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From:   Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date:   Fri, 8 Dec 2023 17:19:10 +0100
Message-ID: <CANiq72k4H2_NZuQcpeKANqyi_9W01fLC0WxXon5cx4z=WsgeXQ@mail.gmail.com>
Subject: Re: [PATCH 5/7] rust: file: add `Kuid` wrapper
To:     comex <comexk@gmail.com>
Cc:     Christian Brauner <brauner@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alice Ryhl <aliceryhl@google.com>,
        Miguel Ojeda <ojeda@kernel.org>,
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On Fri, Dec 8, 2023 at 6:28=E2=80=AFAM comex <comexk@gmail.com> wrote:
>
> Regarding the issue of wrappers not being inlined, it's possible to get L=
LVM 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=3Dx86_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 b=
e redundant in cases where kernel-wide LTO is enabled.
>
> Using this approach might slow down compilation a bit due to needing to p=
ass the LLVM bitcode between multiple commands, but probably not very much.
>
> Just chiming in as someone not involved in Rust for Linux but familiar wi=
th these tools.  Perhaps this has been considered before and rejected for s=
ome reason; I wouldn=E2=80=99t 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 Bj=C3=B6rn
perhaps).

If LLVM folks think LLVM-wise nothing will break, then we are happy to
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