Ask HN: How to get POPCNT into the next RISC-V base or “C” set?

RISC-V is important, but its bit-manipulation capabilities are woefully weak. The proposed Bitmanip ("B") extension is very powerful, but large, so even if ultimately ratified it might never be supported in lower-cost implementations.

A huge part of the value of the Bitmanip extension would be provided by a single, minimal but fast POPCNT instruction. (NSA demanded POPCNT in its Atlas, Stretch, CDC, and Cray machines for good reasons.)

How does one go about getting POPCNT into the next release of the base or "C" compressed instruction set? Is there any possibility of getting it into "C" set but not the base set? Of Bitmanip claiming any part of the "C" instruction code space?