Re: [PATCH v2] xen: Use auto as per C23

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 2025-08-15 10:17, Andrew Cooper wrote:
> On 15/08/2025 8:20 am, Nicola Vetrini wrote:
> > On 2025-08-15 00:25, Andrew Cooper wrote:
> > > In macros it is common to declare local variables using typeof(param)
> > > in order
> > > to ensure that side effects are only evaluated once.  A consequence
> > > of this is
> > > double textural expansion of the parameter, which can get out of hand
> > > very
> > > quickly with nested macros.
> > >
> > > In C23, the auto keyword has been repurposed to perform type 
> > > inference.
> > > A GCC extension, __auto_type, is now avaialble in the new toolchain
> > > baseline
> > > and avoids the double textural expansion.
> > >
> > > Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Reviewed-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> Thankyou.
>
> > > diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
> > > index 88bf26bc5109..38ef5d82ad95 100644
> > > --- a/xen/include/xen/compiler.h
> > > +++ b/xen/include/xen/compiler.h
> > > @@ -64,6 +64,20 @@
> > >  # define asm_inline asm
> > >  #endif
> > >
> > > +/*
> > > + * In C23, the auto keyword has been repurposed to perform type
> > > inference.
> > > + *
> > > + * This behaviour is available via the __auto_type extension in
> > > supported
> > > + * toolchains.
> > > + *
> > > + *
> > > https://www.gnu.org/software/c-intro-and-ref/manual/html_node/Auto-Type.html
> > > + * https://clang.llvm.org/docs/LanguageExtensions.html#auto-type
> > > + */
> > > +#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 202311L
> > > +/* SAF-3-safe MISRA C Rule 20.4: Giving the keyword it's C23
> > > meaning. */
> > > +#define auto __auto_type
> > > +#endif
> > > +
> > A more detailed explanation should live in deviations.rst under this
> > bullet point
> >
> >    * - R20.4
> >      - The override of the keyword \"inline\" in xen/compiler.h is
> > present so
> >        that section contents checks pass when the compiler chooses not 
> > to
> >        inline a particular function.
> >      - Comment-based deviation.
> >
> > as described in the SAF entry:
> >
> >         {
> >             "id": "SAF-3-safe",
> >             "analyser": {
> >                 "eclair": "MC3A2.R20.4"
> >             },
> >             "name": "MC3A2.R20.4: allow the definition of a macro with
> > the same name as a keyword in some special cases",
> >             "text": "The definition of a macro with the same name as a
> > keyword can be useful in certain configurations to improve the
> > guarantees that can be provided by Xen. See docs/misra/deviations.rst
> > for a precise rationale for all such cases."
> >         },
> Ah right.  What about this:
>
> "Xen does not use the \"auto\" keyword as a storage qualifier.  The
> override of the keyword \"auto\" in xen/compiler.h is to give it it's
> C23 behaviour of type inference."
>
> ?
Seems good to me. Maybe this should be spelled out in ./CODING_STYLE as 
well, so that newcomers don't trip over this?
> ~Andrew
>
> P.S. I thought that final fix was too easy.  Your instructions for the
> main config were spot on and worked first time, except for getting the
> R20.4 violation too.
Yeah, forgot about that one; it's not so common to override keywords.

--
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253