Interfacing Ada multidimensional arrays with Fortran.

[David Sauvage <sauvage.david@gmail.com>]

Hi,

Concerning multidimensional arrays, Ada use row-major order [0] while
Fortran use column-major order [1].

It seems to exist two possibilities to handle this issue when calling
Fortran procedures from Ada, and three possibilities when calling BLAS
[2] or LAPACK [3] Fortran specific routines.

1 - Use the pragma Convention (Fortran, T_Real_Matrix)
on the array type, so that the array will be stored in column-major
order, as needed by Fortran.
Any feedbacks concerning the successful use of this pragma Convention
(Fortran, type) & GNATGPL 2010 or FSF GCC would be appreciate.

2 - Transpose the array in the Ada side when calling the Fortran code,
so that the Fortran code will load the array properly.

3 - Concerning BLAS & LAPACK routines, there are parameters to
indicates the form of the array (transposed or not) to the Fortran
code, so that the Fortran code load the array properly.

In Interfaces.Fortran.BLAS, there are Multidimensional arrays types
that are already defined ;
(Real_Matrix, Double_Precision_Matrix, Complex_Matrix,
Double_Complex_Matrix)
But the corresponding pragma Convention (Fortran, type) are not
defined for them. So any user that would re-use those types can not
use pragma Convention, and use possibility 2 or 3 above,

It would be interesting to know the story behind the scene of why
array types declared in  Interfaces.Fortran.BLAS do not use pragma
Convention (Fortran, *) ?

What are the performance issues by using the three possibilities
above ? (may be the community already had some interesting information
about this)

Some would think the pragma Convention (Fortran, type) should be more
efficient, but may be it is not that simple, as apparently the choice
has been made to avoid [4] pragma Convention (Fortran, type).


Cheers


[0] http://en.wikipedia.org/wiki/Row-major_order
[1] http://www.adaic.org/resources/add_content/standards/05rm/html/RM-B-5.html
[2] http://www.netlib.org/blas/
[3] http://www.netlib.org/lapack/
[4] i-forbla.ads