linux-kernel - Re: [PATCH v5 6/9] bug.h: Prevent double evaulation of in BUILD_BUG

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Message-ID: <20121115150726.GD4956@x1.osrc.amd.com>
Date:	Thu, 15 Nov 2012 16:07:27 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Daniel Santos <daniel.santos@...ox.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christopher Li <sparse@...isli.org>,
	David Daney <david.daney@...ium.com>,
	David Howells <dhowells@...hat.com>,
	Joe Perches <joe@...ches.com>,
	Josh Triplett <josh@...htriplett.org>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	linux-sparse@...r.kernel.org,
	Michel Lespinasse <walken@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Pavel Pisa <pisa@....felk.cvut.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v5 6/9] bug.h: Prevent double evaulation of in
 BUILD_BUG_ON

On Tue, Nov 13, 2012 at 04:13:38PM -0600, danielfsantos@....net wrote:
> When calling BUILD_BUG_ON in an optimized build using gcc 4.3 and later,
> the condition will be evaulated twice, possibily with side-effects.
> This patch eliminates that error.
> 
> Signed-off-by: Daniel Santos <daniel.santos@...ox.com>
> ---
>  include/linux/bug.h |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/bug.h b/include/linux/bug.h
> index 1b2465d..ccd44ce 100644
> --- a/include/linux/bug.h
> +++ b/include/linux/bug.h
> @@ -58,8 +58,9 @@ struct pt_regs;
>  extern int __build_bug_on_failed;
>  #define BUILD_BUG_ON(condition)					\
>  	do {							\
> -		((void)sizeof(char[1 - 2*!!(condition)]));	\
> -		if (condition) __build_bug_on_failed = 1;	\
> +		bool __cond = !!(condition);			\
> +		((void)sizeof(char[1 - 2*!!(__cond)]));		\

No need for the !!(__cond) here because you've done it in the line
above. IOW,

	bool __cond = !!(condition);
	((void)sizeof(char[1 - 2 * __cond]));

which is even a bit more readable, as a matter of fact.

> +		if (__cond) __build_bug_on_failed = 1;		\
>  	} while(0)
>  #endif

Thanks.

-- 
Regards/Gruss,
Boris.
--
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