Class KeepAlive

mypyc/ir/ops.py:1821–1866

A no-op operation that ensures source values aren't freed. This is sometimes useful to avoid decref when a reference is still being held but not seen by the compiler. A typical use case is like this (C-like pseudocode): ptr = &x.item r = *ptr keep_alive x # x must n

@final
class KeepAlive(RegisterOp):
    """A no-op operation that ensures source values aren't freed.

    This is sometimes useful to avoid decref when a reference is still
    being held but not seen by the compiler.

    A typical use case is like this (C-like pseudocode):

      ptr = &x.item
      r = *ptr
      keep_alive x  # x must not be freed here
      # x may be freed here

    If we didn't have "keep_alive x", x could be freed immediately
    after taking the address of 'item', resulting in a read after free
    on the second line.

    If 'steal' is true, the value is considered to be stolen at
    this op, i.e. it won't be decref'd. You need to ensure that
    the value is freed otherwise, perhaps by using borrowing
    followed by Unborrow.

    Be careful with steal=True -- this can cause memory leaks.
    """

    error_kind = ERR_NEVER

    def __init__(self, src: list[Value], line: int = -1, *, steal: bool = False) -> None:
        super().__init__(line)
        assert src
        self.src = src
        self.steal = steal

    def sources(self) -> list[Value]:
        return self.src.copy()

    def stolen(self) -> list[Value]:
        if self.steal:
            return self.src.copy()
        return []

    def set_sources(self, new: list[Value]) -> None:
        self.src = new[:]

    def accept(self, visitor: OpVisitor[T]) -> T:
        return visitor.visit_keep_alive(self)


@final