Increased build time and dependencies due to rust-lld in bootstrap builds

r_learn · July 22, 2025, 10:54am

We observed that recent changes in Rust have enabled and built rust-lld as part of the bootstrap process and has significant improvement in build and linking times.

github.com/rust-lang/rust

Use lld by default on `x86_64-unknown-linux-gnu` stable

master ← lqd:stabilize-lld

opened 06:27PM - 30 Apr 25 UTC

lqd

+353 -176

This PR and stabilization report is joint work with @Kobzol. #### Use LLD on …`x86_64-unknown-linux-gnu` by default, and stabilize `-Clinker-features=-lld` and `-Clink-self-contained=-linker` This PR proposes making LLD the default linker on the `x86_64-unknown-linux-gnu` target for the artifacts we distribute, and also stabilizing the `-Clinker-features=-lld` and `-Clink-self-contained=-linker` codegen options to make it possible to opt out. LLD has been used as the default linker on nightly and CI on this target since May 2024 ([PR](https://github.com/rust-lang/rust/pull/124129), [blog post](https://blog.rust-lang.org/2024/05/17/enabling-rust-lld-on-linux.html)), and it seems like it is working fine, so we would like to propose stabilizing it. The main motivation for using LLD instead of the default BFD linker is improving [compilation times](https://perf.rust-lang.org/compare.html?start=b3e117044c7f707293edc040edb93e7ec5f7040a&end=baed03c51a68376c1789cc373581eea0daf89967&stat=instructions%3Au&tab=compile). For example, in the linked benchmark, it makes incremental recompilation of `ripgrep` in `debug` more than twice faster. Another benefit is that Rust compilation becomes more consistent and self-contained, because we will use a known version of the LLD linker, rather than "whatever GNU ld version is on the user's system". Due to the performance benefit being so huge, many people already opt into using LLD (or other fast linkers, such as mold) using various approaches ([1](https://github.com/search?type=code&q=%2Flinker-flavor%5B%3D+%5Dgnu-lld-cc%2F), [2](https://github.com/search?type=code&q=%2Flinker-features%5B%3D+%5D%5C%2Blld%2F), [3](https://github.com/search?type=code&q=language%3Atoml+%22-fuse-ld%3Dlld%22), [4](https://github.com/search?type=code&q=language%3Arust+%22-fuse-ld%3Dlld%22)). By making LLD the default linker on the `x86_64-unknown-linux-gnu` target, we will be able to speed up Rust compilation out of the box, without users having to opt in or know about it. > You can find an extended version of this stabilization report which includes analysis of crater results and more data [here](https://hackmd.io/tFDifkUcSLGoHPBRIl0z8w?view). ## What is being stabilized - `rust-lld` being used as the default linker on the `x86_64-unknown-linux-gnu` target. - Note that `rust-lld` is being enabled by default in the compiler artifacts distributed by our CI/rustup. It is still possible to use the system linker by default using `rust.lld = false` in `bootstrap.toml`, which can be useful e.g. for some Linux distros that might not want to use the LLD we distribute. - This is done by activating the LLD linker feature and using the self-contained linker on that target. Both of which are also usable on the CLI, if some opt outs are necessary, as described below. - `-Clinker-features=-lld` on the `x86_64-unknown-linux-gnu` target. This codegen option tells rustc to disable using the LLD linker. - Note that other options for this codegen flag (`cc`) remain unstable. - Note that only the opt-out is being stabilized, and only for `x86_64-unknown-linux-gnu`: opting in, or using the flag on other targets would still need to pass `-Zunstable-options`. - This flag is being stabilized so that users can opt out of LLD on stable, which would it turn also opt out of using the self-contained linker (since it's an LLD). - `-Clink-self-contained=-linker`. This codegen option tells rustc to use the self-contained linker. It's not particularly useful to turn it on by itself, but when enabled and combined with `-Clinker-features=+lld`, rustc will use the `rust-lld` linker wrapper shipped with the compiler toolchain, instead of some `lld` binary that the linker driver will find in the `PATH`. - Note that other options for this codegen flag (other than the previously stable `y/yes/n/no`). - Note that only the opt-out is being stabilized, and only for `x86_64-unknown-linux-gnu`: opting in, or using this flag on other targets would still need to pass `-Zunstable-options`. - This flag is being stabilized so that users can opt out of using self-contained linking on stable. Doing this would then fall back to using the system `lld`. To opt out of using LLD, `RUSTFLAGS="-Clinker-features=-lld"` would be used. To opt out of using `rust-lld`, falling back to the LLD installed on the system, `RUSTFLAGS="-Clink-self-contained=-linker"` would be used. ## Tests When enabling `rust-lld` on nightly, we also switched x64 linux to use it at stage >= 1, meaning that all tests have been running with lld since May 2024, on CI as well as contributors' machines. (Post opt-dist tests also had been using it when running their test subset earlier than that). There are also a few tests dedicated to the CLI behavior, or ensuring the default linker is indeed the one we expect: - [link-self-contained-consistency](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/ui/linking/link-self-contained-consistency.rs): Checks that `-Clink-self-contained` options are not inconsistent (i.e. that passing both `+linker` and `-linker` is an error). - [link-self-contained-unstable](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/ui/linking/link-self-contained-unstable.rs): Checks that only the `-linker` and `y/yes/n/no` options for `-Clink-self-contained` are stable. - [linker-features-unstable-cc](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/ui/linking/linker-features-unstable-cc.rs): Checks that only the non-lld options of `-Clinker-features` are unstable. - [linker-features-lld-disallowed](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/ui/linking/linker-features-lld-disallowed.rs): Checks that `-Clinker-features=-lld` is only stable on `x86_64-unknown-linux-gnu`. - [link-self-contained-linker-disallowed](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/ui/linking/link-self-contained-linker-disallowed.rs): Checks that `-Clink-self-contained=-linker` is only stable on `x86_64-unknown-linux-gnu`. - [no-gc-encapsulation-symbols](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/ui/linking/no-gc-encapsulation-symbols.rs): Checks that that linker encapsulation symbols are not garbage collected by LLD, so that crates like [linkme](https://github.com/dtolnay/linkme) still work. - [rust-lld](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/run-make/rust-lld): Checks that LLD is actually used when enabled with `-Clinker-features=+lld` and `-Clink-self-contained=+linker`. - [rust-lld-x86_64-unknown-linux-gnu](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/run-make/rust-lld-x86_64-unknown-linux-gnu): Checks that LLD is used by default on `x86_64-unknown-linux-gnu` when the bootstrap `rust.lld` config option is `true`. - [rust-lld-x86_64-unknown-linux-gnu-dist](https://github.com/rust-lang/rust/blob/1117bc1e6ce049495b0044dfe756afafc817d2d7/tests/run-make/rust-lld-x86_64-unknown-linux-gnu-dist): Dist test that checks that our distributed `x86_64-unknown-linux-gnu` archives actually use LLD by default. ## Ecosystem impact As already stated, LLD has been used as the default linker on x64 Linux on nightly for almost a year, and we haven't seen any blockers to stabilization in that time. There were a handful of issues reported, these are discussed later below. Furthermore, two crater runs ([November 2023](https://crater-reports.s3.amazonaws.com/pr-117684-2/index.html), [February 2025](https://crater-reports.s3.amazonaws.com/pr-137044-3/index.html)), were performed to test the impact of using LLD as the default linker. A triage of the earlier crater run was previously done [here](https://hackmd.io/OAJxlxc6Te6YUot9ftYSKQ), but all the important findings from both crater runs are reported below. Below is a list of compatibility differences between BFD and LLD that we have encountered. There is a more thorough list of differences in [this post](https://maskray.me/blog/2020-12-19-lld-and-gnu-linker-incompatibilities) from the current LLD maintainer. From that post, "99.9% pieces of software work with ld.lld