ID: <4672D89D.7010801@zytor.com>
Date:	Fri, 15 Jun 2007 11:21:17 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vegard Nossum <vegard.nossum@...il.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Optimize is_power_of_2().

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?

	-hpa
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