Why popcount?
it's honestly pretty baffling that RISC-V doesnt have it (perils of design-by-academia)
I get that there are a lot of narrow-use instructions but popcount is a pretty well-known and common operation.
(edit: also it seems that ARM has just cnt.v8 for counting 8-bit lanes in NEON and no 64-bit scalar instruction version, interesting. Being part of NEON also means it's an optional part on ARM)
Because it provides order-of magnitude speed improvements for numerous algorithms. Because it is the basis for integer log-base-2.
If you don't know what popcount is good for, or badly miss it where it is lacking, your education has suffered. That is remediable.