linux-kernel - Re: [PATCH] fix BUILD_BUG_ON() and a couple of bogus uses of it

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Date:	Wed, 23 Sep 2009 10:27:00 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Jan Beulich" <JBeulich@...ell.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix BUILD_BUG_ON() and a couple of bogus uses of it

On Wed, 19 Aug 2009 01:29:25 am Jan Beulich wrote:
> gcc permitting variable length arrays makes the current construct
> used for BUILD_BUG_ON() useless, as that doesn't produce any diagnostic
> if the controlling expression isn't really constant. Instead, this
> patch makes it so that a bit field gets used here. Consequently, those
> uses where the condition isn't really constant now also need fixing.
> 
> Note that in the gfp.h, kmemcheck.h, and virtio_config.h cases
> MAYBE_BUILD_BUG_ON() really just serves documentation purposes - even
> if the expression is compile time constant (__builtin_constant_p()
> yields true), the array is still deemed of variable length by gcc, and
> hence the whole expression doesn't have the intended effect.
> 
> Signed-off-by: Jan Beulich <jbeulich@...ell.com>

We used to use an undefined symbol here; diagnostics are worse but it catches
more stuff.

Perhaps a hybrid is the way to go?

#ifndef __OPTIMIZE__
#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
#else
/* If it's a constant, catch it at compile time, otherwise at link time. */
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;		\
	} while(0)
#endif

Thanks,
Rusty.
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