mypyc/irbuild/nonlocalcontrol.py

"""Helpers for dealing with nonlocal control such as 'break' and 'return'.

Model how these behave differently in different contexts.
"""

from __future__ import annotations

from abc import abstractmethod
from typing import TYPE_CHECKING

from mypyc.ir.ops import (
    NO_TRACEBACK_LINE_NO,
    BasicBlock,
    Branch,
    Goto,
    Integer,
    Register,
    Return,
    SetMem,
    Unreachable,
    Value,
)
from mypyc.ir.rtypes import object_rprimitive
from mypyc.irbuild.targets import AssignmentTarget
from mypyc.primitives.exc_ops import restore_exc_info_op, set_stop_iteration_value

if TYPE_CHECKING:
    from mypyc.irbuild.builder import IRBuilder


class NonlocalControl:
    """ABC representing a stack frame of constructs that modify nonlocal control flow.

    The nonlocal control flow constructs are break, continue, and
    return, and their behavior is modified by a number of other
    constructs.  The most obvious is loop, which override where break
    and continue jump to, but also `except` (which needs to clear
    exc_info when left) and (eventually) finally blocks (which need to
    ensure that the finally block is always executed when leaving the
    try/except blocks).
    """

    @abstractmethod
    def gen_break(self, builder: IRBuilder, line: int) -> None:
        pass

    @abstractmethod
    def gen_continue(self, builder: IRBuilder, line: int) -> None:
        pass

    @abstractmethod
    def gen_return(self, builder: IRBuilder, value: Value, line: int) -> None:
        pass


class BaseNonlocalControl(NonlocalControl):
    """Default nonlocal control outside any statements that affect it."""

    def gen_break(self, builder: IRBuilder, line: int) -> None:
        assert False, "break outside of loop"

    def gen_continue(self, builder: IRBuilder, line: int) -> None:
        assert False, "continue outside of loop"

    def gen_return(self, builder: IRBuilder, value: Value, line: int) -> None:
        builder.add(Return(value))


class LoopNonlocalControl(NonlocalControl):
    """Nonlocal control within a loop."""

    def __init__(
        self, outer: NonlocalControl, continue_block: BasicBlock, break_block: BasicBlock
    ) -> None:
        self.outer = outer
        self.continue_block = continue_block
        self.break_block = break_block

    def gen_break(self, builder: IRBuilder, line: int) -> None:
        builder.add(Goto(self.break_block))

    def gen_continue(self, builder: IRBuilder, line: int) -> None:
        builder.add(Goto(self.continue_block))

    def gen_return(self, builder: IRBuilder, value: Value, line: int) -> None:
        self.outer.gen_return(builder, value, line)


class GeneratorNonlocalControl(BaseNonlocalControl):
    """Default nonlocal control in a generator function outside statements."""

    def gen_return(self, builder: IRBuilder, value: Value, line: int) -> None:
        # Assign an invalid next label number so that the next time
        # __next__ is called, we jump to the case in which
        # StopIteration is raised.
        builder.assign(builder.fn_info.generator_class.next_label_target, Integer(-1), line)

        # Raise a StopIteration containing a field for the value that
        # should be returned. Before doing so, create a new block
        # without an error handler set so that the implicitly thrown
        # StopIteration isn't caught by except blocks inside of the
        # generator function.
        builder.builder.push_error_handler(None)
        builder.goto_and_activate(BasicBlock())

        # Skip creating a traceback frame when we raise here, because
        # we don't care about the traceback frame and it is kind of
        # expensive since raising StopIteration is an extremely common
        # case.  Also we call a special internal function to set
        # StopIteration instead of using RaiseStandardError because
        # the obvious thing doesn't work if the value is a tuple
        # (???).

        true, false = BasicBlock(), BasicBlock()
        stop_iter_reg = builder.fn_info.generator_class.stop_iter_value_reg
        assert stop_iter_reg is not None

        builder.add(Branch(stop_iter_reg, true, false, Branch.IS_ERROR))

        builder.activate_block(true)
        # The default/slow path is to raise a StopIteration exception with
        # return value.
        builder.call_c(set_stop_iteration_value, [value], NO_TRACEBACK_LINE_NO)
        builder.add(Unreachable())
        builder.builder.pop_error_handler()

        builder.activate_block(false)
        # The fast path is to store return value via caller-provided pointer
        # instead of raising an exception. This can only be used when the
        # caller is a native function.
        builder.add(SetMem(object_rprimitive, stop_iter_reg, value))
        builder.add(Return(Integer(0, object_rprimitive)))


class CleanupNonlocalControl(NonlocalControl):
    """Abstract nonlocal control that runs some cleanup code."""

    def __init__(self, outer: NonlocalControl) -> None:
        self.outer = outer

    @abstractmethod
    def gen_cleanup(self, builder: IRBuilder, line: int) -> None: ...

    def gen_break(self, builder: IRBuilder, line: int) -> None:
        self.gen_cleanup(builder, line)
        self.outer.gen_break(builder, line)

    def gen_continue(self, builder: IRBuilder, line: int) -> None:
        self.gen_cleanup(builder, line)
        self.outer.gen_continue(builder, line)

    def gen_return(self, builder: IRBuilder, value: Value, line: int) -> None:
        self.gen_cleanup(builder, line)
        self.outer.gen_return(builder, value, line)


class TryFinallyNonlocalControl(NonlocalControl):
    """Nonlocal control within try/finally."""

    def __init__(self, target: BasicBlock) -> None:
        self.target = target
        self.ret_reg: None | Register | AssignmentTarget = None

    def gen_break(self, builder: IRBuilder, line: int) -> None:
        builder.error("break inside try/finally block is unimplemented", line)

    def gen_continue(self, builder: IRBuilder, line: int) -> None:
        builder.error("continue inside try/finally block is unimplemented", line)

    def gen_return(self, builder: IRBuilder, value: Value, line: int) -> None:
        if self.ret_reg is None:
            if builder.fn_info.is_generator:
                self.ret_reg = builder.make_spill_target(builder.ret_types[-1])
            else:
                self.ret_reg = Register(builder.ret_types[-1])
        # assert needed because of apparent mypy bug... it loses track of the union
        # and infers the type as object
        assert isinstance(self.ret_reg, (Register, AssignmentTarget)), self.ret_reg
        builder.assign(self.ret_reg, value, line)

        builder.add(Goto(self.target))


class ExceptNonlocalControl(CleanupNonlocalControl):
    """Nonlocal control for except blocks.

    Just makes sure that sys.exc_info always gets restored when we leave.
    This is super annoying.
    """

    def __init__(self, outer: NonlocalControl, saved: Value | AssignmentTarget) -> None:
        super().__init__(outer)
        self.saved = saved

    def gen_cleanup(self, builder: IRBuilder, line: int) -> None:
        builder.call_c(restore_exc_info_op, [builder.read(self.saved)], line)


class FinallyNonlocalControl(CleanupNonlocalControl):
    """Nonlocal control for finally blocks.

    Just makes sure that sys.exc_info always gets restored when we
    leave and the return register is decrefed if it isn't null.
    """

    def __init__(self, outer: NonlocalControl, saved: Value) -> None:
        super().__init__(outer)
        self.saved = saved

    def gen_cleanup(self, builder: IRBuilder, line: int) -> None:
        # Restore the old exc_info
        target, cleanup = BasicBlock(), BasicBlock()
        builder.add(Branch(self.saved, target, cleanup, Branch.IS_ERROR))
        builder.activate_block(cleanup)
        builder.call_c(restore_exc_info_op, [self.saved], line)
        builder.goto_and_activate(target)