without a change". --- ### `.ctors/.dtors` sections [#128286](https://github.com/rust-lang/rust/issues/128286) reported an issue where LLD was unable to link certain CUDA library was using these sections that were using the `.ctors/.dtors` ELF sections. These were deprecated a long time ago (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46770), replaced with a more modern `.init_array/.fini_array` sections. LLD doesn't (and won't) support these sections ([1](https://github.com/llvm/llvm-project/issues/68071), [2](https://github.com/llvm/llvm-project/issues/30572)), so if they appear in input object files, the linked artifact might produce incorrect behavior, because e.g. some global variables might not get initialized properly. However, the usage of `.ctors/.dtors` should be very rare in practice. We have performed a [crater run](https://github.com/rust-lang/rust/pull/137044) to test this. It has identified only 8 crates where the `.ctors/.dtors` section is occurring in the final linked artifact. It was caused by a few crates using the `.ctors` link section manually, and by using a very (~6 year) old version of the [ctor](https://crates.io/crates/ctor) crate. [Crater run analysis](https://hackmd.io/tFDifkUcSLGoHPBRIl0z8w?view#ctorsdtors-sections) **Possible workaround** It is possible to [detect](https://github.com/rust-lang/rust/commit/e5e2316712e19036eb40e0bf59150f25e35f9880) if `.ctors/.dtors` section is present in the final linked artifact (LLD will keep it there, but it won't be populated), and warn users about it. This check is very cheap and doesn't even appear on [perf](https://github.com/rust-lang/rust/pull/112049#issuecomment-2661125340). We have benchmarked the check on a 240 MiB Chrome binary, where it took 0.8ms with page cache flushed, and 0.06ms with page cache primed (which should be the common case, as the linked artifact is written to disk just before the check is performed). In theory, this could be also solved with a linker script that moves `.ctors` to `.init_array`. We think that these sections should be so rare that it is not worth it to implement any workarounds for now. --- ### Different garbage collection behavior [#130397](https://github.com/rust-lang/rust/issues/130397) reported an issue where LLD prunes a local symbol, so it is missing in the linked artifact. However, BFD keeps the same symbol, so it is a regression. This is caused by a difference in linker garbage collection. Rust uses `--gc-sections` and puts each function into a separate linker section, which prunes unused code. There is some code (specifically the somewhat popular [linkme](https://github.com/dtolnay/linkme) crate) that (arguably ab-)uses so called linker encapsulation symbols to achieve distributed slices. BFD (2.37+) uses a conservative linking mode that works as intended with this behavior, but it might slightly increase binary size of the linked artifact. LLD does not use this workaround by default, which causes the sections to be eliminated, but it can be made to use the conservative mode using [`-z nostart-stop-gc`](https://lld.llvm.org/ELF/start-stop-gc.html#z-start-stop-gc). To avoid this issue, we told LLD to use the [conservative mode](https://github.com/rust-lang/rust/pull/137685), which maintains backwards compatibility with BFD. We found that it has [no effect](https://github.com/rust-lang/rust/pull/112049#issuecomment-2666028775) on compilation performance and binary size in our benchmark suite. With this change, `linkme` works. Since then, rust-lang/rust#140872 removed `linkme` distributed slice's dependence on conservative GC behavior, so this PR also removes that conservative mode: no transition period is necessary, as the PR immediately fixed the crate with no source changes. [Crater run analysis](https://hackmd.io/tFDifkUcSLGoHPBRIl0z8w?view#Different-garbage-collection-behavior) --- ### Various uncommon issues A small number of issues that only occurred in a handful of instances were found in crater, and it is unclear if LLD is at fault or if there is some other issue that was not detected with BFD. You can examine these [here](https://hackmd.io/tFDifkUcSLGoHPBRIl0z8w?view#Various-uncommon-issues). --- ### Missing jobserver