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Stabilize linker-plugin based LTO (aka cross-language LTO) #58057
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r? @estebank (rust_highfive has picked a reviewer for you, use r? to override) |
As discussed in the tracking issue, we'll ask @rust-lang/compiler to sign off on it. |
Team member @michaelwoerister has proposed to merge this. The next step is review by the rest of the tagged team members:
No concerns currently listed. Once a majority of reviewers approve (and none object), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up! See this document for info about what commands tagged team members can give me. |
Looks reasonable to me! Do you know if the way to use this is written down somewhere? I presume it's almost "just set |
☔ The latest upstream changes (presumably #57937) made this pull request unmergeable. Please resolve the merge conflicts. |
Yeah, I ran into a similar problem with path-prefix-remapping a while back. There seems to be no canonical place for documenting niche-features like these. Should I create a page on https://forge.rust-lang.org/? @rust-lang/docs might have a better suggestion. |
The reference? |
The reference looks like it's more about the language, not about the compiler. |
O, I just found this: https://doc.rust-lang.org/rustc/index.html It looks a bit unmaintained though... |
I knocked out the basics, and we'd like to continue to expand it, but just need people to do the work, as always :) It was created to exactly be the place for this kind of information. |
OK, I'll just add a subsection for the feature. It's complicated enough to warrant its own page, I think. |
@michaelwoerister Is there any mechanism to ensure matching LLVM IR versions are used? I feel like this option should be |
🔔 This is now entering its final comment period, as per the review above. 🔔 |
What do you have in mind? Only the linker sees both versions of the LLVM IR, so don't think that
I'm open for suggestions regarding a different name for the command line option. I don't like |
Are there any plans to integrate this as a cargo flag? Or is this intended to stay as a RUSTFLAG? This would be great to have on in most cases for embedded, especially if this would inline external assembly files as well. Edit: specifically as a profile flag that could be set in a Cargo.toml, not a cargo CLI flag |
Is Firefox using it to compile its jemalloc-sys fork somewhere ? |
Not that I'm aware of. @rust-lang/cargo would have to decide about this.
Mabye Ted knows something about this? cc @luser |
@jamesmunns FWIW this unfortunately wouldn't work for external assembly files, linker-based LTO only works for code compiled by the same compiler to the same IR, such as C and Rust compiled both to LLVM IR. External assembly files are compiled directly to object files and skip the LLVM IR step, so they can't be optimized/inlined together. I think we'll probably want to stabilize this first and then see how it plays out to figure out how to best add it to Cargo |
It might be best to add it to the That is, we probably want |
r=me when this is green on CI |
@bors r=alexcrichton |
📌 Commit 3a9d171 has been approved by |
First of all, this is awesome, and I'd love to see it happen. Second, while I wouldn't want to block this on it, we need to take this change into account in any future designs for better ways of handling builds of hybrid C/Rust projects. |
