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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.08.2025 10:33, Nicola Vetrini wrote:
> 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?

I'm not sure newcomers would look there, but in the absence of any better
place that's perhaps indeed where to mention this.

I am, however, still not quite sure if we're doing ourselves a favor with
the re-purposing of auto (while not using C23 in general). It'll certainly
hit me as unexpected, for some time, to see uses of the keyword. It simply
has been for too long that it had its earlier meaning (of very limited use).

Jan