Do not enable ThinLTO on stable, beta, or nightly builds.

config.toml.example

@@ -235,6 +235,11 @@
 # compiler.
 #codegen-units = 1
 
+# Whether to enable ThinLTO (and increase the codegen units to either a default
+# or the configured value). On by default. If we want the fastest possible
+# compiler, we should disable this.
+#thinlto = true
+
 # Whether or not debug assertions are enabled for the compiler and standard
 # library. Also enables compilation of debug! and trace! logging macros.
 #debug-assertions = false

src/bootstrap/builder.rs

@@ -509,10 +509,6 @@ impl<'a> Builder<'a> {
              })
              .env("TEST_MIRI", self.config.test_miri.to_string())
              .env("RUSTC_ERROR_METADATA_DST", self.extended_error_dir());
-        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);
@@ -679,18 +675,31 @@ impl<'a> Builder<'a> {
         if self.is_very_verbose() {
             cargo.arg("-v");
         }
+
+        // This must be kept before the thinlto check, as we set codegen units
+        // to 1 forcibly there.
+        if let Some(n) = self.config.rust_codegen_units {
+            cargo.env("RUSTC_CODEGEN_UNITS", n.to_string());
+        }
+
         if self.config.rust_optimize {
             // FIXME: cargo bench does not accept `--release`
             if cmd != "bench" {
                 cargo.arg("--release");
             }
 
             if self.config.rust_codegen_units.is_none() &&
-               self.build.is_rust_llvm(compiler.host)
-            {
+               self.build.is_rust_llvm(compiler.host) &&
+               self.config.rust_thinlto {
                 cargo.env("RUSTC_THINLTO", "1");
+            } else if self.config.rust_codegen_units.is_none() {
+                // Generally, if ThinLTO has been disabled for some reason, we
+                // want to set the codegen units to 1. However, we shouldn't do
+                // this if the option was specifically set by the user.
+                cargo.env("RUSTC_CODEGEN_UNITS", "1");
             }
         }
+
         if self.config.locked_deps {
             cargo.arg("--locked");
         }

src/bootstrap/config.rs

@@ -81,6 +81,7 @@ pub struct Config {
     // rust codegen options
     pub rust_optimize: bool,
     pub rust_codegen_units: Option<u32>,
+    pub rust_thinlto: bool,
     pub rust_debug_assertions: bool,
     pub rust_debuginfo: bool,
     pub rust_debuginfo_lines: bool,
@@ -261,6 +262,7 @@ impl Default for StringOrBool {
 struct Rust {
     optimize: Option<bool>,
     codegen_units: Option<u32>,
+    thinlto: Option<bool>,
     debug_assertions: Option<bool>,
     debuginfo: Option<bool>,
     debuginfo_lines: Option<bool>,
@@ -412,6 +414,7 @@ impl Config {
 
         // Store off these values as options because if they're not provided
         // we'll infer default values for them later
+        let mut thinlto = None;
         let mut llvm_assertions = None;
         let mut debuginfo_lines = None;
         let mut debuginfo_only_std = None;
@@ -455,6 +458,7 @@ impl Config {
             optimize = rust.optimize;
             ignore_git = rust.ignore_git;
             debug_jemalloc = rust.debug_jemalloc;
+            thinlto = rust.thinlto;
             set(&mut config.rust_optimize_tests, rust.optimize_tests);
             set(&mut config.rust_debuginfo_tests, rust.debuginfo_tests);
             set(&mut config.codegen_tests, rust.codegen_tests);
@@ -539,6 +543,7 @@ impl Config {
             "stable" | "beta" | "nightly" => true,
             _ => false,
         };
+        config.rust_thinlto = thinlto.unwrap_or(true);
         config.rust_debuginfo_lines = debuginfo_lines.unwrap_or(default);
         config.rust_debuginfo_only_std = debuginfo_only_std.unwrap_or(default);
 

src/bootstrap/configure.py

@@ -70,6 +70,7 @@ def v(*args):
 # Optimization and debugging options. These may be overridden by the release
 # channel, etc.
 o("optimize", "rust.optimize", "build optimized rust code")
+o("thinlto", "rust.thinlto", "build Rust with ThinLTO enabled")
 o("optimize-llvm", "llvm.optimize", "build optimized LLVM")
 o("llvm-assertions", "llvm.assertions", "build LLVM with assertions")
 o("debug-assertions", "rust.debug-assertions", "build with debugging assertions")

src/ci/run.sh

@@ -46,6 +46,7 @@ export RUST_RELEASE_CHANNEL=nightly
 if [ "$DEPLOY$DEPLOY_ALT" != "" ]; then
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --release-channel=$RUST_RELEASE_CHANNEL"
   RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-llvm-static-stdcpp"
+  RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-thinlto"
 
   if [ "$NO_LLVM_ASSERTIONS" = "1" ]; then
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --disable-llvm-assertions"