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I’ve heard really good things about SuperH (sh-2, sh-4 etc.), designed by Hitachi for their processors including those used in the Sega Saturn and Sega Dreamcast. Really high code density was one big thing, but it was covered by patents until recently. There was a group trying to make a modern open source processor in VHDL for FPGAs and later on ASICs based on it, but I think it may have mostly fizzled out.

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I do Dreamcast homebrew, and my opinion is the SH ISA is way over-optimized for 90's embedded applications. It wastes the tiny instruction space on useless things like read-modify-write bit operations, intended for setting hardware registers, and has a laughably small displacement for function calls (+/-2KB, with pretty much all calls requiring manually loading a function pointer from RAM). There are parts that are still nice compared to something like RISC-V, like post-increment loads and pre-decrement stores (but hardware designers seem to hate things like that, since they require an extra write port on the register file), and the code density can be pretty good (although GCC's output is awful), but there are so many ways the ISA could be easily improved.

(sorry for being off-topic) regarding https://news.ycombinator.com/item?id=16308439, I wonder how is this project going?

I haven't done much on it since then. While it's something I would like to finish, I realized that there are other, higher priority things that I should do first.

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