…alexcrichton Stabilize linker-plugin based LTO (aka cross-language LTO) This PR stabilizes [linker plugin based LTO](rust-lang#49879), also known as "cross-language LTO" because it allows for doing inlining and other optimizations across language boundaries in mixed Rust/C/C++ projects. As described in the tracking issue, it works by making `rustc` emit LLVM bitcode instead of machine code, the same as `clang` does. A linker with the proper plugin (like LLD) can then run (Thin)LTO across all modules. The feature has been implemented over a number of pull requests and there are various [codegen](https://github.com/rust-lang/rust/blob/master/src/test/codegen/no-dllimport-w-cross-lang-lto.rs) and [run](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-clang)-[make](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-upstream-rlibs) [tests](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto) that make sure that it keeps working. It also works for building big projects like [Firefox](https://treeherder.mozilla.org/#/jobs?repo=try&revision=2ce2d5ddcea6fbff790503eac406954e469b2f5d). The PR makes the feature available under the `-C linker-plugin-lto` flag. As discussed in the tracking issue it is not cross-language specific and also not LLD specific. `-C linker-plugin-lto` is descriptive of what it does. If someone has a better name, let me know `:)`
…alexcrichton Stabilize linker-plugin based LTO (aka cross-language LTO) This PR stabilizes [linker plugin based LTO](rust-lang#49879), also known as "cross-language LTO" because it allows for doing inlining and other optimizations across language boundaries in mixed Rust/C/C++ projects. As described in the tracking issue, it works by making `rustc` emit LLVM bitcode instead of machine code, the same as `clang` does. A linker with the proper plugin (like LLD) can then run (Thin)LTO across all modules. The feature has been implemented over a number of pull requests and there are various [codegen](https://github.com/rust-lang/rust/blob/master/src/test/codegen/no-dllimport-w-cross-lang-lto.rs) and [run](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-clang)-[make](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-upstream-rlibs) [tests](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto) that make sure that it keeps working. It also works for building big projects like [Firefox](https://treeherder.mozilla.org/#/jobs?repo=try&revision=2ce2d5ddcea6fbff790503eac406954e469b2f5d). The PR makes the feature available under the `-C linker-plugin-lto` flag. As discussed in the tracking issue it is not cross-language specific and also not LLD specific. `-C linker-plugin-lto` is descriptive of what it does. If someone has a better name, let me know `:)`
Rollup of 13 pull requests Successful merges: - #57693 (Doc rewording) - #57815 (Speed up the fast path for assert_eq! and assert_ne!) - #58034 (Stabilize the time_checked_add feature) - #58057 (Stabilize linker-plugin based LTO (aka cross-language LTO)) - #58137 (Cleanup: rename node_id_to_type(_opt)) - #58166 (allow shorthand syntax for deprecation reason) - #58196 (Add specific feature gate error for const-unstable features) - #58200 (fix str mutating through a ptr derived from &self) - #58273 (Rename rustc_errors dependency in rust 2018 crates) - #58289 (impl iter() for dyn Error) - #58387 (Disallow `auto` trait alias syntax) - #58404 (use Ubuntu keyserver for CloudABI ports) - #58405 (Remove some dead code from libcore) Failed merges: r? @ghost
…alexcrichton Stabilize linker-plugin based LTO (aka cross-language LTO) This PR stabilizes [linker plugin based LTO](rust-lang#49879), also known as "cross-language LTO" because it allows for doing inlining and other optimizations across language boundaries in mixed Rust/C/C++ projects. As described in the tracking issue, it works by making `rustc` emit LLVM bitcode instead of machine code, the same as `clang` does. A linker with the proper plugin (like LLD) can then run (Thin)LTO across all modules. The feature has been implemented over a number of pull requests and there are various [codegen](https://github.com/rust-lang/rust/blob/master/src/test/codegen/no-dllimport-w-cross-lang-lto.rs) and [run](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-clang)-[make](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto-upstream-rlibs) [tests](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/cross-lang-lto) that make sure that it keeps working. It also works for building big projects like [Firefox](https://treeherder.mozilla.org/#/jobs?repo=try&revision=2ce2d5ddcea6fbff790503eac406954e469b2f5d). The PR makes the feature available under the `-C linker-plugin-lto` flag. As discussed in the tracking issue it is not cross-language specific and also not LLD specific. `-C linker-plugin-lto` is descriptive of what it does. If someone has a better name, let me know `:)`
