linux-kernel - Re: [PATCH for -tip 1/2] kernel.h: add MAYBE_BUILD_BUG_ON_NOT_POWER_OF

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Message-ID: <AANLkTingCD78PYQN1f-0jNQYAU1v6W5RXHe73FnpmqDA@mail.gmail.com>
Date:	Sun, 22 Aug 2010 23:16:42 +0300
From:	Anca Emanuel <anca.emanuel@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	ltt-dev@...ts.casi.polymtl.ca,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Alexander Shishkin <virtuoso@...nd.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org,
	Imre Deak <imre.deak@...ia.com>,
	Jamie Lokier <jamie@...reable.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH for -tip 1/2] kernel.h: add MAYBE_BUILD_BUG_ON_NOT_POWER_OF_2

I think about some test at the last bit if it is on, the number is not
a power of 2

On Sun, Aug 22, 2010 at 10:38 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> * Andi Kleen (andi@...stfloor.org) wrote:
>> > +/* Force a compilation error if condition is constant and not a power of 2 */
>> > +#define MAYBE_BUILD_BUG_ON_NOT_POWER_OF_2(n)               \
>> > +   MAYBE_BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))
>>
>> Looks super-ugly. IMHO just writing MAYBE_BUILD_BUG_ON(!n || n & (n - 1)) directly
>> would be clear enough. If you really think that's unclear define a generic
>> is_power_of_two() macro.
>
> There is already a is_power_of_two macro in log2.h, but I fear it might
> incorrectly interact with "MAYBE_BUILD_BUG_ON" (for some reason passing the
> constant result of a static inline is not treated as a constant by the macro,
> and thus it always "passes" the test).
>
> So if everyone object to this new macro, I'd be tempted to just go with your
> suggestion. However the fact that we already have BUILD_BUG_ON_NOT_POWER_OF_2(n)
> made me think that some people prefer to have it done as a macro.
>
> Other opinions ?
>
> Thanks,
>
> Mathieu
>
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
> --
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