1 files changed, 28 insertions, 1 deletions
diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index e1cdd226fd6..2da5e1c5902 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -161,7 +161,33 @@ pub fn std_cargo(builder: &Builder<'_>,
         cargo.env("MACOSX_DEPLOYMENT_TARGET", target);
     }
 
+    // Determine if we're going to compile in optimized C intrinsics to
+    // the `compiler-builtins` crate. These intrinsics live in LLVM's
+    // `compiler-rt` repository, but our `src/llvm-project` submodule isn't
+    // always checked out, so we need to conditionally look for this. (e.g. if
+    // an external LLVM is used we skip the LLVM submodule checkout).
+    //
+    // Note that this shouldn't affect the correctness of `compiler-builtins`,
+    // but only its speed. Some intrinsics in C haven't been translated to Rust
+    // yet but that's pretty rare. Other intrinsics have optimized
+    // implementations in C which have only had slower versions ported to Rust,
+    // so we favor the C version where we can, but it's not critical.
+    //
+    // If `compiler-rt` is available ensure that the `c` feature of the
+    // `compiler-builtins` crate is enabled and it's configured to learn where
+    // `compiler-rt` is located.
+    let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
+    let compiler_builtins_c_feature = if compiler_builtins_root.exists() {
+        cargo.env("RUST_COMPILER_RT_ROOT", &compiler_builtins_root);
+        " compiler-builtins-c".to_string()
+    } else {
+        String::new()
+    };
+
     if builder.no_std(target) == Some(true) {
+        let mut features = "compiler-builtins-mem".to_string();
+        features.push_str(&compiler_builtins_c_feature);
+
         // for no-std targets we only compile a few no_std crates
         cargo
             .args(&["-p", "alloc"])
@@ -170,7 +196,8 @@ pub fn std_cargo(builder: &Builder<'_>,
             .arg("--features")
             .arg("compiler-builtins-mem compiler-builtins-c");
     } else {
-        let features = builder.std_features();
+        let mut features = builder.std_features();
+        features.push_str(&compiler_builtins_c_feature);
 
         if compiler.stage != 0 && builder.config.sanitizers {
             // This variable is used by the sanitizer runtime crates, e.g.