Auto merge of #124129 - lqd:enable-lld, r=Mark-Simulacrum

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

We believe we have done virtually all the internal work and tests we could to prepare 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).

8 files changed, 117 insertions, 14 deletions

diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 8ba945e193c..910c6aeb7d6 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -612,6 +612,12 @@ impl LinkSelfContainedDefault {
             _ => "crt-objects-fallback",
         }
     }
+
+    /// Creates a `LinkSelfContainedDefault` enabling the self-contained linker for target specs
+    /// (the equivalent of `-Clink-self-contained=+linker` on the CLI).
+    pub fn with_linker() -> LinkSelfContainedDefault {
+        LinkSelfContainedDefault::WithComponents(LinkSelfContainedComponents::LINKER)
+    }
 }
 
 bitflags::bitflags! {
diff --git a/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs b/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs
index 11fb28a9aed..bd12d4d8af0 100644
--- a/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs
+++ b/compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs
@@ -18,6 +18,13 @@ pub fn target() -> Target {
         | SanitizerSet::THREAD;
     base.supports_xray = true;
 
+    // When we're asked to use the `rust-lld` linker by default, set the appropriate lld-using
+    // linker flavor, and self-contained linker component.
+    if option_env!("CFG_USE_SELF_CONTAINED_LINKER").is_some() {
+        base.linker_flavor = LinkerFlavor::Gnu(Cc::Yes, Lld::Yes);
+        base.link_self_contained = crate::spec::LinkSelfContainedDefault::with_linker();
+    }
+
     Target {
         llvm_target: "x86_64-unknown-linux-gnu".into(),
         metadata: crate::spec::TargetMetadata {
diff --git a/config.example.toml b/config.example.toml
index 224d079b206..228521747ed 100644
--- a/config.example.toml
+++ b/config.example.toml
@@ -653,9 +653,12 @@
 # when no explicit backend is specified.
 #codegen-backends = ["llvm"]
 
-# Indicates whether LLD will be compiled and made available in the sysroot for
-# rustc to execute.
-#lld = false
+# Indicates whether LLD will be compiled and made available in the sysroot for rustc to execute, and
+# whether to set it as rustc's default linker on `x86_64-unknown-linux-gnu`. This will also only be
+# when *not* building an external LLVM (so only when using `download-ci-llvm` or building LLVM from
+# the in-tree source): setting `llvm-config` in the `[target.x86_64-unknown-linux-gnu]` section will
+# make this default to false. 
+#lld = false in all cases, except on `x86_64-unknown-linux-gnu` as described above, where it is true
 
 # Indicates whether LLD will be used to link Rust crates during bootstrap on
 # supported platforms.
diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index 48a6602e2df..66692a2a2cb 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -1136,6 +1136,11 @@ pub fn rustc_cargo_env(
         cargo.env("CFG_DEFAULT_LINKER", s);
     }
 
+    // Enable rustc's env var for `rust-lld` when requested.
+    if builder.config.lld_enabled {
+        cargo.env("CFG_USE_SELF_CONTAINED_LINKER", "1");
+    }
+
     if builder.config.rust_verify_llvm_ir {
         cargo.env("RUSTC_VERIFY_LLVM_IR", "1");
     }
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index bb51433a3dc..8713eb304d2 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -1532,6 +1532,7 @@ impl Config {
         let mut debuginfo_level_tests = None;
         let mut optimize = None;
         let mut omit_git_hash = None;
+        let mut lld_enabled = None;
 
         if let Some(rust) = toml.rust {
             let Rust {
@@ -1565,7 +1566,7 @@ impl Config {
                 dist_src,
                 save_toolstates,
                 codegen_backends,
-                lld,
+                lld: lld_enabled_toml,
                 llvm_tools,
                 llvm_bitcode_linker,
                 deny_warnings,
@@ -1620,6 +1621,7 @@ impl Config {
             debuginfo_level_std = debuginfo_level_std_toml;
             debuginfo_level_tools = debuginfo_level_tools_toml;
             debuginfo_level_tests = debuginfo_level_tests_toml;
+            lld_enabled = lld_enabled_toml;
 
             config.rust_split_debuginfo_for_build_triple = split_debuginfo
                 .as_deref()
@@ -1653,18 +1655,8 @@ impl Config {
                 config.incremental = true;
             }
             set(&mut config.lld_mode, lld_mode);
-            set(&mut config.lld_enabled, lld);
             set(&mut config.llvm_bitcode_linker_enabled, llvm_bitcode_linker);
 
-            if matches!(config.lld_mode, LldMode::SelfContained)
-                && !config.lld_enabled
-                && flags.stage.unwrap_or(0) > 0
-            {
-                panic!(
-                    "Trying to use self-contained lld as a linker, but LLD is not being added to the sysroot. Enable it with rust.lld = true."
-                );
-            }
-
             config.llvm_tools_enabled = llvm_tools.unwrap_or(true);
             config.rustc_parallel =
                 parallel_compiler.unwrap_or(config.channel == "dev" || config.channel == "nightly");
@@ -1954,6 +1946,43 @@ impl Config {
         config.llvm_plugins = llvm_plugins.unwrap_or(false);
         config.rust_optimize = optimize.unwrap_or(RustOptimize::Bool(true));
 
