[eigen] Re: Map and pointer freeing in unsupported/Eigen/src/SparseExtra/CholmodSupport.h

[Douglas Bates <bates@xxxxxxxxxxxxx>]

On Sat, Jul 30, 2011 at 5:06 PM, Douglas Bates <bates@xxxxxxxxxxxxx> wrote:
>        /** \internal */
>        template<typename Rhs,typename Dest>
>        void _solve(const MatrixBase<Rhs> &b, MatrixBase<Dest> &dest) const
>        {
>            eigen_assert(m_factorizationIsOk && "The decomposition is not in
> a valid state for solving, you must first call either compute() or
> symbolic()/numeric()");
>            const Index size = m_cholmodFactor->n;
>            eigen_assert(size==b.rows());
>
>            // note: cd stands for Cholmod Dense
>            cholmod_dense b_cd = viewAsCholmod(b.const_cast_derived());
>            cholmod_dense* x_cd = cholmod_solve(m_solveType, m_cholmodFactor,
> &b_cd, &m_cholmod);
>            if(!x_cd)
>            {
>                this->m_info = NumericalIssue;
>            }
>            // TODO optimize this copy by swapping when possible (be carreful
> with alignment, etc.)
>            dest = Matrix<Scalar,Dest::RowsAtCompileTime,Dest::ColsAtCompileTime>::Map(reinterpret_cast<Scalar*>(x_cd->x),b.rows(),b.cols());
>            cholmod_free_dense(&x_cd, &m_cholmod);
>        }
>
> It appears to me that the last two lines change the dest argument to
> map the storage from the (x_cd->x) pointer then free that storage
> (cholmod_free_dense frees all the storage pointed to by the members of
> the cholmod_dense struct).

The same seems to be true in the _solve method for a SparseMatrix,
which ends with

	    // TODO optimize this copy by swapping when possible (be carreful
with alignment, etc.)
	    dest = viewAsEigen<DestScalar,DestOptions,DestIndex>(*x_cs);
	    M_cholmod_free_sparse(&x_cs, &m_cholmod);

viewAsEigen will return a MappedSparseMatrix with pointers to contents
that will be freed in the subsequent line, unless I am missing
something subtle.

The comments about swapping lead me to believe that the dest argument
has storage allocated on entry to the function, in which case the
values could be copied from the structs &x_cd or &x_cs.  I must admit
I get a little lost in the details of the sparse_solve_retval struct
so if anyone could tell me the status of the Dest argument when the
_solve method is called, I would appreciate it.  In particular, for a
dense return value, has the storage for the data already been
allocated?