From 1af1c2de36d39c5e24846d74f1ae549499bab70d Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Thu, 19 Oct 2017 20:02:46 -0700
Subject: [PATCH] rustbuild: Compile rustc with ThinLTO

This commit enables ThinLTO for the compiler as well as multiple codegen units.
This is intended to get the benefits of parallel codegen while also avoiding
any major loss of perf. Finally this commit is also intended as further testing
for #45320 and shaking out bugs.
---
 src/bootstrap/bin/rustc.rs |  3 +++
 src/bootstrap/builder.rs   | 14 ++++++++++++--
 src/bootstrap/config.rs    |  7 +++----
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/bootstrap/bin/rustc.rs b/src/bootstrap/bin/rustc.rs
index aeeda85e924ef..16a23eb364a38 100644
--- a/src/bootstrap/bin/rustc.rs
+++ b/src/bootstrap/bin/rustc.rs
@@ -175,6 +175,9 @@ fn main() {
         if let Ok(s) = env::var("RUSTC_CODEGEN_UNITS") {
             cmd.arg("-C").arg(format!("codegen-units={}", s));
         }
+        if stage != "0" && env::var("RUSTC_THINLTO").is_ok() {
+            cmd.arg("-Ccodegen-units=16").arg("-Zthinlto");
+        }
 
         // Emit save-analysis info.
         if env::var("RUSTC_SAVE_ANALYSIS") == Ok("api".to_string()) {
diff --git a/src/bootstrap/builder.rs b/src/bootstrap/builder.rs
index 6480b5a619c03..5891925f7f798 100644
--- a/src/bootstrap/builder.rs
+++ b/src/bootstrap/builder.rs
@@ -471,8 +471,6 @@ impl<'a> Builder<'a> {
              .env("RUSTC", self.out.join("bootstrap/debug/rustc"))
              .env("RUSTC_REAL", self.rustc(compiler))
              .env("RUSTC_STAGE", stage.to_string())
-             .env("RUSTC_CODEGEN_UNITS",
-                  self.config.rust_codegen_units.to_string())
              .env("RUSTC_DEBUG_ASSERTIONS",
                   self.config.rust_debug_assertions.to_string())
              .env("RUSTC_SYSROOT", self.sysroot(compiler))
@@ -486,6 +484,10 @@ impl<'a> Builder<'a> {
              })
              .env("TEST_MIRI", self.config.test_miri.to_string());
 
+        if let Some(n) = self.config.rust_codegen_units {
+            cargo.env("RUSTC_CODEGEN_UNITS", n.to_string());
+        }
+
         if let Some(host_linker) = self.build.linker(compiler.host) {
             cargo.env("RUSTC_HOST_LINKER", host_linker);
         }
@@ -618,6 +620,14 @@ impl<'a> Builder<'a> {
         // FIXME: cargo bench does not accept `--release`
         if self.config.rust_optimize && cmd != "bench" {
             cargo.arg("--release");
+
+            if mode != Mode::Libstd &&
+               self.config.rust_codegen_units.is_none() &&
+               self.build.is_rust_llvm(compiler.host)
+
+            {
+                cargo.env("RUSTC_THINLTO", "1");
+            }
         }
         if self.config.locked_deps {
             cargo.arg("--locked");
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index d6c83e3acfc8a..66e5efcea4e80 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -81,7 +81,7 @@ pub struct Config {
 
     // rust codegen options
     pub rust_optimize: bool,
-    pub rust_codegen_units: u32,
+    pub rust_codegen_units: Option<u32>,
     pub rust_debug_assertions: bool,
     pub rust_debuginfo: bool,
     pub rust_debuginfo_lines: bool,
@@ -307,7 +307,6 @@ impl Config {
         config.submodules = true;
         config.docs = true;
         config.rust_rpath = true;
-        config.rust_codegen_units = 1;
         config.channel = "dev".to_string();
         config.codegen_tests = true;
         config.ignore_git = false;
@@ -470,8 +469,8 @@ impl Config {
             config.musl_root = rust.musl_root.clone().map(PathBuf::from);
 
             match rust.codegen_units {
-                Some(0) => config.rust_codegen_units = num_cpus::get() as u32,
-                Some(n) => config.rust_codegen_units = n,
+                Some(0) => config.rust_codegen_units = Some(num_cpus::get() as u32),
+                Some(n) => config.rust_codegen_units = Some(n),
                 None => {}
             }
         }