hw/ip/otbn/dv/otbnsim/sim/isa.py

# Copyright lowRISC contributors.
# Licensed under the Apache License, Version 2.0, see LICENSE for details.
# SPDX-License-Identifier: Apache-2.0

import sys
from typing import Dict, Optional, Tuple

from shared.insn_yaml import Insn, load_insns_yaml

from .state import OTBNState


# Load the insns.yml file at module load time: we'll use its data while
# declaring the classes. The point is that an OTBNInsn below is an instance of
# a particular Insn object from shared.insn_yaml, so we want a class variable
# on the OTBNInsn that points at the corresponding Insn.
try:
    _INSNS_FILE = load_insns_yaml()
except RuntimeError as err:
    sys.stderr.write('{}\n'.format(err))
    sys.exit(1)


class DecodeError(Exception):
    '''An error raised when trying to decode malformed instruction bits'''
    def __init__(self, msg: str):
        self.msg = msg

    def __str__(self) -> str:
        return 'DecodeError: {}'.format(self.msg)


class DummyInsn(Insn):
    '''A dummy instruction that will never be decoded. Used for the insn class
    variable in the OTBNInsn base class.

    '''
    def __init__(self) -> None:
        fake_yml = {
            'mnemonic': 'dummy-insn',
            'operands': []
        }
        super().__init__(fake_yml, None)


def insn_for_mnemonic(mnemonic: str, num_operands: int) -> Insn:
    '''Look up the named instruction in the loaded YAML data.

    To make sure nothing's gone really wrong, make sure it has the expected
    number of operands. If we fail to find the right instruction, print a
    message to stderr and exit (rather than raising a RuntimeError: this
    happens on module load time, so it's a lot clearer to the user what's going
    on this way).

    '''
    insn = _INSNS_FILE.mnemonic_to_insn.get(mnemonic)
    if insn is None:
        sys.stderr.write('Failed to find an instruction for mnemonic {!r} in '
                         'insns.yml.\n'
                         .format(mnemonic))
        sys.exit(1)

    if len(insn.operands) != num_operands:
        sys.stderr.write('The instruction for mnemonic {!r} in insns.yml has '
                         '{} operands, but we expected {}.\n'
                         .format(mnemonic, len(insn.operands), num_operands))
        sys.exit(1)

    return insn


class OTBNInsn:
    '''A decoded OTBN instruction.

    '''

    # A class variable that holds the Insn subclass corresponding to this
    # instruction.
    insn = DummyInsn()  # type: Insn

    # A class variable that is set by Insn subclasses that represent
    # instructions that affect control flow (and are not allowed at the end of
    # a loop).
    affects_control = False

    def __init__(self, raw: int, op_vals: Dict[str, int]):
        self.raw = raw
        self.op_vals = op_vals

        # Memoized disassembly for this instruction. We store the PC at which
        # we disassembled too (which should be the same next time around, but
        # it can't hurt to check).
        self._disasm = None  # type: Optional[Tuple[int, str]]

    def execute(self, state: OTBNState) -> None:
        raise NotImplementedError('OTBNInsn.execute')

    def disassemble(self, pc: int) -> str:
        '''Generate an assembly listing for this instruction'''
        if self._disasm is not None:
            old_pc, old_disasm = self._disasm
            assert pc == old_pc
            return old_disasm

        disasm = self.insn.disassemble(pc, self.op_vals, 12)
        self._disasm = (pc, disasm)
        return disasm

    @staticmethod
    def as_u32(value: int) -> int:
        '''Interpret the signed value as a 2's complement u32'''
        assert -(1 << 31) <= value < (1 << 31)
        return (1 << 32) + value if value < 0 else value


class RV32RegReg(OTBNInsn):
    '''A general class for register-register insns from the RV32I ISA'''
    def __init__(self, raw: int, op_vals: Dict[str, int]):
        super().__init__(raw, op_vals)
        self.grd = op_vals['grd']
        self.grs1 = op_vals['grs1']
        self.grs2 = op_vals['grs2']


class RV32RegImm(OTBNInsn):
    '''A general class for register-immediate insns from the RV32I ISA'''
    def __init__(self, raw: int, op_vals: Dict[str, int]):
        super().__init__(raw, op_vals)
        self.grd = op_vals['grd']
        self.grs1 = op_vals['grs1']
        self.imm = op_vals['imm']


class RV32ImmShift(OTBNInsn):
    '''A general class for immediate shift insns from the RV32I ISA'''
    def __init__(self, raw: int, op_vals: Dict[str, int]):
        super().__init__(raw, op_vals)
        self.grd = op_vals['grd']
        self.grs1 = op_vals['grs1']
        self.shamt = op_vals['shamt']


def logical_byte_shift(value: int, shift_type: int, shift_bytes: int) -> int:
    '''Logical shift value by shift_bytes to the left or right.

    value should be an unsigned 256-bit value. shift_type should be 0 (shift left)
    or 1 (shift right): matching the encoding of the big number instructions.
    shift_bytes should be a non-negative number of bytes to shift by.

    Returns an unsigned 256-bit value, truncating on an overflowing left shift.

    '''
    mask256 = (1 << 256) - 1
    assert 0 <= value <= mask256
    assert 0 <= shift_type <= 1
    assert 0 <= shift_bytes

    shift_bits = 8 * shift_bytes
    shifted = value << shift_bits if shift_type == 0 else value >> shift_bits
    return shifted & mask256