add announcement / call for testing for lld on 1.90.0

Author: lqd

content/rust-lld-on-stable-1.90.0/index.md

@@ -0,0 +1,65 @@
++++
+path = "2025/08/29/rust-lld-on-1.90.0-stable"
+title = "Faster linking times with 1.90.0 stable on Linux using `rust-lld`"
+authors = ["Rémy Rakic"]
+
+[extra]
+team = "the compiler performance working group"
+team_url = "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 stable, starting in 1.90.0, to significantly reduce linking times. Test it out on beta now, and please report issues if you encounter any.
+
+#### 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). 
+
+To expand our testing, we have enabled rustc to uses `rust-lld` by default on nightly, [in May 2024](https://blog.rust-lang.org/2024/05/17/enabling-rust-lld-on-linux.html). No major issues have been reported since then.
+
+We believe we've done all the internal testing that we could, on CI, crater, on our benchmarking infrastructure and on nightly, and plan to enable `rust-lld` to be the linker used by default on `x86_64-unknown-linux-gnu` for stable builds in 1.90.0.
+
+#### 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: for an incremental rebuild, linking is reduced 7x, resulting in a 40% reduction in end-to-end compilation times. For a from-scratch debug build, it is a 20% improvement.
+
+![Before/after comparison of a `ripgrep` incremental debug build](ripgrep-comparison.png)
+
+Most binaries should see some improvements here, but it's especially significant with e.g. bigger binaries, or for incremental rebuilds, 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.
+
+This testing period did go well indeed, and we can stabilize using this faster linker by default for `x86_64-unknown-linux-gnu` users.
+
+#### 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 `-C 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, though we also expect this to be quite rare. 
+
+Some of the big gains in performance come from parallelism, which could be undesirable in resource-constrained environments, or for heavy projects that are already reaching hardware limits.
+
+#### Summary, and call for testing
+
+rustc will use `rust-lld` on `x86_64-unknown-linux-gnu`, starting with the 1.90.0 stable release, for much improved linking times. This version 1.90.0 will be released next month, on the 18th of September 2025. 
+
+This linker change is already available on the current beta (`1.90.0-beta.6`). To help everyone prepare for this landing on stable, please test your projects on beta and 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 `-C 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 = ["-Clinker-features=-lld"]
+```