Allow linking a prebuilt optimized compiler-rt builtins library

Extend the <target>.optimized-compiler-builtins bootstrap option to accept a
path to a prebuilt compiler-rt builtins library, and update compiler-builtins
to enable optimized builtins without building compiler-rt builtins.

5 files changed, 43 insertions, 24 deletions

diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index 8cad5b920b9..005c849c47a 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -575,25 +575,31 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, cargo: &mut Car
     // `compiler-builtins` crate is enabled and it's configured to learn where
     // `compiler-rt` is located.
     let compiler_builtins_c_feature = if builder.config.optimized_compiler_builtins(target) {
-        // NOTE: this interacts strangely with `llvm-has-rust-patches`. In that case, we enforce `submodules = false`, so this is a no-op.
-        // But, the user could still decide to manually use an in-tree submodule.
-        //
-        // NOTE: if we're using system llvm, we'll end up building a version of `compiler-rt` that doesn't match the LLVM we're linking to.
-        // That's probably ok? At least, the difference wasn't enforced before. There's a comment in
-        // the compiler_builtins build script that makes me nervous, though:
-        // https://github.com/rust-lang/compiler-builtins/blob/31ee4544dbe47903ce771270d6e3bea8654e9e50/build.rs#L575-L579
-        builder.require_submodule(
-            "src/llvm-project",
-            Some(
-                "The `build.optimized-compiler-builtins` config option \
-                 requires `compiler-rt` sources from LLVM.",
-            ),
-        );
-        let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
-        assert!(compiler_builtins_root.exists());
-        // The path to `compiler-rt` is also used by `profiler_builtins` (above),
-        // so if you're changing something here please also change that as appropriate.
-        cargo.env("RUST_COMPILER_RT_ROOT", &compiler_builtins_root);
+        if let Some(path) = builder.config.optimized_compiler_builtins_path(target) {
+            cargo.env("LLVM_COMPILER_RT_LIB", path);
+        } else {
+            // NOTE: this interacts strangely with `llvm-has-rust-patches`. In that case, we enforce
+            // `submodules = false`, so this is a no-op. But, the user could still decide to
+            //  manually use an in-tree submodule.
+            //
+            // NOTE: if we're using system llvm, we'll end up building a version of `compiler-rt`
+            // that doesn't match the LLVM we're linking to. That's probably ok? At least, the
+            // difference wasn't enforced before. There's a comment in the compiler_builtins build
+            // script that makes me nervous, though:
+            // https://github.com/rust-lang/compiler-builtins/blob/31ee4544dbe47903ce771270d6e3bea8654e9e50/build.rs#L575-L579
+            builder.require_submodule(
+                "src/llvm-project",
+                Some(
+                    "The `build.optimized-compiler-builtins` config option \
+                     requires `compiler-rt` sources from LLVM.",
+                ),
+            );
+            let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
+            assert!(compiler_builtins_root.exists());
+            // The path to `compiler-rt` is also used by `profiler_builtins` (above),
+            // so if you're changing something here please also change that as appropriate.
+            cargo.env("RUST_COMPILER_RT_ROOT", &compiler_builtins_root);
+        }
         " compiler-builtins-c"
     } else {
         ""
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index 05c2579ac08..09a1eb5b847 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -1675,10 +1675,18 @@ impl Config {
     pub fn optimized_compiler_builtins(&self, target: TargetSelection) -> bool {
         self.target_config
             .get(&target)
-            .and_then(|t| t.optimized_compiler_builtins)
+            .and_then(|t| t.optimized_compiler_builtins.as_ref())
+            .map(StringOrBool::is_string_or_true)
             .unwrap_or(self.optimized_compiler_builtins)
     }
 
+    pub fn optimized_compiler_builtins_path(&self, target: TargetSelection) -> Option<&str> {
+        match self.target_config.get(&target)?.optimized_compiler_builtins.as_ref()? {
+            StringOrBool::String(s) => Some(s),
+            StringOrBool::Bool(_) => None,
+        }
+    }
+
     pub fn llvm_enabled(&self, target: TargetSelection) -> bool {
         self.enabled_codegen_backends(target).contains(&CodegenBackendKind::Llvm)
     }
diff --git a/src/bootstrap/src/core/config/tests.rs b/src/bootstrap/src/core/config/tests.rs
index 50eba12aba7..c32e4384cf6 100644
--- a/src/bootstrap/src/core/config/tests.rs
+++ b/src/bootstrap/src/core/config/tests.rs
@@ -17,7 +17,7 @@ use crate::core::build_steps::clippy::{LintConfig, get_clippy_rules_in_order};
 use crate::core::build_steps::llvm;
 use crate::core::build_steps::llvm::LLVM_INVALIDATION_PATHS;
 use crate::core::config::toml::TomlConfig;
-use crate::core::config::{LldMode, Target, TargetSelection};
+use crate::core::config::{LldMode, StringOrBool, Target, TargetSelection};
 use crate::utils::tests::git::git_test;
 
 pub(crate) fn parse(config: &str) -> Config {
@@ -212,7 +212,7 @@ runner = "x86_64-runner"
     let darwin = TargetSelection::from_user("aarch64-apple-darwin");
     let darwin_values = Target {
         runner: Some("apple".into()),
-        optimized_compiler_builtins: Some(false),
+        optimized_compiler_builtins: Some(StringOrBool::Bool(false)),
         ..Default::default()
     };
     assert_eq!(
diff --git a/src/bootstrap/src/core/config/toml/target.rs b/src/bootstrap/src/core/config/toml/target.rs
index 2c06fd083a8..3236b5cdbaa 100644
--- a/src/bootstrap/src/core/config/toml/target.rs
+++ b/src/bootstrap/src/core/config/toml/target.rs
@@ -39,7 +39,7 @@ define_config! {
         no_std: Option<bool> = "no-std",
         codegen_backends: Option<Vec<String>> = "codegen-backends",
         runner: Option<String> = "runner",
-        optimized_compiler_builtins: Option<bool> = "optimized-compiler-builtins",
+        optimized_compiler_builtins: Option<StringOrBool> = "optimized-compiler-builtins",
         jemalloc: Option<bool> = "jemalloc",
     }
 }
@@ -71,7 +71,7 @@ pub struct Target {
     pub runner: Option<String>,
     pub no_std: bool,
     pub codegen_backends: Option<Vec<CodegenBackendKind>>,
-    pub optimized_compiler_builtins: Option<bool>,
+    pub optimized_compiler_builtins: Option<StringOrBool>,
     pub jemalloc: Option<bool>,
 }
 
diff --git a/src/bootstrap/src/utils/change_tracker.rs b/src/bootstrap/src/utils/change_tracker.rs
index 54f563fe68c..073954e9337 100644
--- a/src/bootstrap/src/utils/change_tracker.rs
+++ b/src/bootstrap/src/utils/change_tracker.rs
@@ -521,4 +521,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Warning,
         summary: "It is no longer possible to `x dist` or `x install` with stage 0. All dist and install commands have to be on stage 1+.",
     },
+    ChangeInfo {
+        change_id: 143689,
+        severity: ChangeSeverity::Info,
+        summary: "The `optimized-compiler-builtins` option now accepts a path to an existing compiler-rt builtins library.",
+    },
 ];