(Updated 2025-06-02 https://github.com/riscv/riscv-opcodes/tree/d6b540068aeb1ec442f1bb2d30f0b15860984aad)
If you really want to use this janky code:
$ git clone https://github.com/riscv/riscv-opcodes
$ ./parse.py 'rv_*' 'rv64_*'
$ python3 free-space.py