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

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 04/07/23 16:24, Jan Beulich wrote:
> On 26.06.2023 17:37, 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>
> The code change looks okay to me now, but I'd like to double check
> towards the testing you did with this change: While it is clear that
> you will have checked that the tree builds with the adjustments, I
> expect that would have been with a compiler supporting _Static_assert.
> Did you also check with an older compiler, using the alternative
> implementation? Plus did you also check both constructs for actually
> triggering when the supplied condition turns out to be true?
>
> Jan
Besides using the build pipelines in gitlab, I checked just the macro on 
Compiler Explorer using gcc 4.4.7 and clang 16 
(https://godbolt.org/z/1d6vznxcW) and the construct is behaving as expected.
Regards,

--
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)