Re: [XEN PATCH 9/9] xen/compat: address Rule 10.1 for macros CHECK_SIZE

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 6 Oct 2023, Nicola Vetrini wrote:
> The essential type of the result of an inequality operator is
> essentially boolean, therefore it shouldn't be used as an argument of
> the multiplication operator, which expects an integer.
> 
> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> ---
>  xen/include/xen/compat.h | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/include/xen/compat.h b/xen/include/xen/compat.h
> index f2ce5bb3580a..5ffee6a9fed1 100644
> --- a/xen/include/xen/compat.h
> +++ b/xen/include/xen/compat.h
> @@ -151,12 +151,14 @@ CHECK_NAME_(k, n, T)(k xen_ ## n *x, \
>      return x == c; \
>  }
>  
> +#define SIZE_NEQUAL(a, b) \
> +    (sizeof(a) != sizeof(b) ? 1 : 0)
>  #define CHECK_SIZE(name) \
> -    typedef int CHECK_NAME(name, S)[1 - (sizeof(xen_ ## name ## _t) != \
> -                                         sizeof(compat_ ## name ## _t)) * 2]
> +    typedef int CHECK_NAME(name, S)[1 - (SIZE_NEQUAL(xen_ ## name ## _t, \
> +                                                     compat_ ## name ## _t)) 
> * 2]
>  #define CHECK_SIZE_(k, n) \
> -    typedef int CHECK_NAME_(k, n, S)[1 - (sizeof(k xen_ ## n) != \
> -                                          sizeof(k compat_ ## n)) * 2]
> +    typedef int CHECK_NAME_(k, n, S)[1 - (SIZE_NEQUAL(k xen_ ## n, \
> +                                                      k compat_ ## n)) * 2]

I think this style is easier to read but I'll let the x86 maintainers
decide

    typedef int CHECK_NAME(name, S)[(sizeof(xen_ ## name ## _t) == \
                                     sizeof(compat_ ## name ## _t)) ? 1 : -1]

Also am I reading this correctly that we are using -1 as array index? I
must have made a calculation mistake?