[fulafel]

The RISC way is to prove the exception is worth it. There are a lot of possible specialized instructions that are useful for some workloads.

[PeCaN]

ARM has it. DEC Alpha had it (before x86, even).

I get that there are a lot of narrow-use instructions but popcount is a pretty well-known and common operation.

[fulafel]

In the Alpha case it was a very late addition, in the last widely shipping version of the chip and IIRC was speculated to be part of some supercomputer/classified use case. ARM has a history of having quirky un-RISCy instructions.

(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)

[ncmncm]

Late addition is more indicative of value than appearing in first releases. People guess about the base instruction set, but additions happen only in response to high demand.