instr = (instr << 5) | ra
            if opcode == 'grevi' or opcode == 'grevi.':
                assert 0 <= rb_imm < 64
                instr = (instr << 6) | rb_imm
                instr = (instr << 9) | xo
            else:
                assert 0 <= rb_imm < 32
                instr = (instr << 5) | rb_imm
                instr = (instr << 10) | xo