Problem is, there are a lot of RISC-V instruction way longer than that (like th.vslide1down.vx) so hashing is going to be slow.

You could copy the instruction to a 16 byte sized buffer and hash the one/two int64s. Looking at the code sample in the article, there wasn't a single instruction longer than 5 characters, and I suspect that in general instructions with short names are more common than those with long names.

This last fact might actually support the current model, as it grows linearly-ish in the size of the instruction, instead of being constant like hash.

Note th.vslide1down.vx is a T-Head instruction, a vendor custom extension.

It is not part of RISC-V, nor supported by any CPUs outside of that vendors' own.

Is there a handy list of all RISC-V instructions?