support LLD doesn't support the jobserver protocol for limiting the number of threads used, it simply defaults to using all available cores, and is one of the reasons why it's faster than BFD. However, this should mostly be a non-issue, because most of the linking done during high parallelism sections of `cargo build` is linking of build scripts and proc macros, which are typically very fast to link (e.g. ~50ms), and a potential oversubscription of cores thus doesn't hurt that much. When the final artifact is linked (which typically takes the most time), there should be no other sources of parallelism conflicts from compiling other code, so LLD should be able to use all available threads. That being said, it is a difference of behavior, where previously a `-j` flag was generally not using more cpu than the specified limit. It can be impactful in some resource-constrained systems, but to be clear that is already the case today due to [cargo parallelism](https://github.com/rust-lang/cargo/issues/9157). This could be one reason to opt out of using `rust-lld` on some systems. LLD has support for limiting the number of threads to use, so in theory rustc could try to get all the jobserver tokens available and use that as lld's thread limit. It'd still be suboptimal as new tokens would not be dynamically detected, and we could be using less threads than available. We did a benchmark on a real-world crate that shows that using multiple LLD threads for intermediate artifacts doesn't seem to have a performance effect. You can find it [here](https://hackmd.io/tFDifkUcSLGoHPBRIl0z8w?view#Missing-jobserver-support). --- #### Opting out of LLD in the ecosystem We have also examined repositories where people opted out of LLD on nightly, using [this GitHub query](https://github.com/search?q=%22linker-features%3D-lld%22&type=code). The summary can be found below: <details> <summary>Summary of LLD opt outs</summary> > This examination was performed on 2025-03-09. Here we briefly examine the most common reasons why people use `-Zlinker-features=-lld`, based on comments and git history. - Nix/NixOS ([1](https://github.com/rszyma/vscode-kanata/blob/59d703dff5a238b14ab3759cac27f73fb34bbcfe/flake.nix#L33), [2](https://github.com/sbernauer/breakwater/blob/3cc3449fc126c5c99d4a971733fd32be589884e0/.cargo/config.toml#L4), [3](https://github.com/tiiuae/ebpf-firewall/blame/32bdb17cedd1c9bea1ab3482623de458d95da7d0/.cargo/config.toml#L2), [4](https://github.com/jules-sommer/wavetheme-gen/blob/f5f657d014d4a30607625afb70f810c229c0294e/Cargo.toml#L4), [5](https://github.com/LayerTwo-Labs/zside-rust/blob/e4266f5c5571a1b180a9c70cf0939c7070e410c7/.cargo/config.toml#L10), [6](https://github.com/przyjacielpkp/zkml/blob/22a4aef24e9d2c77789229d7c634fc67e9eb1184/README.md?plain=1#L78), [7](https://github.com/LayerTwo-Labs/thunder-rust/blob/2222d53474c8d2d0428b4c56f8157095dced6d5a/.cargo/config.toml#L2), [8](https://github.com/enesoztrk/nixos-tc-aya-test/blob/b2ffa59d3eba8b60fd04b0a4c8bbe047400eb981/.cargo/config.toml#L4), [9](https://github.com/lowRISC/container-hotplug/blob/3ead4ef9c7f79c303392178c99677dbecff1aea6/.cargo/config.toml#L2), [10](https://github.com/Eliah-Lakhin/ad-astra/blob/ca6b8c8a5dba7bb5e894f3f1013f17876962a021/work/examples/lsp-client/src/extension.ts#L94)) - There was an [issue](https://github.com/NixOS/nixpkgs/issues/312661) with LLD, which seems to have been fixed with https://github.com/NixOS/nixpkgs/pull/314268. It's unclear whether that fixed all the Nix issues though. - Issues with linkme ([1](https://github.com/0xPolygonZero/zk_evm/blob/ef388619ffbd5305209519a3a5bc0396185d68ac/.cargo/config.toml#L4), [2](https://github.com/conjure-cp/conjure-oxide/blob/be0fc5827ff90e8486d416cc184b6ce24f73bf01/README.md?plain=1#L20), [3](https://github.com/clchiou/garage/blob/c5d8444d56bb6ee24ca95e5c6b9880ed996f4918/rust/.cargo/config.toml#L6), [4](https://github.com/PonasKovas/craftflow/blob/5b4cc1a5196e08a975368399fefda4b71f3a2f6f/.cargo/config.toml#L3), [5](https://github.com/kezhuw/zookeeper-client-rust/blob/4e27c7de2a7cc5e709af012b791c8fea9bb47f1f/.github/workflows/ci.yml#L82), [6](https://github.com/niklasdewally/conjure-oxide/blob/8fe60c12bca7011a2f9eded4b7c95ad0e77b6f44/.github/workflows/code-coverage.yml#L48), [7](https://github.com/kezhuw/spawns/blob/c8b468379805de9df3287c01b94b4ed3db6b61ed/.github/workflows/ci.yml#L74)) - These