linux-kernel - Re: [PATCH] Add "is_power_of

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Date:	Tue, 30 Jan 2007 11:00:07 -0500 (EST)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Paul Mackerras <paulus@...ba.org>, dhowells@...hat.com,
	galak@...nel.crashing.org
Subject: Re: [PATCH] Add "is_power_of_2" checking to log2.h.

On Tue, 30 Jan 2007, Jan Engelhardt wrote:

>
> On Jan 30 2007 09:45, Robert P. J. Day wrote:

> >seriously, though, there is the potential of breaking something with
> >this change since you can see how there is some inconsistency in how
> >it's done *now* just for powerpc which, in some places, defines its
> >own versions of this:
> >
> >./arch/ppc/mm/pgtable.c:
> >	#define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))
> >./arch/ppc/syslib/ppc85xx_rio.c:
> >	#define is_power_of_2(x) (((x) & ((x) - 1)) == 0)
> >./arch/powerpc/mm/pgtable_32.c:
> >	#define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))
> >
> >note how the first and third macros *won't* consider zero a power of
> >two, while the second one *will*.  hence the need for a single
> >standard for all of this, just to play it safe.
>
> Hmpf. Perhaps a second macro "is_intdivisible_by_power_of_2" or so could
> catch the "am I zero or 2^n" question.

no.  if that's really what the programmer wants, they can code it that
way explicitly.  let's not make this overly obscure.

rday
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