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

[Martin Uecker <uecker@tugraz.at>]

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

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).
> > 
>