Rollup of 12 pull requests Successful merges: - #57693 (Doc rewording) - #57815 (Speed up the fast path for assert_eq! and assert_ne!) - #58034 (Stabilize the time_checked_add feature) - #58057 (Stabilize linker-plugin based LTO (aka cross-language LTO)) - #58137 (Cleanup: rename node_id_to_type(_opt)) - #58166 (allow shorthand syntax for deprecation reason) - #58200 (fix str mutating through a ptr derived from &self) - #58273 (Rename rustc_errors dependency in rust 2018 crates) - #58289 (impl iter() for dyn Error) - #58387 (Disallow `auto` trait alias syntax) - #58404 (use Ubuntu keyserver for CloudABI ports) - #58405 (Remove some dead code from libcore) Failed merges: r? @ghost
Pkgsrc changes: * Bump required rust version to build to 1.33.0. * Adapt patches to changed file locations. * (I worry about 32-bit ports, now that Atomic64 apparently is First-Class; this has been built on NetBSD/amd64 so far) Upstream changes: Version 1.34.0 (2019-04-11) ========================== Language -------- - [You can now use `#[deprecated = "reason"]`][58166] as a shorthand for `#[deprecated(note = "reason")]`. This was previously allowed by mistake but had no effect. - [You can now accept token streams in `#[attr()]`,`#[attr[]]`, and `#[attr{}]` procedural macros.][57367] - [You can now write `extern crate self as foo;`][57407] to import your crate's root into the extern prelude. Compiler -------- - [You can now target `riscv64imac-unknown-none-elf` and `riscv64gc-unknown-none-elf`.][58406] - [You can now enable linker plugin LTO optimisations with `-C linker-plugin-lto`.][58057] This allows rustc to compile your Rust code into LLVM bitcode allowing LLVM to perform LTO optimisations across C/C++ FFI boundaries. - [You can now target `powerpc64-unknown-freebsd`.][57809] Libraries --------- - [The trait bounds have been removed on some of `HashMap<K, V, S>`'s and `HashSet<T, S>`'s basic methods.][58370] Most notably you no longer require the `Hash` trait to create an iterator. - [The `Ord` trait bounds have been removed on some of `BinaryHeap<T>`'s basic methods.][58421] Most notably you no longer require the `Ord` trait to create an iterator. - [The methods `overflowing_neg` and `wrapping_neg` are now `const` functions for all numeric types.][58044] - [Indexing a `str` is now generic over all types that implement `SliceIndex<str>`.][57604] - [`str::trim`, `str::trim_matches`, `str::trim_{start, end}`, and `str::trim_{start, end}_matches` are now `#[must_use]`][57106] and will produce a warning if their returning type is unused. - [The methods `checked_pow`, `saturating_pow`, `wrapping_pow`, and `overflowing_pow` are now available for all numeric types.][57873] These are equivalvent to methods such as `wrapping_add` for the `pow` operation. Stabilized APIs --------------- #### std & core * [`Any::type_id`] * [`Error::type_id`] * [`atomic::AtomicI16`] * [`atomic::AtomicI32`] * [`atomic::AtomicI64`] * [`atomic::AtomicI8`] * [`atomic::AtomicU16`] * [`atomic::AtomicU32`] * [`atomic::AtomicU64`] * [`atomic::AtomicU8`] * [`convert::Infallible`] * [`convert::TryFrom`] * [`convert::TryInto`] * [`iter::from_fn`] * [`iter::successors`] * [`num::NonZeroI128`] * [`num::NonZeroI16`] * [`num::NonZeroI32`] * [`num::NonZeroI64`] * [`num::NonZeroI8`] * [`num::NonZeroIsize`] * [`slice::sort_by_cached_key`] * [`str::escape_debug`] * [`str::escape_default`] * [`str::escape_unicode`] * [`str::split_ascii_whitespace`] #### std * [`Instant::checked_add`] * [`Instant::checked_sub`] * [`SystemTime::checked_add`] * [`SystemTime::checked_sub`] Cargo ----- - [You can now use alternative registries to crates.io.][cargo/6654] Misc ---- - [You can now use the `?