They probably used the CPU -- I remember reading somewhere that Carmack also wanted to implement a beam-tree (3D BSP) for faster culling of shadow surfaces, but

They probably used the CPU -- I remember reading somewhere that Carmack also wanted to implement a beam-tree (3D BSP) for faster culling of shadow surfaces, but I am not sure. I did some experiments with Shadows myself 10 years ago, on that era of CPUs and GPUs and the bottleneck for me was drawing the volumes in the stencil buffer.