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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.08.2025 12:53, Andrew Cooper wrote:
> On 15/08/2025 11:25 am, Jan Beulich wrote:
>> On 15.08.2025 11:51, Andrew Cooper wrote:
>>> On 15/08/2025 10:36 am, Jan Beulich wrote:
>>>> 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.
>>> How about this:
>>>
>>> diff --git a/CODING_STYLE b/CODING_STYLE
>>> index 7bf3848444ad..e33b9d1170cf 100644
>>> --- a/CODING_STYLE
>>> +++ b/CODING_STYLE
>>> @@ -129,6 +129,10 @@ Fixed width types should only be used when a fixed 
>>> width quantity is
>>>  meant (which for example may be a value read from or to be written to a
>>>  register).
>>>  
>>> +Macros which otherwise would use "typeof(arg) newarg =" to avoid double
>>> +evaluation of side effects should use "auto newarg =" per it's C23 
>>> behaviour,
>>> +to also avoid double textural expansion.
>>> +
>>>  Especially with pointer types, whenever the pointed to object is not
>>>  (supposed to be) modified, qualify the pointed to type with "const".
>> That doesn't focus on the pitfall though, in that people shouldn't be using
>> the "auto" keyword (except in said cases).
> 
> /sigh, this is why noone does patches to CODING_STYLE.
> 
> If you don't like the wording, propose some wording that you do like.
> 
> Or I will commit the patch without this hunk, because I'm not going to
> get drawn into the cycle of blind guessing that every change to
> CODING_STYLE seems to get caught in.

I don't care about the wording; what I do care about is to get the caveat
across. Maybe:

'"auto" isn't used in its traditional sense, but rather with its C23 meaning.
 Such uses are intended to be limited to macro-local variables.'

Jan