bootstrap: Link LLVM as a dylib with ThinLTO

When building a distributed compiler on Linux where we use ThinLTO to
create the LLVM shared object this commit switches the compiler to
dynamically linking that LLVM artifact instead of statically linking to
LLVM. The primary goal here is to reduce CI compile times, avoiding two+
ThinLTO builds of all of LLVM. By linking dynamically to LLVM we'll
reuse the one ThinLTO step done by LLVM's build itself.

Lots of discussion about this change can be found [here] and down. A
perf run will show whether this is worth it or not!

[here]: https://github.com/rust-lang/rust/pull/53245#issuecomment-417015334

1 files changed, 8 insertions, 41 deletions

diff --git a/src/bootstrap/compile.rs b/src/bootstrap/compile.rs
index 821bd002e95..0d2546a0e9c 100644
--- a/src/bootstrap/compile.rs
+++ b/src/bootstrap/compile.rs
@@ -19,6 +19,7 @@ use build_helper::{output, mtime, up_to_date};
 use filetime::FileTime;
 use serde_json;
 
+use crate::dist;
 use crate::util::{exe, libdir, is_dylib};
 use crate::{Compiler, Mode, GitRepo};
 use crate::native;
@@ -104,7 +105,6 @@ impl Step for Std {
                 &compiler.host, target));
         run_cargo(builder,
                   &mut cargo,
-                  vec![],
                   &libstd_stamp(builder, compiler, target),
                   false);
 
@@ -365,7 +365,6 @@ impl Step for Test {
                 &compiler.host, target));
         run_cargo(builder,
                   &mut cargo,
-                  vec![],
                   &libtest_stamp(builder, compiler, target),
                   false);
 
@@ -493,7 +492,6 @@ impl Step for Rustc {
                  compiler.stage, &compiler.host, target));
         run_cargo(builder,
                   &mut cargo,
-                  vec![],
                   &librustc_stamp(builder, compiler, target),
                   false);
 
@@ -636,47 +634,18 @@ impl Step for CodegenBackend {
 
         let out_dir = builder.cargo_out(compiler, Mode::Codegen, target);
 
-        let mut cargo = builder.cargo(compiler, Mode::Codegen, target, "rustc");
+        let mut cargo = builder.cargo(compiler, Mode::Codegen, target, "build");
         cargo.arg("--manifest-path")
             .arg(builder.src.join("src/librustc_codegen_llvm/Cargo.toml"));
         rustc_cargo_env(builder, &mut cargo);
 
         let features = build_codegen_backend(&builder, &mut cargo, &compiler, target, backend);
 
-        let mut cargo_tails_args = vec![];
-
-        if builder.config.llvm_thin_lto {
-            cargo_tails_args.push("--".to_string());
-
-            let num_jobs = builder.jobs();
-
-            if !target.contains("msvc") {
-                // Here we assume that the linker is clang. If it's not, there'll
-                // be linker errors.
-                cargo_tails_args.push("-Clink-arg=-fuse-ld=lld".to_string());
-                cargo_tails_args.push("-Clink-arg=-flto=thin".to_string());
-
-                if builder.config.llvm_optimize {
-                    cargo_tails_args.push("-Clink-arg=-O2".to_string());
-                }
-
-                // Let's make LLD respect the `-j` option.
-                let num_jobs_arg = format!("-Clink-arg=-Wl,--thinlto-jobs={}", num_jobs);
-                cargo_tails_args.push(num_jobs_arg);
-            } else {
-                // Here we assume that the linker is lld-link.exe. lld-link.exe
-                // does not need the extra arguments except for num_jobs
-                let num_jobs_arg = format!("-Clink-arg=/opt:lldltojobs={}", num_jobs);
-                cargo_tails_args.push(num_jobs_arg);
-            }
-        }
-
         let tmp_stamp = out_dir.join(".tmp.stamp");
 
         let _folder = builder.fold_output(|| format!("stage{}-rustc_codegen_llvm", compiler.stage));
         let files = run_cargo(builder,
                               cargo.arg("--features").arg(features),
-                              cargo_tails_args,
                               &tmp_stamp,
                               false);
         if builder.config.dry_run {
@@ -749,7 +718,9 @@ pub fn build_codegen_backend(builder: &Builder,
                                          "libstdc++.a");
                 cargo.env("LLVM_STATIC_STDCPP", file);
             }
-            if builder.config.llvm_link_shared {
+            if builder.config.llvm_link_shared ||
+                (builder.config.llvm_thin_lto && backend != "emscripten")
+            {
                 cargo.env("LLVM_LINK_SHARED", "1");
             }
         }
@@ -989,6 +960,8 @@ impl Step for Assemble {
             copy_lld_to_sysroot(builder, target_compiler, &lld_install);
         }
 
+        dist::maybe_install_llvm_dylib(builder, target_compiler.host, &sysroot);
+
         // Link the compiler binary itself into place
         let out_dir = builder.cargo_out(build_compiler, Mode::Rustc, host);
         let rustc = out_dir.join(exe("rustc_binary", &*host));
@@ -1015,7 +988,6 @@ pub fn add_to_sysroot(builder: &Builder, sysroot_dst: &Path, stamp: &Path) {
 
 pub fn run_cargo(builder: &Builder,
                  cargo: &mut Command,
-                 tail_args: Vec<String>,
                  stamp: &Path,
                  is_check: bool)
     -> Vec<PathBuf>
@@ -1038,7 +1010,7 @@ pub fn run_cargo(builder: &Builder,
     // files we need to probe for later.
     let mut deps = Vec::new();
     let mut toplevel = Vec::new();
-    let ok = stream_cargo(builder, cargo, tail_args, &mut |msg| {
+    let ok = stream_cargo(builder, cargo, &mut |msg| {
         let filenames = match msg {
             CargoMessage::CompilerArtifact { filenames, .. } => filenames,
             _ => return,
@@ -1163,7 +1135,6 @@ pub fn run_cargo(builder: &Builder,
 pub fn stream_cargo(
     builder: &Builder,
     cargo: &mut Command,
-    tail_args: Vec<String>,
     cb: &mut dyn FnMut(CargoMessage),
 ) -> bool {
     if builder.config.dry_run {
@@ -1174,10 +1145,6 @@ pub fn stream_cargo(
     cargo.arg("--message-format").arg("json")
          .stdout(Stdio::piped());
 
-    for arg in tail_args {
-        cargo.arg(arg);
-    }
-
     builder.verbose(&format!("running: {:?}", cargo));
     let mut child = match cargo.spawn() {
         Ok(child) => child,