def tdot_blas(mat, out=None):
    """returns np.dot(mat, mat.T), but faster for large 2D arrays of doubles."""
    if (mat.dtype != 'float64') or (len(mat.shape) != 2):
        return np.dot(mat, mat.T)
    nn = mat.shape[0]
    if out is None:
        out = np.zeros((nn, nn))
    else:
        assert(out.dtype == 'float64')
        assert(out.shape == (nn, nn))
        # FIXME: should allow non-contiguous out, and copy output into it:
        assert(8 in out.strides)
        # zeroing needed because of dumb way I copy across triangular answer
        out[:] = 0.0

    # # Call to DSYRK from BLAS
    mat = np.asfortranarray(mat)
    out = blas.dsyrk(alpha=1.0, a=mat, beta=0.0, c=out, overwrite_c=1,
                     trans=0, lower=0)

    symmetrify(out, upper=True)
    return np.ascontiguousarray(out)