Using FLENS-BLAS

FLENS-BLAS is built on top of CXXBLAS and utilizes FLENS matrix/vector types. Main benefits are:

It is easier to use as functions from FLENS-BLAS require less arguments
Technical details like are encapsulated in the implementation and hidden from the user. You pass complete matrix/vector types instead of things like pointers and dimensions. This makes your code less error prone.

Toy Example

We use the same example as before, i.e. we compute \(y = \beta y + \alpha\,A^T x + z\) where \(\alpha, \beta\) are scalars, \(x, y, z\) are (dense) vectors and \(A\) is a (general) matrix (with full storage).

Using FLENS-BLAS for this task we

first call mv which internally calls gemv from CXXBLAS
and subsequently axpy which internnaly calls axpy from CXXBLAS.

As long as you do not compile with -DNDEBUG the FLENS-BLAS layer also does some error-checking on its arguments.

Example Code

flens/examples/tut02-page02-example.cc

#include <cxxstd/iostream.h>

#include <flens/flens.cxx>

using namespace flens;

using namespace std;

int

main()

{

    typedef double                               T;

    typedef DenseVector<Array<T> >               DEVector;

    typedef GeMatrix<FullStorage<T, ColMajor> >  GEMatrix;

    const T  alpha = 1.5,

             beta = 2.5;

    DEVector x(3), y(3), z(3);

    x = 1, 2, 3;

    y = 2, 3, 4;

    z = 3, 4, 5;

    GEMatrix A(3,3);

    A = 1, 2, 3,

        5, 6, 7,

        5, 4, 3;

    blas::mv(Trans, alpha, A, x, beta, y);

    blas::axpy(T(1), z, y);

    cout << "y = " << y << endl;

    return 0;

}

Comments on Example Code

Compute \(y = \beta y + \alpha A^T x\)

blas::mv(Trans, alpha, A, x, beta, y);

Compute the update \(y = y + z\)

blas::axpy(T(1), z, y);

Compile and Run

$shell> cd flens/examples                                                         
$shell> g++ -Wall -std=c++11 -I../.. tut02-page02-example.cc                      
$shell> ./a.out                                                                   
y = 
           47           50.5             54