Re: [XEN PATCH v6] xen/include: avoid using a compiler extension for BUILD_BUG_ON_ZERO

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 26 Jun 2023, Nicola Vetrini wrote:
> Redefine BUILD_BUG_ON_ZERO to avoid using a compiler extension
> that gives an acceptable semantics to C99 undefined behavior 58
> ("A structure or union is defined as containing no named members
> (6.7.2.1)").
> 
> The first definition includes an additional named field of type
> char.
> 
> The chosen ill-formed construct for the second definition is a struct
> with a named bitfield of width 0 when the condition is true,
> which prevents the UB without using the compiler extension while keeping
> the semantic of the construct.
> 
> The choice of the bitwise AND operation to bring the result to 0
> when cond is false boils down to possibly better portability.
> 
> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> Changes in V2:
> - Avoid using a VLA as the compile-time assertion
> - Do not drop _Static_assert
> Changes in V3:
> - Changed the operation to bring the result to 0 when the
> construct does not lead to a compilation error
> Changes in V4:
> - Switched to a shorter construct for the second definition.
> Changes in V5:
> - Dropped 'U's from the macro definitions.
> Changes in V6:
> - Fixed patch submission.
> ---
>  xen/include/xen/lib.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/include/xen/lib.h b/xen/include/xen/lib.h
> index 67fc7c1d7e..a8958ed57b 100644
> --- a/xen/include/xen/lib.h
> +++ b/xen/include/xen/lib.h
> @@ -51,9 +51,10 @@
>     e.g. in a structure initializer (or where-ever else comma expressions
>     aren't permitted). */
>  #define BUILD_BUG_ON_ZERO(cond) \
> -    sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
> +    (sizeof(struct { char c; _Static_assert(!(cond), "!(" #cond ")"); }) & 0)
>  #else
> -#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
> +#define BUILD_BUG_ON_ZERO(cond) \
> +    (sizeof(struct { unsigned u : !(cond); }) & 0)
>  #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
>  #endif
>  
> -- 
> 2.34.1
>