Auto merge of rust-lang#99062 - Kobzol:lld-icf, r=jyn514

Use ICF (identical code folding) for building rustc

It seems that ICF (identical code folding) is able to remove duplicated functions created by monomorphization from binaries, resulting in smaller binary size and better i-cache utilization. Let's see if it helps for `rustc`.

I'm not sure if `lld` is even used for linking `rustc` on the Linux `dist` builder, let's see.

Author: bors

src/bootstrap/compile.rs

@@ -651,6 +651,12 @@ impl Step for Rustc {
             panic!("Cannot use and generate PGO profiles at the same time");
         }
 
+        // With LLD, we can use ICF (identical code folding) to reduce the executable size
+        // of librustc_driver/rustc and to improve i-cache utilization.
+        if builder.config.use_lld {
+            cargo.rustflag("-Clink-args=-Wl,--icf=all");
+        }
+
         let is_collecting = if let Some(path) = &builder.config.rust_profile_generate {
             if compiler.stage == 1 {
                 cargo.rustflag(&format!("-Cprofile-generate={}", path));

src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile

@@ -122,7 +122,8 @@ ENV RUST_CONFIGURE_ARGS \
       --set target.x86_64-unknown-linux-gnu.ranlib=/rustroot/bin/llvm-ranlib \
       --set llvm.thin-lto=true \
       --set llvm.ninja=false \
-      --set rust.jemalloc
+      --set rust.jemalloc \
+      --set rust.use-lld=true
 ENV SCRIPT ../src/ci/pgo.sh python3 ../x.py dist \
     --host $HOSTS --target $HOSTS \
     --include-default-paths \

src/ci/pgo.sh

@@ -195,3 +195,7 @@ rm -r $BUILD_ARTIFACTS/llvm $BUILD_ARTIFACTS/lld
 $@ \
     --rust-profile-use=${RUSTC_PROFILE_MERGED_FILE} \
     --llvm-profile-use=${LLVM_PROFILE_MERGED_FILE}
+
+echo "Rustc binary size"
+ls -la ./build/$PGO_HOST/stage2/bin
+ls -la ./build/$PGO_HOST/stage2/lib