message: Add clarification comments in HypreParMatrix::ExtractSubmatrix
diff: @@ -1538,6 +1538,7 @@ HypreParMatrix *HypreParMatrix::ExtractSubmatrix(const Array<int> &indices,
  // Form hypre CF-splitting array designating submatrix as F-points (-1)
  #ifdef hypre_IntArrayData
  + // hypre_BoomerAMGCoarseParms needs CF_marker to be hypre_IntArray *
  hypre_IntArray *CF_marker;
  CF_marker = hypre_IntArrayCreate(local_num_vars);
  @@ -1554,6 +1555,7 @@ HypreParMatrix *HypreParMatrix::ExtractSubmatrix(const Array<int> &indices,
  MFEM_WARNING("WARNING : " << indices[j] << " > " << local_num_vars);
  }
  #ifdef hypre_IntArrayData
  + // Note: CF_marker is host or UVM memory, so we can modify it on host:
  hypre_IntArrayData(CF_marker)[indices[j]] = -1;
  #else
  CF_marker[indices[j]] = -1;
  