Auto merge of #129295 - Zalathar:profiler-builtins, r=Kobzol

Build `library/profiler_builtins` from `ci-llvm` if appropriate

Running all of `tests/coverage` requires the LLVM profiler runtime, which requires setting `build.profiler = true`.

Historically, doing that has required checking out the entire `src/llvm-project` submodule. For compiler contributors who otherwise don't need that submodule (thanks to `download-ci-vm`), that's quite inconvenient.

However, thanks to #129116, the downloaded CI LLVM tarball now contains a copy of LLVM's `compiler-rt` directory, which includes all the files needed to build the profiler runtime. So with a little bit of extra logic in bootstrap, we can have `library/profiler_builtins` look for the `compiler-rt` files in `ci-llvm` instead of the `src/llvm-project` submodule.

2 files changed, 37 insertions, 12 deletions

diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index 4353cfadd8d..edf18e2ebf3 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -199,16 +199,6 @@ impl Step for Std {
 
         builder.require_submodule("library/stdarch", None);
 
-        // Profiler information requires LLVM's compiler-rt
-        if builder.config.profiler {
-            builder.require_submodule(
-                "src/llvm-project",
-                Some(
-                    "The `build.profiler` config option requires `compiler-rt` sources from LLVM.",
-                ),
-            );
-        }
-
         let mut target_deps = builder.ensure(StartupObjects { compiler, target });
 
         let compiler_to_use = builder.compiler_for(compiler.stage, compiler.host, target);
@@ -466,6 +456,29 @@ pub fn std_crates_for_run_make(run: &RunConfig<'_>) -> Vec<String> {
     }
 }
 
+/// Tries to find LLVM's `compiler-rt` source directory, for building `library/profiler_builtins`.
+///
+/// Normally it lives in the `src/llvm-project` submodule, but if we will be using a
+/// downloaded copy of CI LLVM, then we try to use the `compiler-rt` sources from
+/// there instead, which lets us avoid checking out the LLVM submodule.
+fn compiler_rt_for_profiler(builder: &Builder<'_>) -> PathBuf {
+    // Try to use `compiler-rt` sources from downloaded CI LLVM, if possible.
+    if builder.config.llvm_from_ci {
+        // CI LLVM might not have been downloaded yet, so try to download it now.
+        builder.config.maybe_download_ci_llvm();
+        let ci_llvm_compiler_rt = builder.config.ci_llvm_root().join("compiler-rt");
+        if ci_llvm_compiler_rt.exists() {
+            return ci_llvm_compiler_rt;
+        }
+    }
+
+    // Otherwise, fall back to requiring the LLVM submodule.
+    builder.require_submodule("src/llvm-project", {
+        Some("The `build.profiler` config option requires `compiler-rt` sources from LLVM.")
+    });
+    builder.src.join("src/llvm-project/compiler-rt")
+}
+
 /// Configure cargo to compile the standard library, adding appropriate env vars
 /// and such.
 pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, cargo: &mut Cargo) {
@@ -473,8 +486,15 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
         cargo.env("MACOSX_DEPLOYMENT_TARGET", target);
     }
 
+    // Paths needed by `library/profiler_builtins/build.rs`.
     if let Some(path) = builder.config.profiler_path(target) {
         cargo.env("LLVM_PROFILER_RT_LIB", path);
+    } else if builder.config.profiler_enabled(target) {
+        let compiler_rt = compiler_rt_for_profiler(builder);
+        // Currently this is separate from the env var used by `compiler_builtins`
+        // (below) so that adding support for CI LLVM here doesn't risk breaking
+        // the compiler builtins. But they could be unified if desired.
+        cargo.env("RUST_COMPILER_RT_FOR_PROFILER", compiler_rt);
     }
 
     // Determine if we're going to compile in optimized C intrinsics to
@@ -507,8 +527,8 @@ pub fn std_cargo(builder: &Builder<'_>, target: TargetSelection, stage: u32, car
         );
         let compiler_builtins_root = builder.src.join("src/llvm-project/compiler-rt");
         assert!(compiler_builtins_root.exists());
-        // Note that `libprofiler_builtins/build.rs` also computes this so if
-        // you're changing something here please also change that.
+        // 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/utils/change_tracker.rs b/src/bootstrap/src/utils/change_tracker.rs
index c629f04c00e..51a25104e4c 100644
--- a/src/bootstrap/src/utils/change_tracker.rs
+++ b/src/bootstrap/src/utils/change_tracker.rs
@@ -230,4 +230,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Warning,
         summary: "./x test --rustc-args was renamed to --compiletest-rustc-args as it only applies there. ./x miri --rustc-args was also removed.",
     },
+    ChangeInfo {
+        change_id: 129295,
+        severity: ChangeSeverity::Info,
+        summary: "The `build.profiler` option now tries to use source code from `download-ci-llvm` if possible, instead of checking out the `src/llvm-project` submodule.",
+    },
 ];