Re: [PATCH 04/13] Kbuild: Rust support

[Miguel Ojeda]

On Fri, Apr 16, 2021 at 10:58 PM Willy Tarreau <w@xxxxxx> wrote:
>
> No, two:
>   - ok in %rax (seems like it's "!ok" technically speaking since it
>     returns 1 on !ok and 0 on ok)
>   - foo_or_err in %rdx

Yes, but that is the implementation -- conceptually you only have one
or the other, and Rust won't allow you to use the wrong one.

> However then I'm bothered because Miguel's example showed that regardless
> of OK, EINVAL was always returned in foo_or_err, so maybe it's just
> because his example was not well chosen but it wasn't very visible from
> the source:

That is the optimizer being fancy since the error can be put
unconditionally in `rdx`.

If you compile:

    pub fn it_is_ok() -> KernelResult<Bar> {
        Ok(Bar)
    }

you will see it just clears `rax`.

Cheers,
Miguel