hw/ip/otbn/util/shared/constants.py

#!/usr/bin/env python3
# Copyright lowRISC contributors.
# Licensed under the Apache License, Version 2.0, see LICENSE for details.
# SPDX-License-Identifier: Apache-2.0

from typing import Dict, Optional

from .insn_yaml import Insn
from .operand import RegOperandType

def get_op_val_str(insn: Insn, op_vals: Dict[str, int], opname: str) -> str:
    '''Get the value of the given (register) operand as a string.'''
    op = insn.name_to_operand[opname]
    assert isinstance(op.op_type, RegOperandType)
    return op.op_type.op_val_to_str(op_vals[opname], None)

class ConstantContext:
    '''Represents known-constant GPRs.

    This datatype is used to track and evaluate GPR pointers for indirect
    references.
    '''
    def __init__(self, values: Dict[str,int]):
        # The x0 register needs to always be 0
        assert values.get('x0', None) == 0
        self.values = values.copy()

    @staticmethod
    def empty() -> 'ConstantContext':
        '''Represents a context with no known constants.'''
        return ConstantContext({'x0': 0})

    def set(self, gpr: str, value: int) -> None:
        '''Set the value of a GPR in the context.'''
        if gpr == 'x0':
            # Ignore writes to x0; it's read-only.
            return
        self.values[gpr] = value

    def get(self, gpr: str) -> Optional[int]:
        '''Get the value of a GPR in the context.'''
        return self.values.get(gpr, None)

    def __contains__(self, gpr: str) -> bool:
        return gpr in self.values

    def copy(self) -> 'ConstantContext':
        return ConstantContext(self.values)

    def intersect(self, other: 'ConstantContext') -> 'ConstantContext':
        '''Returns a new context with only values on which self/other agree.

        Does not modify self or other.
        '''
        out = {}
        for k,v in self.values.items():
            if other.get(k) == v:
                out[k] = v
        return ConstantContext(out)

    def update_insn(self, insn: Insn, op_vals: Dict[str, int]) -> None:
        '''Updates to new known constant values GPRs after the instruction.

        Currently, this procedure supports only a limited set of instructions.
        Since constant values only need to be known in order to decode indirect
        references to WDRs and loop counts, this set is chosen based on operations
        likely to happen to those registers: `addi`, `lui`, and bignum instructions
        containing `_inc` op_vals.
        '''
        new_values = {}
        if insn.mnemonic == 'addi':
            grs1_name = get_op_val_str(insn, op_vals, 'grs1')
            if grs1_name in self.values:
                grd_name = get_op_val_str(insn, op_vals, 'grd')
                # Operand is a constant; add/update grd
                new_values[grd_name]= self.values[grs1_name] + op_vals['imm']
        elif insn.mnemonic == 'lui':
            grd_name = get_op_val_str(insn, op_vals, 'grd')
            new_values[grd_name] = op_vals['imm'] << 12
        else:
            # If the instruction has any op_vals ending in _inc,
            # assume we're incrementing the corresponding register
            for op in insn.operands:
                if op.name.endswith('_inc'):
                    # If reg to be incremented is a constant, increment it
                    inc_op = op.name[:-(len('_inc'))]
                    inc_name = get_op_val_str(insn, op_vals, inc_op)
                    if inc_name in self.values:
                        new_values[inc_name] = self.values[inc_name] + 1

        # If the instruction's information-flow graph indicates that we updated any
        # constant register other than the ones handled above, the value of that
        # register can no longer be determined; remove it from the constants
        # dictionary.
        iflow = insn.iflow.evaluate(op_vals, self.values)
        for sink in iflow.all_sinks():
            # Remove from self.values if key exists
            self.values.pop(sink, None)

        self.values.update(new_values)