Hacker News Reader

Comment by 1f60c | Hacker News Reader

I'm pretty sure catching this in all cases would involve solving the Halting Problem.

If you're allowed to have false positives, it's not so hard. Like "this function allocates if A or B is true" just becomes "this function can allocate". It's not so different from finding functions that can panic.

I’m fairly certain that this does not require solving the halting problem. Fundamentally, every line of code can be transformed to the underlying code constructs, and those could be checked for allocations. Doing this in an efficient manner is probably hard, but certainly not impossible

    fn maybe_alloc() -> Vec<usize> {
        if collatz_conjecture_is_true() {
            vec![42]
        } else {
            vec![]
        }
    }

    fn main() {
        let v = maybe_alloc(); // Does this allocate?
    }

You would just assume it does. Really you want to detect all places that might allocate.

You are right. It’s definitely possible if you accept false positives.

I think if we wanted a concrete answer you’d be right.

But I feel like a static analyzer could clearly identify “this line can allocate” just by knowing what language features can allocate. And for third party library methods “does this function have anything inside of it that could allocate?”

You can absolutely statically determine where allocations happen.

You can't know for sure if all of those places are reachable in a running program, but that's not necessary to solve this problem.

Reply on news.ycombinator.com