Add announcement for rust-lld on linux nightlies

posts/2024-05-17-enabling-rust-lld-on-linux.md

@@ -0,0 +1,94 @@
+---
+layout: post
+title: Faster linking times on nightly on Linux using `rust-lld`
+author: Rémy Rakic
+team: the compiler performance working group <https://www.rust-lang.org/governance/teams/compiler#team-wg-compiler-performance>
+---
+
+TL;DR: rustc will use `rust-lld` by default on `x86_64-unknown-linux-gnu` on nightly to
+significantly reduce linking times.
+
+#### Some context
+
+Linking time is often a big part of compilation time. When rustc needs to build a binary or a shared
+library, it will usually call the default linker installed on the system to do that (this can be
+changed on the command-line or by the target for which the code is compiled).
+
+The linkers do an important job, with concerns about stability, backwards-compatibility and so on.
+For these and other reasons, on the most popular operating systems they usually are older programs,
+designed when computers only had a single core. So, they usually tend to be slow on a modern
+machine. For example, when building ripgrep 13 in debug mode on Linux, roughly half of the time is
+actually spent in the linker.
+
+There are different linkers, however, and the usual advice to improve linking times is to use one of
+these newer and faster linkers, like LLVM's [`lld`](https://lld.llvm.org/) or Rui Ueyama's
+[`mold`](https://github.com/rui314/mold).
+
+Some of Rust's wasm and aarch64 targets already use `lld` by default. When using rustup, rustc ships
+with a version of `lld` for this purpose. When CI builds LLVM to use in the compiler, it also builds
+the linker and packages it. It's referred to as `rust-lld` to avoid colliding with any `lld` already
+installed on the user's machine.
+
+Since improvements to linking times are substantial, it would be a good default to use in the most
+popular targets. This has been discussed for a long time, for example in issues
+[#39915](https://github.com/rust-lang/rust/issues/39915) and
+[#71515](https://github.com/rust-lang/rust/issues/71515), and rustc already offers nightly flags to
+use `rust-lld`.
+
+By now, we believe we've done all the internal testing that we could, on CI, crater, and our
+benchmarking infrastructure. We would now like to expand testing and gather real-world feedback and
+use-cases. Therefore, we will enable `rust-lld` to be the linker used by default on
+`x86_64-unknown-linux-gnu` for nightly builds.
+
+#### Benefits
+
+While this also enables the compiler to use more linker features in the future, the most immediate
+benefit is much improved linking times.
+
+Here are more details from the ripgrep example mentioned above: linking is reduced 7x, resulting in
+a 40% reduction in end-to-end compilation times.
+
+![Before/after comparison of a `ripgrep` debug build](../../../../images/2024-05-17-enabling-rust-lld-on-linux/ripgrep-comparison.png)
+
+Most binaries should see some improvements here, but it's especially significant with e.g. bigger
+binaries, or when involving debuginfo. These usually see bottlenecks in the linker.
+
+Here's [a
+link](https://perf.rust-lang.org/compare.html?start=b3e117044c7f707293edc040edb93e7ec5f7040a&end=baed03c51a68376c1789cc373581eea0daf89967&stat=instructions%3Au&tab=compile)
+to the complete results from our benchmarks.
+
+If testing goes well, we can then stabilize using this faster linker by default for
+`x86_64-unknown-linux-gnu` users, before maybe looking at other targets.
+
+#### Possible drawbacks
+
+From our prior testing, we don't really expect issues to happen in practice. It is a drop-in
+replacement for the vast majority of cases, but `lld` is not _bug-for-bug_ compatible with GNU ld.
+
+In any case, using `rust-lld` can be disabled if any problem occurs: use the `-Z
+linker-features=-lld` flag to revert to using the system's default linker.
+
+Some crates somehow relying on these differences could need additional link args. For example, we
+saw <20 crates in the crater run failing to link because of a different default about [encapsulation
+symbols](https://lld.llvm.org/ELF/start-stop-gc): these could require
+`-Clink-arg=-Wl,-z,nostart-stop-gc` to match the legacy GNU ld behavior.
+
+Some of the big gains in performance come from parallelism, which could be undesirable in
+resource-constrained environments.
+
+#### Summary
+
+rustc will use `rust-lld` on `x86_64-unknown-linux-gnu` nightlies, for much improved linking times,
+starting in tomorrow's rustup nightly (`nightly-2024-05-18`).
+Let us know if you encounter problems, by [opening an
+issue](https://github.com/rust-lang/rust/issues/new/choose) on GitHub.
+
+If that happens, you can revert to the default linker with the `-Z linker-features=-lld` flag.
+Either by adding it to the usual `RUSTFLAGS` environment variable, or to a project's
+[`.cargo/config.toml`](https://doc.rust-lang.org/cargo/reference/config.html) configuration file,
+like so:
+
+```toml
+[target.x86_64-unknown-linux-gnu]
+rustflags = ["-Zlinker-features=-lld"]
+```