linux-kernel - Re: [PATCH] BUILD_BUG_ON: make it handle more cases

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Message-Id: <20091105112020.33f02552.sfr@canb.auug.org.au>
Date:	Thu, 5 Nov 2009 11:20:20 +1100
From:	Stephen Rothwell <sfr@...b.auug.org.au>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Hollis Blanchard <hollisb@...ibm.com>,
	Jan Beulich <JBeulich@...ell.com>, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org, kvm-ppc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-next@...r.kernel.org
Subject: Re: [PATCH] BUILD_BUG_ON: make it handle more cases

Hi Rusty,

On Tue, 20 Oct 2009 14:15:33 +1030 Rusty Russell <rusty@...tcorp.com.au> wrote:
>
> +#ifndef __OPTIMIZE__
> +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> +#else
> +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
> +#define MAYBE_BUILD_BUG_ON(condition) BUILD_BUG_ON(condition)
> +

I decided to try this in linux-next, but an x86_64 allmodconfig build
gave this (gcc 4.4.0):

ERROR: "__build_bug_on_failed" [drivers/net/virtio_net.ko] undefined!
ERROR: "__build_bug_on_failed" [drivers/block/virtio_blk.ko] undefined!

I assume that this is caused by the "MAYBE_BUILD_BUG_ON(fbit >= 32)" in
virtio_has_feature() (in include/linux/virtio_config.h) which is called
all over the place.  Unfortunately, virtio_has_feature() gets uninlined
in those two files ...

I have taken the patch back out again for today.
-- 
Cheers,
Stephen Rothwell                    sfr@...b.auug.org.au
http://www.canb.auug.org.au/~sfr/

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