Another interesting application of popcount is in computer vision, namely in matching keypoints that use binary descriptors for 3D reconstruction in SLAM/TRN etc
Horror story: I was once developing a TRN system for a spacecraft instrument which uses an ancient x86 processor that does not have popcnt, ended up using a look-up table instead...
It seems that look up tables are still faster when memory allows it though (http://www.dalkescientific.com/writings/diary/archive/2008/0...)