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

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Message-ID: <466DD696.8080901@zytor.com>
Date:	Mon, 11 Jun 2007 16:11:18 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
CC:	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	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.

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.
> 

I suspect the reason the test for zero was omitted is because the author
didn't want the extra cost (the test for zero needs an extra branch on a
lot of architectures.)

	-hpa
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