[gsmecher]

Note that you can't compare LUT4 results (ZPU @ 440 LUTs) against LUT6 results (PicoRV32 @ 750 LUTs). The ZPU is remarkably small, and it's a bigger gap than a direct comparison shows.

SERV is a fair comparison, since it's architected for 4LUTs and I suspect the synthesis results come from iCE40 tools.

I have a contender in the "very small" space, too [1], although I don't claim it's as mature or complete as SERV. (If Minimax was excluded from your post on the basis of insanity, I'm OK with that.)

[1] https://github.com/gsmecher/minimax