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

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

> 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?

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).
>>> 
>> 
>