I thought we had filed a Rust issue on this but apparently not. There's some
good discussion in this issue:
https://github.com/rust-lang/rust/issues/37530
For Firefox, simply having an equivalent to `-fvisilbility=hidden` so that all
the symbols in the generated static library are hidden would be sufficient to
meet our current needs, since we don't need to export any actual public APIs
from Rust code. For that feature to be usable for more use cases you'd probably
also want an equivalent to `__attribute__ ((visibility ("default")))`, but I
don't know what exactly that'd look like.
-Ted
On Wed, Aug 29, 2018, at 3:38 AM, Michael Woerister wrote:
> For some background: The Rust compiler will mark everything as `hidden` in
> LLVM IR unless it is exported via a C interface (via either #[no_mangle] or
> #[export_name]). So the FFI symbols in gkrust will have default visibility.
> LLD will thus probably retain and re-export them, even if they are not
> called from within libXUL. But we should double-check that.
>
> It might be a good idea to provide a way to tell the Rust compiler to make
> everything `hidden` by default.
>
> On Tue, Aug 28, 2018 at 3:56 PM, Till Schneidereit <
> t...@tillschneidereit.net> wrote:
>
> > CC'ing mw, who is working on cross-language LTO.
> >
> > On Tue, Aug 28, 2018 at 3:20 PM Mike Hommey <m...@glandium.org> wrote:
> >
> >> On Tue, Aug 28, 2018 at 09:14:02AM -0400, Jeff Muizelaar wrote:
> >> > We don't LTO yet on Mac.
> >>
> >> We don't LTO across languages on any platform yet. Rust is LTOed on all
> >> platforms, which removes a bunch of its symbols. Everything that is
> >> exposed for C/C++ from Rust, though, is left alone. That's likely to
> >> stay true even with cross-language LTO, because as far as the linker is
> >> concerned, those FFI symbols might be used by code that link against
> >> libxul, so it would still export them. We'd essentially need the
> >> equivalent to -fvisibility=hidden for Rust for that to stop being true.
> >>
> >> Mike
> >>
> >>
> >> > On Tue, Aug 28, 2018 at 5:17 AM, Emilio Cobos Álvarez <emi...@crisal.io>
> >> wrote:
> >> > >
> >> > >
> >> > > On 8/28/18 9:35 AM, Henri Sivonen wrote:
> >> > >>
> >> > >> Does some lld mechanism successfully remove dead code when gkrust
> >> > >> exports some FFI function that the rest of Gecko never ends up
> >> > >> calling?
> >> > >
> >> > >
> >> > > I would expect LTO to get rid of it, but haven't checked myself.
> >> > >
> >> > >> I.e. in terms of code size, is it OK to vendor an FFI-exposing Rust
> >> > >> crate where not every FFI function is used (at least right away)?
> >> > >>
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