Re: Thoughts on Gengtype and Single Inheritance

[Laurynas Biveinis <laurynas dot biveinis at gmail dot com>]

>>> GRAMMAR
>>>
>>> Support adding a second discriminator.  This support is not for
>>> multiple inheritance, but for single inheritance when a second
>>> discriminator is used to further refine it.  Look at struct
>>> tree_omp_clause.  It contains a sub union.  We can represent the
>>> hierarchy like:
>>>
>>> struct tree_omp_clause : tree_common {
>>>   location_t locus;
>>>   enum omp_clause_code code;
>>> };
>>>
>>> struct tree_omp_default_clause : tree_omp_clause {
>>>   enum omp_clause_default_kind default_kind;
>>> };
>>>
>>> struct tree_omp_schedule_clause : tree_omp_clause {
>>>   enum omp_clause_schedule_kind schedule_kind;
>>> };
>>>
>>> struct tree_omp_reduction_clause : tree_omp_clause {
>>>   enum tree_code reduction_code;
>>> };
>>>
>>> We use TREE_CODE to understand that we have at least a tree_omp_clause
>>> and then we use tree_common.code to to distinguish these last three.
>>>
>>> Another possible case is tree_type_symtab inside tree_type_common.
>>>
>>> The syntax would be something like the following.
>>>
>>> enum F { F1, F2, F3, F4, F5 };
>>>
>>> class CTYPE GTY ((desc ("%h.kind"), tag ("F1")))
>>> : GTY ((tag ("EC"))) public BTYPE
>>> { public: enum F kind; something *pq; ... };
>>>
>>> class FTYPE : GTY ((tag ("F2"))) public CTYPE { ... };
>>
>> I wonder if the second discriminator support is easily generalizable
>> to enabling any derived class being a root class on it own with its
>> own subtree? If I understand correctly, the GTY syntax would be the
>> same.
>
> If I understand correctly, you are suggesting multiple inheritance
> via enums.  I think it is possible, but I think the tag syntax
> would need to be changed to more directly associate the tag with
> the variable.
>
> --
> Lawrence Crowl

I was trying to talk about single inheritance, not multiple
inheritance nor composition here, but perhaps I misunderstood it
myself.

As I saw it, there is a hierarchy rooted at tree_common. For its child
tree_omp_clause there is further sub-hierarchy. It's all single
inheritance, and the second discriminator here would be the first
discriminator, if tree_omp_clause were not a child of other class.

-- 
Laurynas