>>> 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