Re: Interfacing Ada to C for linear algebra

["vincent.diemunsch@gmail.com" <vincent.diemunsch@gmail.com>]

On 2 sep, 16:53, Simon Wright <si...@pushface.org> wrote:
> "vincent.diemun...@gmail.com" <vincent.diemun...@gmail.com> writes:
> > Hello everybody,
>
> > I would like to use in Ada some linear algebra libraries written in C,
> > especially the SuiteSparse :
> >  http://www.cise.ufl.edu/research/sparse/SuiteSparse/
> > These libraries handle Sparse Matrices that are in the format used
> > internally by MatLab : a column-compressed version of the standard
> > matrices.
>
> > Ada defines Generic_Real_Arrays to handle vectors and matrices. So I
> > would like to simply create a Generic_Real_Sparse_Arrays package to
> > define MatLab compatible matrices. Since the internal format of MatLab
> > matrices is in C, in need to do a conversion between my Sparse_Matrix
> > and the C arrays that represent the Sparse Matrix in MatLab.
>
> > Looking at the implementation of Generic_Real_Arrays in GNAT, I see
> > that they internally call the BLAS and LAPACK packages, written in
> > Fortran, through the use of two packages : System.Generic_Real_BLAS
> > and System.Generic_Real_LAPACK. These package do exactly the same as
> > what I want to do : they convert Ada arrays of Ada floating point type
> > in Fortran arrays of Fortran Float type and back, without using
> > copying, so that the compiler produces no code for this conversion
> > (using pragma Inline_Always and so on). This is important because if
> > we use optimized versions of linear algebra packages it is because we
> > manipulate huge arrays and we don't want to loose time in copying
> > values from on representation to another !
>
> The LAPACK interface used in Generic_Real_Arrays deals with matrices in
> Ada (row-major) format and - usually, anyway - transposes in order to
> call the Fortran implementations. Unlike BLAS, LAPACK itself doesn't
> have options to deal with row-major matrices.
>
> We did some timings and found that it's quite a bit quicker to declare
> the BLAS/LAPACK interfaces in terms of Fortran (column-major) matrices
> (declared using pragma Convention (Fortran)) and have the compiler
> implement the necessary moves on assignment, rather than using a library
> Transpose subprogram. But make no mistake, the Generic_Real_Arrays code
> definitely copies! (look at the body for confirmation).
>
> > I suppose that since C is very common, there must already exist a very
> > good implementation of conversion of Ada arrays of floating point
> > types in C arrays of C.Float, idealy using the GNAT
> > implementation. This could save me a lot of time doing interfaces...
>
> GNAT and GCC share the same compiler backend, so the underlying numeric
> types are the same. Just tell the compiler what to import:
>
>    procedure dgeev
>      (Jobv_L :        Character;
>       Jobv_R :        Character;
>       N      :        Positive;
>       A      : in out Real_Arrays.Real_Matrix;
>       Ld_A   :        Positive;
>       W_R    :    out Real_Arrays.Real_Vector;
>       W_I    :    out Real_Arrays.Real_Vector;
>       V_L    :    out Real_Arrays.Real_Matrix;
>       Ld_V_L :        Integer;
>       V_R    :    out Real_Arrays.Real_Matrix;
>       Ld_V_R :        Integer;
>       Work   :    out Real_Arrays.Real_Vector;
>       L_Work :        Integer;
>       Info   :    out Integer);
>    pragma Import (Fortran, dgeev, "dgeev_");
>
> (Real_Arrays.Real_Matrix etc from Generic_Real_Arrays). The Fortran
> convention means that the Character and Integer arguments are passed by
> reference without your needing to fake up accesses.
>
> More of this athttps://sourceforge.net/projects/gnat-math-extn/.

Thank you Siomon for your answer.
You are right I will take some inspiration from your packages !
Since Gnat is so close to C, I hope I won't need copying.
Regards,

Vincent