Re: [Qemu-devel] [PATCH v3 2/4] compiler: rework BUG_ON using a struct

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

* Michael S. Tsirkin (mst@redhat.com) wrote:
> There are theoretical concerns that some compilers might not trigger
> build failures on attempts to define an array of size -1 and make it a
> variable sized array instead. Let rewrite using a struct with a negative
> bit field size instead as there are no dynamic bit field sizes.  This is
> similar to what Linux does.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/qemu/compiler.h | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> index 7512082..c6f673e 100644
> --- a/include/qemu/compiler.h
> +++ b/include/qemu/compiler.h
> @@ -85,9 +85,12 @@
>  #define typeof_field(type, field) typeof(((type *)0)->field)
>  #define type_check(t1,t2) ((t1*)0 - (t2*)0)
>  
> -#define QEMU_BUILD_BUG_ON(x) \
> -    typedef char glue(qemu_build_bug_on__, __LINE__)[(x) ? -1 : 1] \
> -        __attribute__((unused))
> +#define QEMU_BUILD_BUG_ON_STRUCT(x) \
> +    struct { \
> +        int qemu_build_bug_on : (x) ? -1 : 1; \
> +    }

The problem with this is it can't be used as an expression, where as
your previous version could.
I've got a similar case (see previous reply) that needed an expression
bug-on that would evaluate to zero.

Dave

> +#define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
> +    glue(qemu_build_bug_on__, __LINE__) __attribute__((unused))
>  
>  #if defined __GNUC__
>  # if !QEMU_GNUC_PREREQ(4, 4)
> -- 
> MST
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK