implement named return value optimization in the frontend

[steveklabnik]

Issue by mahkoh
Monday Oct 27, 2014 at 13:24 GMT

For earlier discussion, see rust-lang/rust#18363

This issue was labelled with: I-enhancement, I-slow in the Rust repository

---

extern crate test;

pub struct X {
    x: [u8, ..1 << 10],
}

pub fn f() -> X {
    let mut x: X = unsafe { std::mem::uninitialized() };
    for i in range(0, x.x.len()) {
        x.x[i] = i as u8;
    }
    x
}

fn main() {
    let x = f();
    test::black_box(&x);
}

This function needs 2k stack space even though f is being inlined.

[michaelwu]

Having this would help a bunch with the spidermonkey upgrade in servo. Spidermonkey needs us to provide pointers to structs on the stack, but we can't easily do it in the constructor function since the struct is moved when returned. We're currently using return_address() to work around this, but this doesn't scale well when constructors are nested.

[briansmith]

Wasn't it a mistake to move this to the RFC repo? It is just a missed optimization. I don't think it requires an RFC.

[emberian]

this should be closed in favor of rust-lang/rust#32966