` operator in your documentation tests without manually adding `fn main() -> Result<(), _> {}`.][56470] Compatibility Notes ------------------- - [`Command::before_exec` is now deprecated in favor of the unsafe method `Command::pre_exec`.][58059] - [Use of `ATOMIC_{BOOL, ISIZE, USIZE}_INIT` is now deprecated.][57425] As you can now use `const` functions in `static` variables. [58370]: rust-lang/rust#58370 [58406]: rust-lang/rust#58406 [58421]: rust-lang/rust#58421 [58166]: rust-lang/rust#58166 [58044]: rust-lang/rust#58044 [58057]: rust-lang/rust#58057 [58059]: rust-lang/rust#58059 [57809]: rust-lang/rust#57809 [57873]: rust-lang/rust#57873 [57604]: rust-lang/rust#57604 [57367]: rust-lang/rust#57367 [57407]: rust-lang/rust#57407 [57425]: rust-lang/rust#57425 [57106]: rust-lang/rust#57106 [56470]: rust-lang/rust#56470 [cargo/6654]: rust-lang/cargo#6654 [`Any::type_id`]: https://doc.rust-lang.org/std/any/trait.Any.html#tymethod.type_id [`Error::type_id`]: https://doc.rust-lang.org/std/error/trait.Error.html#tymethod.type_id [`atomic::AtomicI16`]: https://doc.rust-lang.org/std/atomic/struct.AtomicI16.html [`atomic::AtomicI32`]: https://doc.rust-lang.org/std/atomic/struct.AtomicI32.html [`atomic::AtomicI64`]: https://doc.rust-lang.org/std/atomic/struct.AtomicI64.html [`atomic::AtomicI8`]: https://doc.rust-lang.org/std/atomic/struct.AtomicI8.html [`atomic::AtomicU16`]: https://doc.rust-lang.org/std/atomic/struct.AtomicU16.html [`atomic::AtomicU32`]: https://doc.rust-lang.org/std/atomic/struct.AtomicU32.html [`atomic::AtomicU64`]: https://doc.rust-lang.org/std/atomic/struct.AtomicU64.html [`atomic::AtomicU8`]: https://doc.rust-lang.org/std/atomic/struct.AtomicU8.html [`convert::Infallible`]: https://doc.rust-lang.org/std/convert/enum.Infallible.html [`convert::TryFrom`]: https://doc.rust-lang.org/std/convert/trait.TryFrom.html [`convert::TryInto`]: https://doc.rust-lang.org/std/convert/trait.TryInto.html [`iter::from_fn`]: https://doc.rust-lang.org/std/iter/fn.from_fn.html [`iter::successors`]: https://doc.rust-lang.org/std/iter/fn.successors.html [`num::NonZeroI128`]: https://doc.rust-lang.org/std/num/struct.NonZeroI128.html [`num::NonZeroI16`]: https://doc.rust-lang.org/std/num/struct.NonZeroI16.html [`num::NonZeroI32`]: https://doc.rust-lang.org/std/num/struct.NonZeroI32.html [`num::NonZeroI64`]: https://doc.rust-lang.org/std/num/struct.NonZeroI64.html [`num::NonZeroI8`]: https://doc.rust-lang.org/std/num/struct.NonZeroI8.html [`num::NonZeroIsize`]: https://doc.rust-lang.org/std/num/struct.NonZeroIsize.html [`slice::sort_by_cached_key`]: https://doc.rust-lang.org/std/slice/fn.sort_by_cached_key [`str::escape_debug`]: https://doc.rust-lang.org/std/primitive.str.html#method.escape_debug [`str::escape_default`]: https://doc.rust-lang.org/std/primitive.str.html#method.escape_default [`str::escape_unicode`]: https://doc.rust-lang.org/std/primitive.str.html#method.escape_unicode [`str::split_ascii_whitespace`]: https://doc.rust-lang.org/std/primitive.str.html#method.split_ascii_whitespace [`Instant::checked_add`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.checked_add [`Instant::checked_sub`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.checked_sub [`SystemTime::checked_add`]: https://doc.rust-lang.org/std/time/struct.SystemTime.html#method.checked_add [`SystemTime::checked_sub`]: https://doc.rust-lang.org/std/time/struct.SystemTime.html#method.checked_sub
