Auto merge of #94381 - Kobzol:llvm-bolt, r=Mark-Simulacrum

Use BOLT in CI to optimize LLVM

This PR adds an optimization step in the Linux `dist` CI pipeline that uses [BOLT](https://github.com/llvm/llvm-project/tree/main/bolt) to optimize the `libLLVM.so` library built by boostrap.

Steps:
- [x] Use LLVM 15 as a bootstrap compiler and use it to build BOLT
- [x] Compile LLVM with support for relocations (`-DCMAKE_SHARED_LINKER_FLAGS="-Wl,-q"`)
- [x] Gather profile data using instrumented LLVM
- [x] Apply profile to LLVM that has already been PGOfied
- [x] Run with BOLT profiling on more benchmarks
- [x] Decide on the order of optimization (PGO -> BOLT?)
- [x] Decide how we should get `bolt` (currently we use the host `bolt`)
- [x] Clean up

The latest perf results can be found [here](https://github.com/rust-lang/rust/pull/94381#issuecomment-1258269440). The current CI build time with BOLT applied is around 1h 55 minutes.

src/bootstrap/native.rs

@@ -16,6 +16,7 @@ use std::io;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
+use crate::bolt::{instrument_with_bolt_inplace, optimize_library_with_bolt_inplace};
 use crate::builder::{Builder, RunConfig, ShouldRun, Step};
 use crate::channel;
 use crate::config::TargetSelection;
@@ -403,6 +404,12 @@ impl Step for Llvm {
         if let Some(path) = builder.config.llvm_profile_use.as_ref() {
             cfg.define("LLVM_PROFDATA_FILE", &path);
         }
+        if builder.config.llvm_bolt_profile_generate
+            || builder.config.llvm_bolt_profile_use.is_some()
+        {
+            // Relocations are required for BOLT to work.
+            ldflags.push_all("-Wl,-q");
+        }
 
         // Disable zstd to avoid a dependency on libzstd.so.
         cfg.define("LLVM_ENABLE_ZSTD", "OFF");
@@ -571,12 +578,34 @@ impl Step for Llvm {
             }
         }
 
+        // After LLVM is built, we modify (instrument or optimize) the libLLVM.so library file
+        // in place. This is fine, because currently we do not support incrementally rebuilding
+        // LLVM after a configuration change, so to rebuild it the build files have to be removed,
+        // which will also remove these modified files.
+        if builder.config.llvm_bolt_profile_generate {
+            instrument_with_bolt_inplace(&get_built_llvm_lib_path(&build_llvm_config));
+        }
+        if let Some(path) = &builder.config.llvm_bolt_profile_use {
+            optimize_library_with_bolt_inplace(
+                &get_built_llvm_lib_path(&build_llvm_config),
+                &Path::new(path),
+            );
+        }
+
         t!(stamp.write());
 
         build_llvm_config
     }
 }
 
+/// Returns path to a built LLVM library (libLLVM.so).
+/// Assumes that we have built LLVM into a single library file.
+fn get_built_llvm_lib_path(llvm_config_path: &Path) -> PathBuf {
+    let mut cmd = Command::new(llvm_config_path);
+    cmd.arg("--libfiles");
+    PathBuf::from(output(&mut cmd).trim())
+}
+
 fn check_llvm_version(builder: &Builder<'_>, llvm_config: &Path) {
     if !builder.config.llvm_version_check {
         return;