+        // We make `x86_64-unknown-linux-gnu` use the self-contained linker by default, so we will
+        // build our internal lld and use it as the default linker, by setting the `rust.lld` config
+        // to true by default:
+        // - on the `x86_64-unknown-linux-gnu` target
+        // - on the `dev` and `nightly` channels
+        // - when building our in-tree llvm (i.e. the target has not set an `llvm-config`), so that
+        //   we're also able to build the corresponding lld
+        // - or when using an external llvm that's downloaded from CI, which also contains our prebuilt
+        //   lld
+        // - otherwise, we'd be using an external llvm, and lld would not necessarily available and
+        //   thus, disabled
+        // - similarly, lld will not be built nor used by default when explicitly asked not to, e.g.
+        //   when the config sets `rust.lld = false`
+        if config.build.triple == "x86_64-unknown-linux-gnu"
+            && config.hosts == [config.build]
+            && (config.channel == "dev" || config.channel == "nightly")
+        {
+            let no_llvm_config = config
+                .target_config
+                .get(&config.build)
+                .is_some_and(|target_config| target_config.llvm_config.is_none());
+            let enable_lld = config.llvm_from_ci || no_llvm_config;
+            // Prefer the config setting in case an explicit opt-out is needed.
+            config.lld_enabled = lld_enabled.unwrap_or(enable_lld);
+        } else {
+            set(&mut config.lld_enabled, lld_enabled);
+        }
+
+        if matches!(config.lld_mode, LldMode::SelfContained)
+            && !config.lld_enabled
+            && flags.stage.unwrap_or(0) > 0
+        {
+            panic!(
+                "Trying to use self-contained lld as a linker, but LLD is not being added to the sysroot. Enable it with rust.lld = true."
+            );
+        }
+
         let default = debug == Some(true);
         config.rust_debug_assertions = debug_assertions.unwrap_or(default);
         config.rust_debug_assertions_std =
diff --git a/src/bootstrap/src/utils/change_tracker.rs b/src/bootstrap/src/utils/change_tracker.rs
index a1c9ddebb5d..2f9eaf51c34 100644
--- a/src/bootstrap/src/utils/change_tracker.rs
+++ b/src/bootstrap/src/utils/change_tracker.rs
@@ -185,4 +185,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: r#"The compiler profile now defaults to rust.debuginfo-level = "line-tables-only""#,
     },
+    ChangeInfo {
+        change_id: 124129,
+        severity: ChangeSeverity::Warning,
+        summary: "`rust.lld` has a new default value of `true` on `x86_64-unknown-linux-gnu`. Starting at stage1, `rust-lld` will thus be this target's default linker. No config changes should be necessary.",
+    },
 ];
diff --git a/tests/run-make/rust-lld-by-default/main.rs b/tests/run-make/rust-lld-by-default/main.rs
new file mode 100644
index 00000000000..e9f655fc09e
--- /dev/null
+++ b/tests/run-make/rust-lld-by-default/main.rs
@@ -0,0 +1,5 @@
+// Test linking using `cc` with `rust-lld`, which is on by default on the x86_64-unknown-linux-gnu
+// target.
+// See https://github.com/rust-lang/compiler-team/issues/510 for more info
+
+fn main() {}
diff --git a/tests/run-make/rust-lld-by-default/rmake.rs b/tests/run-make/rust-lld-by-default/rmake.rs
new file mode 100644
index 00000000000..876968727f3
--- /dev/null
+++ b/tests/run-make/rust-lld-by-default/rmake.rs
@@ -0,0 +1,43 @@
+// Ensure that rust-lld is used as the default linker on `x86_64-unknown-linux-gnu`, and that it can
+// also be turned off with a CLI flag.
+
+//@ needs-rust-lld
+//@ only-x86_64-unknown-linux-gnu
+
+extern crate run_make_support;
+
+use run_make_support::regex::Regex;
+use run_make_support::rustc;
+use std::process::Output;
+
+fn main() {
+    // A regular compilation should use rust-lld by default. We'll check that by asking the linker
+    // to display its version number with a link-arg.
+    let output = rustc()
+        .env("RUSTC_LOG", "rustc_codegen_ssa::back::link=info")
+        .link_arg("-Wl,-v")
+        .input("main.rs")
+        .run();
+    assert!(
+        find_lld_version_in_logs(output),
+        "the LLD version string should be present in the output logs"
+    );
+
+    // But it can still be disabled by turning the linker feature off.
+    let output = rustc()
+        .env("RUSTC_LOG", "rustc_codegen_ssa::back::link=info")
+        .link_arg("-Wl,-v")
+        .arg("-Zlinker-features=-lld")
+        .input("main.rs")
+        .run();
+    assert!(
+        !find_lld_version_in_logs(output),
+        "the LLD version string should not be present in the output logs"
+    );
+}
+
+fn find_lld_version_in_logs(output: Output) -> bool {
+    let lld_version_re = Regex::new(r"^LLD [0-9]+\.[0-9]+\.[0-9]+").unwrap();
+    let stderr = std::str::from_utf8(&output.stderr).unwrap();
+    stderr.lines().any(|line| lld_version_re.is_match(line))
+}