Re: [PATCH v2 0/3] Inline helpers into Rust without full LTO

[Miguel Ojeda]

On Tue, Feb 3, 2026 at 12:34 PM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> Currently the only way for Rust code to call a static inline function is
> to go through a helper in rust/helpers/. This introduces performance
> costs due to additional function calls and also clutters backtraces and
> flame graphs with helper symbols.
>
> To get rid of these helper symbols, provide functionality to inline
> helpers into Rust using llvm-link. This option complements full LTO, by
> being much cheaper and avoiding incompatibility with BTF.

Applied to `rust-next` -- thanks everyone!

    [ Fixed typo. - Miguel ]

    [ Andreas writes:

        For the rnull driver, enabling helper inlining with this patch
        gives an average speedup of 2% over the set of 120 workloads that
        we publish on [2].

        Link: https://rust-for-linux.com/null-block-driver [2]

      This series also uncovered a pre-existing UB instance thanks to an
      `objtool` warning which I noticed while testing the series (details
      in the mailing list).

          - Miguel ]

    [ Some changes, apart from the rebase:

      - Added "(EXPERIMENTAL)" to Kconfig as the commit mentions.

      - Added `depends on ARM64 || X86_64` and `!UML` for now, since this is
        experimental, other architectures may require other changes (e.g.
        the issues I mentioned in the mailing list for ARM and UML) and they
        are not really tested so far. So let arch maintainers pick this up
        if they think it is worth it.

      - Gated the `cmd_ld_single` step also into the new mode, which also
        means that any possible future `objcopy` step is done after the
        translation, as expected.

      - Added `.gitignore` for `.bc` with exception for existing script.

      - Added `part-of-*` for helpers bitcode files as discussed, and
        dropped `$(if $(filter %_module.bc,$@),-DMODULE)` since `-DMODULE`
        is already there (would be duplicated otherwise).

      - Moved `LLVM_LINK` to keep binutils list alphabetized.

      - Fixed typo in title.

      - Dropped second `cmd_ld_single` commit message paragraph.

          - Miguel ]

...to boldly go where no Rust build has gone before.

Cheers,
Miguel