Re: [PATCH v8 1/5] Provide counted_by attribute to flexible array member field (PR108896)

[Qing Zhao <qing.zhao@oracle.com>]

> On Apr 11, 2024, at 02:02, Martin Uecker <uecker@tugraz.at> wrote:
> 
> Am Mittwoch, dem 10.04.2024 um 19:35 +0000 schrieb Qing Zhao:
>> 
>>> On Apr 10, 2024, at 15:05, Martin Uecker <uecker@tugraz.at> wrote:
>>> 
>>> Am Mittwoch, dem 10.04.2024 um 20:25 +0200 schrieb Martin Uecker:
>>>> Am Mittwoch, dem 10.04.2024 um 17:35 +0000 schrieb Joseph Myers:
>>>>> On Fri, 29 Mar 2024, Qing Zhao wrote:
>>>>> 
>>>>>> +  /* Issue error when there is a counted_by attribute with a different
>>>>>> +     field as the argument for the same flexible array member field.  */
>>>>> 
>>>>> There's another case of this to consider, though I'm not sure where best 
>>>>> to check for it (Martin might have suggestions) - of course this case will 
>>>>> need testcases as well.
>>>>> 
>>>>> Suppose, as allowed in C23, a structure is defined twice in the same 
>>>>> scope, but the two definitions of the structure use inconsistent 
>>>>> counted_by attributes.  I'd say that, when the declarations are in the 
>>>>> same scope (thus required to be consistent), it should be an error for the 
>>>>> two definitions of what is meant to be the same structure to use 
>>>>> incompatible counted_by attributes (even though the member declarations 
>>>>> are otherwise the same).
>>>> 
>>>> I think the right place could be comp_types_attributes in
>>>> attributes.cc.  It may be sufficient to set the
>>>> affects_type_identify flag.
>>>> 
>>>> This should then give a redefinition error as it should do for
>>>> "packed".
>>> 
>>> Thinking about this a bit more, this will not work here, because
>>> the counted_by attribute is not applied to the struct type but
>>> one of the members.
>>> 
>>> So probably there should be a check added directly
>>> to tagged_types_tu_compatible_p
>> 
>> 
>> There are two cases we will check:
>> 
>>  A. Both definitions are in the same scope;
>>      Then if the 2nd definition has a counted-by attribute different from the 1st definition, the 2nd definition will be given a redefinition error; 
>> 
>>  B. These two definitions are in different scope;
>>      When these two definitions are used in a way need to be compatible, an incompatible error need to be issued at that
>> Point;
>> 
>> 
>> My question is, Will the routine “tagged_types_tu_compatible_p” can handle both A and B?
> 
> Yes, changing this function should address both cases if I am
> not missing something.
> 
Thanks for the help.
Will study this routine in more details and update the patch.

Qing
> Martin
> 
>> 
>> Thanks.
>> 
>> Qing
>>> 
>>> Martin
>>> 
>>>> 
>>>>> 
>>>>> In C23 structures defined with the same tag in different scopes are 
>>>>> compatible given requirements including compatible types for corresponding 
>>>>> elements.  It would seem most appropriate to me for such structures with 
>>>>> incompatible counted_by attributes to be considered *not* compatible types 
>>>>> (but it would be valid to define structures with the same tag, different 
>>>>> scopes, and elements the same except for counted_by - just not to use them 
>>>>> in any way requiring them to be compatible).
>>>> 
>>>> Another option might be to warn about the case when those types
>>>> are then used together in a way where they are required to
>>>> be compatible.  Then comp_types_attributes would have to return 2.
>>>> 
>>>> 
>>>> Martin
>>>> 
>>>>> 
>>>>>> +The @code{counted_by} attribute may be attached to the C99 flexible array
>>>>>> +member of a structure.  It indicates that the number of the elements of the
>>>>>> +array is given by the field "@var{count}" in the same structure as the
>>>>> 
>>>>> As noted previously, the "" quotes should be removed there (or replaced by 
>>>>> ``'' quotes).
>>>>> 
>>>> 
>>> 
>> 
>