Pkgsrc changes: * Bump required rust version to build to 1.33.0. * Adapt patches to changed file locations. * (I worry about 32-bit ports, now that Atomic64 apparently is First-Class; this has been built on NetBSD/amd64 so far) Upstream changes: Version 1.34.0 (2019-04-11) ========================== Language -------- - [You can now use `#[deprecated = "reason"]`][58166] as a shorthand for `#[deprecated(note = "reason")]`. This was previously allowed by mistake but had no effect. - [You can now accept token streams in `#[attr()]`,`#[attr[]]`, and `#[attr{}]` procedural macros.][57367] - [You can now write `extern crate self as foo;`][57407] to import your crate's root into the extern prelude. Compiler -------- - [You can now target `riscv64imac-unknown-none-elf` and `riscv64gc-unknown-none-elf`.][58406] - [You can now enable linker plugin LTO optimisations with `-C linker-plugin-lto`.][58057] This allows rustc to compile your Rust code into LLVM bitcode allowing LLVM to perform LTO optimisations across C/C++ FFI boundaries. - [You can now target `powerpc64-unknown-freebsd`.][57809] Libraries --------- - [The trait bounds have been removed on some of `HashMap<K, V, S>`'s and `HashSet<T, S>`'s basic methods.][58370] Most notably you no longer require the `Hash` trait to create an iterator. - [The `Ord` trait bounds have been removed on some of `BinaryHeap<T>`'s basic methods.][58421] Most notably you no longer require the `Ord` trait to create an iterator. - [The methods `overflowing_neg` and `wrapping_neg` are now `const` functions for all numeric types.][58044] - [Indexing a `str` is now generic over all types that implement `SliceIndex<str>`.][57604] - [`str::trim`, `str::trim_matches`, `str::trim_{start, end}`, and `str::trim_{start, end}_matches` are now `#[must_use]`][57106] and will produce a warning if their returning type is unused. - [The methods `checked_pow`, `saturating_pow`, `wrapping_pow`, and `overflowing_pow` are now available for all numeric types.][57873] These are equivalvent to methods such as `wrapping_add` for the `pow` operation. Stabilized APIs --------------- #### std & core * [`Any::type_id`] * [`Error::type_id`] * [`atomic::AtomicI16`] * [`atomic::AtomicI32`] * [`atomic::AtomicI64`] * [`atomic::AtomicI8`] * [`atomic::AtomicU16`] * [`atomic::AtomicU32`] * [`atomic::AtomicU64`] * [`atomic::AtomicU8`] * [`convert::Infallible`] * [`convert::TryFrom`] * [`convert::TryInto`] * [`iter::from_fn`] * [`iter::successors`] * [`num::NonZeroI128`] * [`num::NonZeroI16`] * [`num::NonZeroI32`] * [`num::NonZeroI64`] * [`num::NonZeroI8`] * [`num::NonZeroIsize`] * [`slice::sort_by_cached_key`] * [`str::escape_debug`] * [`str::escape_default`] * [`str::escape_unicode`] * [`str::split_ascii_whitespace`] #### std * [`Instant::checked_add`] * [`Instant::checked_sub`] * [`SystemTime::checked_add`] * [`SystemTime::checked_sub`] Cargo ----- - [You can now use alternative registries to crates.io.][cargo/6654] Misc ---- - [You can now use the `?` operator in your documentation tests without manually adding `fn main() -> Result<(), _> {}`.][56470] Compatibility Notes ------------------- - [`Command::before_exec` is now deprecated in favor of the unsafe method `Command::pre_exec`.][58059] - [Use of `ATOMIC_{BOOL, ISIZE, USIZE}_INIT` is now deprecated.][57425] As you can now use `const` functions in `static` variables. [58370]: rust-lang/rust#58370 [58406]: rust-lang/rust#58406 [58421]: rust-lang/rust#58421 [58166]: rust-lang/rust#58166 [58044]: rust-lang/rust#58044 [58057]: rust-lang/rust#58057 [58059]: rust-lang/rust#58059 [57809]: rust-lang/rust#57809 [57873]: rust-lang/rust#57873 [57604]: rust-lang/rust#57604 [57367]: rust-lang/rust#57367 [57407]: rust-lang/rust#57407 [57425]: rust-lang/rust#57425 [57106]: rust-lang/rust#57106 [56470]: rust-lang/rust#56470 [cargo/6654]: rust-lang/cargo#6654 [`Any::type_id`]: https://doc.rust-lang.org/std/any/trait.Any.html#tymethod.type_id [`Error::type_id`]: https://doc.rust-lang.org/std/error/trait.Error.html#tymethod.type_id [`atomic::AtomicI16`]: https://doc.rust-lang.org/std/atomic/struct.AtomicI16.html [`atomic::AtomicI32`]: https://doc.rust-lang.org/std/atomic/struct.AtomicI32.html [`atomic::AtomicI64`]: https://doc.rust-lang.org/std/atomic/struct.AtomicI64.html [`atomic::AtomicI8`]: https://doc.rust-lang.org/std/atomic/struct.AtomicI8.html [`atomic::AtomicU16`]: https://doc.rust-lang.org/std/atomic/struct.AtomicU16.html [`atomic::AtomicU32`]: https://doc.rust-lang.org/std/atomic/struct.AtomicU32.html [`atomic::AtomicU64`]: https://doc.rust-lang.org/std/atomic/struct.AtomicU64.html [`atomic::AtomicU8`]: https://doc.rust-lang.org/std/atomic/struct.AtomicU8.html [`convert::Infallible`]: https://doc.rust-lang.org/std/convert/enum.Infallible.html [`convert::TryFrom`]: https://doc.rust-lang.org/std/convert/trait.TryFrom.html [`convert::TryInto`]: https://doc.rust-lang.org/std/convert/trait.TryInto.html [`iter::from_fn`]: https://doc.rust-lang.org/std/iter/fn.from_fn.html [`iter::successors`]: https://doc.rust-lang.org/std/iter/fn.successors.html [`num::NonZeroI128`]: https://doc.rust-lang.org/std/num/struct.NonZeroI128.html [`num::NonZeroI16`]: https://doc.rust-lang.org/std/num/struct.NonZeroI16.html [`num::NonZeroI32`]: https://doc.rust-lang.org/std/num/struct.NonZeroI32.html [`num::NonZeroI64`]: https://doc.rust-lang.org/std/num/struct.NonZeroI64.html [`num::NonZeroI8`]: https://doc.rust-lang.org/std/num/struct.NonZeroI8.html [`num::NonZeroIsize`]: https://doc.rust-lang.org/std/num/struct.NonZeroIsize.html [`slice::sort_by_cached_key`]: https://doc.rust-lang.org/std/slice/fn.sort_by_cached_key [`str::escape_debug`]: https://doc.rust-lang.org/std/primitive.str.html#method.escape_debug [`str::escape_default`]: https://doc.rust-lang.org/std/primitive.str.html#method.escape_default [`str::escape_unicode`]: https://doc.rust-lang.org/std/primitive.str.html#method.escape_unicode [`str::split_ascii_whitespace`]: https://doc.rust-lang.org/std/primitive.str.html#method.split_ascii_whitespace [`Instant::checked_add`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.checked_add [`Instant::checked_sub`]: https://doc.rust-lang.org/std/time/struct.Instant.html#method.checked_sub [`SystemTime::checked_add`]: https://doc.rust-lang.org/std/time/struct.SystemTime.html#method.checked_add [`SystemTime::checked_sub`]: https://doc.rust-lang.org/std/time/struct.SystemTime.html#method.checked_sub
I would like to bring to your attention that stabilizing this requires the LLVM version used in Rust to be a non-arbitrary version of LLVM trunk. It does happen that LLVM IR breaks compatibility on trunk (and it ends up being fixed at some point), and IIRC, it has happened that Rust picked a version of LLVM that happened to be broken. |
Why? This option is disabled by default. If a user enables it, and the LLVM used is incompatible, we could:
AFAICT, this option enables and optimization and that can be "best-effort". We should document the behavior, and that's it. If the current behavior is neither of these (e.g. unsound, an LLVM error is emitted, etc.), you can open an issue about this. That's bad and should be fixed. |
@glandium While this is true in general, rust is currently using (and has been since its release) the LLVM 8 branch, not trunk. If you're seeing issues, they cannot be related to trunk changes. |
@nikic users might be using the system LLVM (up to 6.0), depending on their platform (linux distribution, etc.). This case needs to be handled somehow. |
@michaelwoerister -flto=full never works |
This PR stabilizes linker plugin based LTO, also known as "cross-language LTO" because it allows for doing inlining and other optimizations across language boundaries in mixed Rust/C/C++ projects.
As described in the tracking issue, it works by making
rustc
emit LLVM bitcode instead of machine code, the same asclang
does. A linker with the proper plugin (like LLD) can then run (Thin)LTO across all modules.The feature has been implemented over a number of pull requests and there are various codegen and run-make tests that make sure that it keeps working.
It also works for building big projects like Firefox.
The PR makes the feature available under the
-C linker-plugin-lto
flag. As discussed in the tracking issue it is not cross-language specific and also not LLD specific.-C linker-plugin-lto
is descriptive of what it does. If someone has a better name, let me know:)