[Bug fortran/53971] New: [OOP] Class container - reverse order of _vtab and _data

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53971

             Bug #: 53971
           Summary: [OOP] Class container - reverse order of _vtab and
                    _data
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org


Created attachment 27798
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27798
Draft patch - should be sufficient, but isn't.

The main purpose of the proposed change is the handling of assumed-rank arrays,
both implementation wise and by allowing to bump the MAX_DIMENSION without
breaking the ABI.

The attached patch should be sufficient, but it causes many test suite
failures, e.g. by having a __vtype in the call to class.c's
find_typebound_proc_uop. For that argument, derived->f2k_derived == NULL, which
triggers an assert for gfortran.dg/abstract_type_6.f03.



Currently, the first element in a class container is _data, followed by _vptr.

_data can be either directly a memory address or array descriptor of rank n.
Hence, the byte-offset between the base address of the class container and the
_vptr depends on the kind of data (normal scalar vs. array) and the rank.

For assumed-shape arrays, that implies that one always needs to have an array
descriptor of MAX_DIMENSION – and changing MAX_DIMENSION will break the arrays.

If one reorder _vptr and _data, _vptr can be always accessed and one can
directly pass class containers (with descriptor) of any rank – the size of the
array container is then known at run time based on the rank information.

In C syntax, that's

{
   void *_vpt;
   {
     /* First fields of the array descriptor.  */
     array_dim_triplet dim[ ];
   }
}

Where the "[ ]" allows for different lengths; it is created by creating a
tree-decl for an array of size 0.

The current assumed-rank patch, has always MAX_DIMENSION and copies all fields
including the dimension triplet (the latter via range-ref) over.