[thrtythreeforty]

For those curious, Xtensa is a similar embeddable architecture (known especially for its use in the ESP32 microcontroller) that allows broad latitude to the designer to customize its instruction set with custom acceleration. The integration is very good, the compiler recognizes the new intrinsics and the designer has control over how the instruction is pipelined into the main processor.

Unfortunately it's very proprietary, and as far as I know there isn't an at-home version you can play with on FPGAs. But this kind of thing does exist if you can afford it - you don't have to roll your own RTL.