linux-kernel - Re: [PATCH] Optimize is_power_of

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Message-ID: <20070615145420.3a824477@localhost>
Date:	Fri, 15 Jun 2007 14:54:20 -0500
From:	"David M. Lloyd" <dmlloyd@...rg.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Robert P. J. Day" <rpjday@...dspring.com>
Subject: Re: [PATCH] Optimize is_power_of_2().

On Fri, 15 Jun 2007 21:47:50 +0200 (CEST)
Jan Engelhardt <jengelh@...putergmbh.de> wrote:

> On Jun 15 2007 18:56, Vegard Nossum wrote:
> > bool is_power_of_2(unsigned long n)
> > {
> >-	return (n != 0 && ((n & (n - 1)) == 0));
> >+	return n * !(n & (n - 1));
> > }
> 
> There is a third way which uses neither * nor &&, but []:

I assume using something GCC-specific is right out?

bool is_power_of_to(unsigned long n)
{
	return __builtin_ffsl(n) == 1;
}

- DML
-
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