qd-lapack-getrf.cc (flens/examples/qd-lapack-getrf.cc)

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#include <cxxstd/iostream.h>

///
///  Include the qd-headers
///
#include <qd/qd_real.h>
#include <qd/fpu.h>

#include <flens/flens.cxx>

using namespace std;
using namespace flens;

///
///  For using quad-double precision in this example change the typedef to
///
typedef qd_real   T;

///
///  For understanding the next code lines we first take a look at the
///  __QD Library__ documentation:
///
///  *--[BOX]------------------------------------------------------------*
///  |                                                                   |
///  | The algorithms in the QD library assume IEEE double precision     |
///  | floating point arithmetic. Since Intel x86 processors have        |
///  | extended (80-bit) floating point registers, the round-to-double   |
///  | flag must be enabled in the control word of the FPU for this      |
///  | library to function properly under x86 processors. The function   |
///  | `fpu_fix_start` turns on the round-to-double bit in the FPU       |
///  | control word, while `fpu_fix_end` will restore the original       |
///  | state.                                                            |
///  |                                                                   |
///  *-------------------------------------------------------------------*
///
///  So the first thing we do in main is turning on the correct rounding ...
///
int
main()
{
    unsigned int old_cw;
    fpu_fix_start(&old_cw);

    typedef GeMatrix<FullStorage<T> >           Matrix;
    typedef Matrix::IndexType                   IndexType;
    typedef DenseVector<Array<IndexType> >      IndexVector;

    const Underscore<IndexType> _;

    const IndexType m = 4,
                    n = 5;

    Matrix            Ab(m, n);
    IndexVector       piv(m);

    Ab =  2,   3,  -1,   0,  20,
         -6,  -5,   0,   2, -33,
          2,  -5,   6,  -6, -43,
          4,   6,   2,  -3,  49;

    cout << "Ab = " << Ab << endl;

    lapack::trf(Ab, piv);

    const auto A = Ab(_,_(1,m));
    auto       B = Ab(_,_(m+1,n));

    blas::sm(Left, NoTrans, T(1), A.upper(), B);

    cout << "X = " << B << endl;

    ///
    ///  ... and at the end restore FPU rounding behavior as mentioned above.
    ///
    fpu_fix_end(&old_cw);
}

///
///  :links: __QD Library__ -> http://crd-legacy.lbl.gov/~dhbailey/mpdist/
///