Re: [Qemu-devel] [PATCH 3/4] compiler: expression version of QEMU_BUILD_BUG_ON

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Michael S. Tsirkin (mst@redhat.com) wrote:
> QEMU_BUILD_BUG_ON uses a typedef in order to be safe
> to use outside functions, but sometimes it's useful
> to have a version that can be used within an expression.
> Following what Linux does, introduce QEMU_BUILD_BUG_ON_ZERO
> that return zero after checking condition at build time.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/qemu/compiler.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> index 1a9eeb9..d2b05dd 100644
> --- a/include/qemu/compiler.h
> +++ b/include/qemu/compiler.h
> @@ -88,6 +88,8 @@
>  #define QEMU_BUILD_BUG_ON(x) \
>      typedef char glue(qemu_build_bug_on__,__LINE__)[(x)?-1:1] __attribute__((unused))
>  
> +#define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(int[(x)?-1:1]) - sizeof(int))
> +

Yes, pretty much the same as:
https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg07090.html

I'd used -sizeof(int[1])   in the right hand size to match the left, but
I think it should always be the same.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

>  #if defined __GNUC__
>  # if !QEMU_GNUC_PREREQ(4, 4)
>     /* gcc versions before 4.4.x don't support gnu_printf, so use printf. */
> -- 
> MST
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK