  An emerging phase of matter among the class of topological materials is nodal
line semimetal, possessing symmetry-protected one-dimensional gapless lines at
the (or close to) the Fermi level in $k$-space. When the $k$-dispersion of the
nodal line is weak, van Hove singularities generated by the almost flat nodal
lines may be prone to instabilities introduced by additional perturbations such
as spin-orbit coupling or magnetism. Here, we study Cr-based ferromagnetic
chalcospinel compound CuCr$_2$Se$_4$ (CCS) via first-principles electronic
structure methods and reveal the true origin of its dissipationless anomalous
Hall conductivity, which was not well understood previously. We find that CCS
hosts nodal lines protected by nonsymmorphic symmetries, located in the
vicinity of Fermi level, and that such nodal lines are the origin of the
previously observed distinct behavior of the anomalous Hall signature in the
presence of electron doping. The splitting of nodal line via spin-orbit
coupling produces a large Berry curvature, which leads to a significant
response in anomalous Hall conductivity. Upon electron doping via chemical
substitution or gating, or rotation of magnetization via external magnetic
field, steep change of anomalous Hall behavior occurs, which makes CCS a
promising compound for low energy spintronics applications.
