A number of companies have asked for popcount to NOT be included in the base spec for B (certainly Zbb for embedded), because of the expense of implementing it

A number of companies have asked for popcount to NOT be included in the base spec for B (certainly Zbb for embedded), because of the expense of implementing it vs the relatively small speedup it givesfor quite specialized use-cases. Of course popcount will have an opcode allocated and binutils and gcc will (already do) know how to use it if it is present, so anyone who wants/needs it can implement it.