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Message-ID: <45BFEF2A.5070907@yahoo.com.au>
Date: Wed, 31 Jan 2007 12:21:46 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: "Robert P. J. Day" <rpjday@mindspring.com>
CC: Linux kernel mailing list <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>, Paul Mackerras <paulus@samba.org>,
       dhowells@redhat.com, galak@kernel.crashing.org
Subject: Re: [PATCH] Add "is_power_of_2" checking to log2.h.
References: <Pine.LNX.4.64.0701300543420.17991@CPE00045a9c397f-CM001225dbafb6> <45BF3954.8050106@yahoo.com.au> <Pine.LNX.4.64.0701300753170.23302@CPE00045a9c397f-CM001225dbafb6>
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Robert P. J. Day wrote:
> On Tue, 30 Jan 2007, Nick Piggin wrote:
> 
> 
>>Robert P. J. Day wrote:
>>
>>>  Add the inline function "is_power_of_2()" to log2.h, where the value
>>>zero is *not* considered to be a power of two.
>>
>>>Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>
>>>
>>> /*
>>>+ *  Determine whether some value is a power of two, where zero is
>>>+ * *not* considered a power of two.
>>>+ */
>>
>>Why the qualifier? Zero *is* not a power of 2, is it?
> 
> 
> no, but it bears repeating since some developers might think it *is*.
> if you peruse the current kernel code, you'll find some tests of the
> simpler form:
> 
> 	((n & (n - 1)) == 0))
> 
> which is clearly testing for "power of twoness" but which will return
> true for a value of zero.  that's wrong, and it's why it's emphasized
> in the comment.

I would have thought you'd comment the broken ones, but that's just me.

-- 
SUSE Labs, Novell Inc.
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