lkml.org 
[lkml]   [2009]   [Nov]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] BUILD_BUG_ON: make it handle more cases
Hi Rusty,

On Thu, 5 Nov 2009 16:58:36 +1030 Rusty Russell <rusty@rustcorp.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@canb.auug.org.au
http://www.canb.auug.org.au/~sfr/
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2009-11-05 07:41    [W:0.095 / U:25.956 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site