should be resolved with the conservative garbage collection ([#137685](https://github.com/rust-lang/rust/pull/137685)). - Bazel ([1](https://github.com/google-parfait/confidential-federated-compute/blob/1823f69ed8f5f4f819f7bfa21da1ca629fdc826b/.bazelrc#L71)), WASM ([1](https://github.com/Eliah-Lakhin/ad-astra/blob/ca6b8c8a5dba7bb5e894f3f1013f17876962a021/work/examples/wasm-build.sh#L37), [2](https://github.com/yacineb/pgrx-wasi-test/blob/2bf99037ca1b650b2cbc35f1257a87fb6ead0920/build.sh#L21)), uncategorized ([2](https://github.com/nbdd0121/r2vm/blob/5118be6b9e757c6fef2f019385873f403c23c548/.cargo/config.toml#L3), [3](https://github.com/Wyvern/Img/blame/45020c7e1dc4926c8129647014c708db0c13f463/.cargo/config.toml#L209), [4](https://github.com/arnaudpoullet/leptos-i18n-compile-error/blob/042eb835f7ca0dc36be67cf7fe65b35b22b6059f/README.md?plain=1#L89), [5](https://github.com/JonLeeCon/numerical-rust-cpu/blob/fd0b3006768ed81c56147044dc05c92b11b7b6f0/exercises/.cargo/config.toml#L13), [6](https://github.com/PonasKovas/shallowclone/blob/be65f2ec923cac6ceedbc8db520c89969ebfce7c/.github/workflows/rust.yml#L20)) - Reason unclear. </details> ## History The idea to use a faster linker by default has been on the radar for quite some time ([#39915](https://github.com/rust-lang/rust/issues/39915), [#71515](https://github.com/rust-lang/rust/issues/71515)). There were [very early attempts](https://github.com/rust-lang/rust/pull/29974) to use the gold linker by default, but these had to be [reverted](https://github.com/rust-lang/rust/pull/30913) because of compatibility issues. Support for LLD was implemented back in [2017](https://github.com/rust-lang/rust/pull/40018), but it has not been made default yet, except for some more niche targets, such as [WASM](https://github.com/rust-lang/rust/pull/48125), [ARM Cortex](https://github.com/rust-lang/rust/pull/53648) or [RISC-V](https://github.com/rust-lang/rust/pull/53822). It took quite some time to figure out how should the interface for selecting the linker (and the way it is invoked) look like, as it differs a lot between different platforms, linkers and compiler drivers. During that time, LLD has matured and achieved [almost perfect compatibility](https://maskray.me/blog/2020-12-19-lld-and-gnu-linker-incompatibilities) with the default Linux linker (BFD). - [#56351](https://github.com/rust-lang/rust/pull/56351) stabilized `-Clinker-flavor`, which is used to determine how to invoke the linker. It is especially useful on targets where selecting the linker directly with `-Clinker` is not possible or is impractical. - December 2018, author @davidtwco, reviewer @nagisa - [#76158](https://github.com/rust-lang/rust/pull/76158) stabilized `-Clink-self-contained=[y|n]`, which allows overriding the compiler's heuristic for deciding whether it should use self-contained or external tools (linker, sanitizers, libc, etc.). It only allowed using the self-contained mode either for everything (`y`) or nothing (`n`), but did not allow granular choice. - September 2020, author @mati864, reviewer @petrochenkov - [#85961](https://github.com/rust-lang/rust/pull/85961) implemented the `-Zgcc-ld` flag, which was a hacky way of opting into LLD usage. - June 2021, author @sledgehammervampire, reviewer @petrochenkov - [MCP 510](https://github.com/rust-lang/compiler-team/issues/510) proposed stabilizing the behavior of `-Zgcc-ld` using more granular flags (`-Clink-self-contained=linker -Clinker-flavor=gcc-lld`). - Initially implemented in [#96827](https://github.com/rust-lang/rust/pull/96827), but @petrochenkov [suggested](https://github.com/rust-lang/rust/pull/96827#issuecomment-1208441595) a slightly different approach. - The PR was split into [#96884](https://github.com/rust-lang/rust/pull/96884), where it was decided what will be the individual components of `-Clink-self-contained=linker`. - And [#96401](https://github.com/rust-lang/rust/pull/96401), which implemented the `-Clinker-flavor` part. - The MCP was finally implemented in [#112910](https://github.com/rust-lang/rust/pull/112910). - [#116514](https://github.com/rust-lang/rust/pull/116514) then removed `-Zgcc-ld`, as it was replaced by `-Clinker-flavor=gnu-lld-cc` + `-Clink-self-contained=linker`. - April 2022 - October 2023, author @lqd, reviewer @petrochenkov - Various linker handling refactorings were performed in the meantime: [#97375](https://github.com/rust-lang/rust/pull/97375), [#98212](https://github.com/rust-lang/rust/pull/98212), [#100126](https://github.com/rust-lang/rust/pull/100126), [#100552](https://github.com/rust-lang/rust/pull/100552), [#102836](https://github.com/rust-lang/rust/pull/102836), [#110807](https://github.com/rust-lang/rust/pull/110807), [#101988](https://github.com/rust-lang/rust/pull/101988), [#116515](https://github.com/rust-lang/rust/pull/116515) - The implementation of linker flavors with LLD was causing a sort of a combinatorial explosion of various options. [#119906](https://github.com/rust-lang/rust/pull/119906) suggested a different approach for linker flavors (described [here](https://github.com/rust-lang/rust/pull/119906#issuecomment-1894088306)), where the individual flavors could be enabled separately using `+/-` (e.g. `+lld`). - After some back and forth, this idea was moved to `-Clinker-features` (see [comment 1](https://github.com/rust-lang/rust/pull/119906#issuecomment-1895693162) and [comment 2](https://github.com/rust-lang/rust/pull/119906#issuecomment-1980801438)), which was implemented in [#123656](https://github.com/rust-lang/rust/pull/123656). - April 2024, author @lqd, reviewer @petrochenkov - [#124129](https://github.com/rust-lang/rust/pull/124129) enabled LLD by default on nightly. - April 2024, author @lqd, reviewer @petrochenkov - [#137685](https://github.com/rust-lang/rust/pull/137685), [#137926](https://github.com/rust-lang/rust/pull/137926) enabled the conservative gargage collection mode (`-znostart-stop-gc`) to improve compatibility with BFD. - February 2025, author @lqd, reviewer @petrochenkov (implementation), author @kobzol, reviewer @lqd (test) - [#96025](https://github.com/rust-lang/rust/pull/96025) (April 2022), [#117684](https://github.com/rust-lang/rust/pull/117684) (November 2023), [#137044](https://github.com/rust-lang/rust/pull/137044) (February 2025): crater runs. ## Unresolved questions/concerns - Is changing the linker considered a breaking change? In (hopefully very rare) cases, it might break some existing code. It should mostly only affect the final linked artifact, so it should be easy to opt out. - Similarly, is the single-threaded behavior of such tools encompassed in our stability guarantee: it can be observed via the `-j` job limit (though I believe we have/had some open issues on sometimes using more CPU resources than the job count limit implied). As mentioned above, LLD does not support the jobserver protocol. - A concern [was raised](https://github.com/rust-lang/rust/issues/71515#issuecomment-2612370229) about increased memory usage of LLD. We should probably let users know about the possibly increased memory usage, and jobserver incompatibility: we did so when announcing this landing on nightly. - LLD seems to produce [slightly larger](https://perf.rust-lang.org/compare.html?start=b3e117044c7f707293edc040edb93e7ec5f7040a&end=baed03c51a68376c1789cc373581eea0daf89967&stat=size%3Alinked_artifact&tab=compile) binary artifacts. This can be partially clawed back using Identical Code Folding (`-Clink-args=-Wl,--icf=all`). - Should we detect the outdated `.ctors/.dtors` sections to provide a better error message, even if that should be rare in practice? --- ### Next steps After the FCP completes: - we should land this PR at the beginning of a beta cycle, to maximize time for testing - keep an eye on the beta crater run results for possible linker issues (or do a dedicated beta crater run with only this change) - release a blog post announcing the change, and asking for testing feedback of the appropriate beta - depending on feedback, or if a period of testing of 6 weeks is not long enough, we could keep this change on beta for another cycle --- Development, testing, try builds were done in https://github.com/rust-lang/rust/pull/138645. r? @petrochenkov @rustbot label +needs-fcp +T-compiler

