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

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Date:	Sat, 21 Aug 2010 14:40:15 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Alexey Dobriyan <adobriyan@...il.com>,
	Jan Beulich <JBeulich@...ell.com>,
	Roland Dreier <rdreier@...co.com>
Cc:	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>,
	Andi Kleen <andi@...stfloor.org>,
	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>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH for -tip 1/2] kernel.h: add
	MAYBE_BUILD_BUG_ON_NOT_POWER_OF_2

* Alexey Dobriyan (adobriyan@...il.com) wrote:
> On Sat, Aug 21, 2010 at 10:17:51AM -0400, Mathieu Desnoyers 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))
> 
> Sorry, this is tasteless macro.

Let's look at the surrounding where I added this macro in kernel.h:


/* Force a compilation error if condition is true */
#define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))

/* Force a compilation error if condition is constant and true */
#define MAYBE_BUILD_BUG_ON(cond) ((void)sizeof(char[1 - 2 * !!(cond)]))

/* Force a compilation error if a constant expression is not a power of 2 */
#define BUILD_BUG_ON_NOT_POWER_OF_2(n)			\
	BUILD_BUG_ON((n) == 0 || (((n) & ((n) - 1)) != 0))

/* Force a compilation error if condition is true, but also produce a
   result (of value 0 and type size_t), so the expression can be used
   e.g. in a structure initializer (or where-ever else comma expressions
   aren't permitted). */
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
#define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))


So I am guessing that you plan to rewrite all of these ? Or perharps you have
other suggestions ? Commit cc8ef6eb21e964b1c5eb97b2d0e8ac9893e1bf86 introduced
"BUILD_BUG_ON_NOT_POWER_OF_2()" btw.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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