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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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."

?

~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.