github.com/rust-lang/rust

Enable `rust-lld` on nightly `x86_64-unknown-linux-gnu`

master ← lqd:enable-lld

opened 06:34PM - 18 Apr 24 UTC

lqd

+117 -14

We believe we have done virtually all the internal work and tests we could to pr…epare for using `lld` as the default linker (at least on Linux). We're IMHO at a point where we'd need to expand testing and coverage in order to make progress on this effort. Therefore, for further testing and gathering real-world feedback, unexpected issues and use-cases, this PR enables `rust-lld` as the default linker: - on nightly only (and dev channel) - on `x86_64-unknown-linux-gnu` only - when not using an external LLVM (except `download-ci-llvm`), so that distros are not impacted as described in more detail in this [zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Enabling.20.60rust-lld.60.20on.20nightly.20.60x86_64-unknown-linux-gnu.60/near/433709343). In case any issues happen to users, as e.g. lld is not bug-for-bug compatible with GNU ld, it's easy to disable with `-Zlinker-features=-lld` to revert to using the system's default linker. --- I don't know who should review this kind of things, as it's somewhat of a crosscutting effort. Compiler contributor, compiler performance WG and infra member sounds perfect, so r? @Mark-Simulacrum. The last crater run encountered a low number (44) of mainly avoidable issues, like small incompatibilities, user errors, and a difference between the two linkers about which default to use with `--gc-sections`. [Here's the triage report](https://hackmd.io/OAJxlxc6Te6YUot9ftYSKQ?view), categorizing the issues, with some analyses and workarounds. I'd appreciate another set of eyes looking at these results. The changes in this PR have been test-driven for CI changes, try builds with tests enabled, rustc-perf with bootstrapping, in PR #113382. For infra, about the CI change: this PR forces `rust.lld` to false on vanilla LLVM builders, just to make sure we have coverage without `rust-lld`. Though to be clear, just using an external LLVM is already enough to keep `rust.lld` to false, in turn reverting everything to using the system's default linker. cc @rust-lang/bootstrap for the bootstrap and config change cc @petrochenkov for the small compiler change cc @rust-lang/wg-compiler-performance The blog post announcing the change, that we expect to merge around the same time as we merge this PR, is open [on the blog repo](https://github.com/rust-lang/blog.rust-lang.org/pull/1319). Bootstrap change history: this PR changes the default of a config option on `x86_64-unknown-linux-gnu`. It's, however, not expected to cause issues, or require any changes to existing configurations. It's a big enough change that people should at least know about it, in case it causes unexpected problems. If that happens, set `rust.lld = false` in your `config.toml` (and open an issue).

While integrating Rust into our Yocto-based project, we attempted to support and build rust-lld as part of the Rust build process.
During this effort, we encountered a few issues and would like to confirm whether they are expected or indicative of a regression.

Observations:

Additional Build Dependencies
Enabling rust-lld introduced new dependencies not previously required for the bootstrap build:
ninja, openssl, pkg-config, bash (A few of them are mentioned in rust/INSTALL.md at master · rust-lang/rust · GitHub)

Following errors were encountered:

1.
   error: failed to run custom build command for `openssl-sys v0.9.106`
   | Could not find openssl via pkg-config:
   |   The pkg-config command could not be found.

   2.
   | Building LLD for x86_64-unknown-linux-gnu
   |
   | Couldn't find required command: ninja (or ninja-build)
   |
   | You should install ninja as described at
   | <https://github.com/ninja-build/ninja/wiki/Pre-built-Ninja-packages>,
   | or set `ninja = false` in the `[llvm]` section of `config.toml`.
   | Alternatively, set `download-ci-llvm = true` in that `[llvm]` section
   | to download LLVM rather than building it.

Increase in Build Time and Filesystem Usage
We observed significant increases in both build time and disk usage during the bootstrap process:

Build time increased from ~32 minutes to ~45 minutes (+40%)
Disk usage increased from ~10 GB to ~13 GB (+30%)

Questions & Concerns:

Is there a comprehensive document or reference for rust-lld that outlines its purpose, requirements, and implications for downstream build systems?
Are these additional dependencies and the observed increases in build time and space expected as part of rust-lld support, or could this be a regression?
Based on our understanding, rust-lld is not yet fully stable and is currently enabled by default only for x86_64-unknown-linux-gnu. We also noticed (this commit, Do not enable LLD by default in the dist profile by Kobzol · Pull Request #143255 · rust-lang/rust · GitHub) disables rust-lld by default in distributed Rust sources.

Given the impact on various downstream distributions and the current stability status of rust-lld, is it recommended to enable and build rust-lld in place of the system/default linker for Rust-based projects?

