Re: [Qemu-devel] [PATCH v2 3/4] compiler: expression version of QEMU_BUILD_BUG_ON

[Markus Armbruster <armbru@redhat.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> 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.

Following Linux's example makes sense, but I can't help but wonder
whether we need both QEMU_BUILD_BUG_ON_ZERO() and QEMU_BUILD_BUG_ON().

> 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 2882470..f4cf13b 100644
> --- a/include/qemu/compiler.h
> +++ b/include/qemu/compiler.h
> @@ -89,6 +89,8 @@
>      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))
> +
>  #if defined __GNUC__
>  # if !QEMU_GNUC_PREREQ(4, 4)
>     /* gcc versions before 4.4.x don't support gnu_printf, so use printf. */

More so since your QEMU_BUILD_BUG_ON_ZERO seems easier to understand: no
token pasting.

Anyway,
Reviewed-by: Markus Armbruster <armbru@redhat.com>