Re: [PATCH v2 2/3] kbuild: rust: provide an option to inline C helpers into Rust

[Andreas Hindborg]

"Gary Guo" <gary@xxxxxxxxxxx> writes:

> A new Kconfig option, `RUST_INLINE_HELPERS` is added to allow C helpers
> (which was created to allow Rust to call into inline/macro C functions
> without having to re-implement the logic in Rust) to be inlined into
> Rust crates without performing a global LTO.
>
> If the option is enabled, the following is performed:
> * For helpers, instead of compiling them to object file to be linked
>   into vmlinux, we compile them to LLVM IR.
> * The LLVM IR is patched to add `linkonce_odr` linkage. This linkage
>   means that the function is inlineable (effect of `_odr`), and the
>   symbols generated will have weak linkage if emitted into object file
>   (important since as later described, we might have multiple copies of
>   the same symbol) and it will may be discarded if it is not invoked or
>   all invocations are inlined.
> * The LLVM IR is compiled to bitcode (This is step is not necessary, but
>   is a performance optimisation to prevent LLVM from always have to
>   reparse the same IR).
> * When a Rust crate is compiled, instead of generating object file, we
>   ask LLVM bitcode to be generated.
> * llvm-link is invoked to combine the helper bitcode with the crate
>   bitcode. This step is similar to LTO, but this is much faster since it
>   only needs to inline the helpers.
> * clang is invoked to turn the combined bitcode into object file.
>
> Some caveats with the option:
> * clang and Rust doesn't have the exact target string. Manual inspection
>   shows that they should be compatible, but since they are not exactly
>   the same LLVM seems to prefer not inlining them. This is bypassed with
>   `--ignore-tti-inline-compatible`.
> * LLVM doesn't want to inline functions combined with
>   `-fno-delete-null-pointer-checks` with code compiled without. So we
>   remove this command when compiling helpers. I think this should be
>   okay since this is one of the hardening features and we shouldn't have
>   null pointer dereferences in these helpers.
>
> The checks can also be bypassed with force inlining (`__always_inline`)
> but the behaviour is the same with extra options.
>
> Co-developed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>


Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
Tested-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>

Tested with downstream rnull tree, works as expected.

Best regards,
Andreas Hindborg