We would appreciate any clarification or guidance on the intended usage and roadmap for rust-lld, especially for embedded and cross-compilation use cases.
Thanks.

bjorn3 · July 22, 2025, 11:16am

This has nothing to do with rust-lld. Rust-lld is a renamed lld, which does not use any rust code. This error is for the rust openssl-sys crate however.

r_learn:

2.
   | Building LLD for x86_64-unknown-linux-gnu
   |
   | Couldn't find required command: ninja (or ninja-build)
   |
   | You should install ninja as described at
   | <https://github.com/ninja-build/ninja/wiki/Pre-built-Ninja-packages>,
   | or set `ninja = false` in the `[llvm]` section of `config.toml`.
   | Alternatively, set `download-ci-llvm = true` in that `[llvm]` section
   | to download LLVM rather than building it.

Do you have ninja installed? If not follow the suggestion in the error to disable ninja usage.

Lld is a much faster linker than ld.bfd, thus using it for building rust programs reduces build time. And it is necessary when compiling for wasm.

Lld depends on LLVM. I'm guessing you were previously using the system LLVM install, but because rust-lld got enabled, (part of) the in-tree LLVM gets built too. If you use a system LLVM install, you may want to disable building rust-lld.

r_learn · July 22, 2025, 11:32am

No, currently we don't build ninja. Is it recommended for building lld?

Currently, we set download-ci-llvm to false in config.toml. There's a separate rust-llvm in our sources which is used to provide llvm support for build rust. Perhaps similar to what you are referring to as system llvm.

Will that internally also build rust-lld? If yes, then we can disable building it as you suggested. Also, is it recommended to build and use rust-lld for cross-compilation target builds in downstream projects?

bjorn3 · July 22, 2025, 11:34am

No, I mean you can point the llvm-config config key to the location of llvm-config in an existing LLVM installation. Are you doing this? If so then disable rust-lld building. If not, it should already have been complaining about missing ninja when building LLVM if you didn't disable ninja usage.

Can you post your config.toml?

r_learn · July 22, 2025, 12:14pm

I believe you are referring to llvm-config set in [target.x86_64-unknown-linux-gnu] and [target.x86_64-poky-linux-gnu].

So, we need not build rust-lld and can disable it right?
As per the given config.toml, are we already building and using rust-lld from the path mentioned in llvm-config installation?
Could you please confirm?

Here's the config.toml:

change-id = 116881

[target.x86_64-poky-linux-gnu]
llvm-config = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/recipe-sysroot/usr/lib/llvm-rust/bin/llvm-config"
cxx = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/wrapper/target-rust-cxx"
cc = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/wrapper/target-rust-cc"
linker = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/wrapper/target-rust-ccld"

[target.x86_64-unknown-linux-gnu]
llvm-config = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/recipe-sysroot-native/usr/lib/llvm-rust/bin/llvm-config"
cxx = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/wrapper/build-rust-cxx"
cc = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/wrapper/build-rust-cc"
linker = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/wrapper/build-rust-ccld"

[llvm]
static-libstdcpp = false
download-ci-llvm = false

[rust]
rpath = true
remap-debuginfo = true
download-rustc = false
llvm-tools = false
channel = "nightly"
optimize = true
verbose-tests = true

[build]
submodules = false
docs = false
rustc = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/recipe-sysroot-native/usr/bin/rustc"
cargo = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/recipe-sysroot-native/usr/bin/cargo"
extended = false
vendor = true
target = ["x86_64-poky-linux-gnu"]
host = ["x86_64-poky-linux-gnu"]
build = "x86_64-unknown-linux-gnu"

[install]
prefix = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/image/usr"
bindir = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/image/usr/bin"
libdir = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/image/usr/lib"
datadir = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/image/usr/share"
mandir = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/image/usr/share/man"
sysconfdir = "/home/poky/build/tmp/work/core2-64-poky-linux/rust/1.88.0/image/etc"

bjorn3 · July 22, 2025, 12:18pm

Yes

Yes, that is what I would suggest.

r_learn · July 22, 2025, 12:20pm

What about this?
Thank you for the information and your help.

bjorn3 · July 23, 2025, 9:34am

You can configure whether or not you build lld when building LLVM. I don't know what you have configured. In any case the executable would be called lld, not rust-lld when doing that. We rename lld to rust-lld to prevent conflicting with a system lld install, but there is no reason to do the rename when building against an already existing LLVM.

system · October 21, 2025, 9:35am

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Increased build time and dependencies due to rust-lld in bootstrap builds

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