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

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Date:	Thu, 5 Nov 2009 17:38:42 +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 Thu, 5 Nov 2009 16:58:36 +1030 Rusty Russell <rusty@...tcorp.com.au> wrote:
>
> Huh?  virtio_has_feature does:
> 
> 	if (__builtin_constant_p(fbit))
> 		BUILD_BUG_ON(fbit >= 32);
> 	else
> 		BUG_ON(fbit >= 32);

In Linus' tree (and linux-next) it looks like this:

static inline bool virtio_has_feature(const struct virtio_device *vdev,
				      unsigned int fbit)
{
	/* Did you forget to fix assumptions on max features? */
	MAYBE_BUILD_BUG_ON(fbit >= 32);

	if (fbit < VIRTIO_TRANSPORT_F_START)
		virtio_check_driver_offered_feature(vdev, fbit);

	return test_bit(fbit, vdev->features);
}

> So, if it's not a constant, gcc should throw away that first branch.  If it
> is, it should optimize it away (and no, these are not real bugs AFAICT).

Your version above may well fix the problem.  Alternatively marking it
__always_inline way work as well.

> I did a 4.3.3 allmodconfig with this patch on 64-bit a few days back and all
> was fine.  Will try 4.4.0 now.

4.4 is more problematic.

-- 
Cheers,
Stephen Rothwell                    sfr@...b.auug.org.au
http://www.canb.auug.org.au/~sfr/

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