Re: [PATCH 5/7] rust: file: add `Kuid` wrapper

[Nick Desaulniers]

On Fri, Dec 8, 2023 at 8:19 AM Miguel Ojeda
<miguel.ojeda.sandonis@xxxxxxxxx> wrote:
>
> On Fri, Dec 8, 2023 at 6:28 AM comex <comexk@xxxxxxxxx> wrote:
> >
> > Regarding the issue of wrappers not being inlined, it's possible to get LLVM to optimize C and Rust code together into an object file, with the help of a compatible Clang and LLD:
> >
> > @ rustc -O --emit llvm-bc a.rs
> > @ clang --target=x86_64-unknown-linux-gnu -O2 -c -emit-llvm -o b.bc b.c
> > @ ld.lld -r -o c.o a.bc b.bc
> >
> > Basically LTO but within the scope of a single object file.  This would be redundant in cases where kernel-wide LTO is enabled.
> >
> > Using this approach might slow down compilation a bit due to needing to pass the LLVM bitcode between multiple commands, but probably not very much.
> >
> > Just chiming in as someone not involved in Rust for Linux but familiar with these tools.  Perhaps this has been considered before and rejected for some reason; I wouldn’t know.
>
> Thanks comex for chiming in, much appreciated.
>
> Yeah, this is what we have been calling the "local-LTO hack" and it
> was one of the possibilities we were considering for non-LTO kernel
> builds for performance reasons originally. I don't recall who
> originally suggested it in one of our meetings (Gary or Björn
> perhaps).
>
> If LLVM folks think LLVM-wise nothing will break, then we are happy to

On paper, nothing comes to mind.  No promises though.

>From a build system perspective, I'd rather just point users towards
LTO if they have this concern.  We support full and thin lto.  This
proposal would add a third variant for just rust drivers.  Each
variation on LTO has a maintenance cost and each have had their own
distinct fun bugs in the past.  Not sure an additional variant is
worth the maintenance cost, even if it's technically feasible.

> go ahead with that (since it also solves the performance side), but it
> would be nice to know if it will always be OK to build like that, i.e.
> I think Andreas actually tried it and it seemed to work and boot, but
> the worry is whether there is something subtle that could have bad
> codegen in the future.
>
> (We will also need to worry about GCC.)
>
> Cheers,
> Miguel



-- 
Thanks,
~Nick Desaulniers