Message-ID: <4B460F98.9000208@s5r6.in-berlin.de>
Date: Thu, 07 Jan 2010 17:45:12 +0100
From: Stefan Richter <stefanr@s5r6.in-berlin.de>
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To: "Robert P. J. Day" <rpjday@crashcourse.ca>
CC: Roland Dreier <rdreier@cisco.com>,
       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       Bart Van Assche <bart.vanassche@gmail.com>,
       David Dillow <dave@thedillows.org>
Subject: Re: [PATCH] Add BUILD_BUG_ON_NOT_POWER_OF_2()
References: <ada3a2jugqu.fsf@roland-alpha.cisco.com> <20100106123306.ac85e557.akpm@linux-foundation.org> <aday6kbt14a.fsf@roland-alpha.cisco.com> <20100106134232.0025e818.akpm@linux-foundation.org> <adaeim2supb.fsf_-_@roland-alpha.cisco.com> <alpine.LFD.2.00.1001070332060.16874@localhost>
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Robert P. J. Day wrote:
>   my normal pedantry coming to the surface, but can we at least assume
> that people will use this test to *legitimately* test whether or not
> something is a power of two, and not that there is a single bit set
> (in the case of mask bits where all settings must be mutually
> exclusive)?
> 
>   there's a lot of this sort of thing throughout the kernel:
> 
>     (n) != 0 && (((n) & ((n) - 1))
> 
> but it's sometimes unclear whether someone is testing for a) power of
> two, or b) single bit set.  if you're going to introduce that kind of
> BUILD BUG (which is a good idea), let's try to not immediately abuse
> it semantically. :-)

It's merely about math, not about semantics.  Plus, its application is
restricted to build-time checks (of defined constants) anyway.

Hence I would argue that "check at build time whether a defined
bitmask's Hamming weight is 1; abort build if it isn't" can be
legitimately and sufficiently readably implemented by means of the new
BUILD_BUG_ON_NOT_POWER_OF_2().
-- 
Stefan Richter
-=====-==-=- ---= --===
http://arcgraph.de/sr/
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