Re: Thoughts on Gengtype and Single Inheritance

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

Lawrence -

I don't have an opinion from the rest of compiler point of view, but
gengtype implementation-wise, it certainly looks doable. One minor
comment below -

> 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.

-- 
Laurynas