linux-kernel - Re: [PATCH] Optimize is_power_of

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Date:	Fri, 15 Jun 2007 14:57:23 -0500
From:	"David M. Lloyd" <dmlloyd@...rg.com>
To:	"David M. Lloyd" <dmlloyd@...rg.com>
Cc:	Jan Engelhardt <jengelh@...putergmbh.de>,
	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 14:54:20 -0500
"David M. Lloyd" <dmlloyd@...rg.com> wrote:

> 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;

Pretend I typed this instead:

bool is_power_of_2(unsigned long n)
{
	return __builtin_popcountl(n) == 1;
}

All I can say is, it's really hot here :P

- DML
-
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