Support static linking LLVM with ThinLTO

Author: tmandry

src/bootstrap/builder.rs

@@ -787,6 +787,33 @@ impl<'a> Builder<'a> {
             }
         }
 
+        // If we're statically linking in an LLVM compiled for ThinLTO,
+        // configure the linker accordingly.
+        if Mode::Rustc == mode && self.config.llvm_thin_lto && !self.config.llvm_link_shared {
+            if target.contains("msvc") {
+                // Here we assume the linker is lld-link.exe.
+                rustflags.arg(&format!("-Clink-arg=/opt:lldtojobs={}", self.jobs()));
+            } else {
+                // Here we assume the linker is clang and that lld is available.
+                // If not, there'll be linker errors.
+                rustflags.arg("-Clink-arg=-fuse-ld=lld");
+                rustflags.arg("-Clink-arg=-flto=thin");
+
+                // Copy the optimization flags LLVM uses for its Release and
+                // RelWithDebugInfo builds.
+                if self.config.llvm_optimize {
+                    if self.config.llvm_release_debuginfo {
+                        rustflags.arg("-Clink-arg=-O2");
+                    } else {
+                        rustflags.arg("-Clink-arg=-O3");
+                    }
+                }
+
+                // Make LLD respect the `-j` option.
+                rustflags.arg(&format!("-Clink-arg=-Wl,--thinlto-jobs={}", self.jobs()));
+            }
+        }
+
         // This tells Cargo (and in turn, rustc) to output more complete
         // dependency information.  Most importantly for rustbuild, this
         // includes sysroot artifacts, like libstd, which means that we don't

src/bootstrap/compile.rs

@@ -567,9 +567,12 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: Interne
             let file = compiler_file(builder, builder.cxx(target).unwrap(), target, "libstdc++.a");
             cargo.env("LLVM_STATIC_STDCPP", file);
         }
-        if builder.config.llvm_link_shared || builder.config.llvm_thin_lto {
+        if builder.config.llvm_link_shared {
             cargo.env("LLVM_LINK_SHARED", "1");
         }
+        if builder.config.llvm_thin_lto {
+            cargo.env("LLVM_LTO", "thin");
+        }
         if builder.config.llvm_use_libcxx {
             cargo.env("LLVM_USE_LIBCXX", "1");
         }

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

@@ -101,6 +101,7 @@ ENV RUST_CONFIGURE_ARGS \
       --set target.x86_64-unknown-linux-gnu.linker=clang \
       --set target.x86_64-unknown-linux-gnu.ar=/rustroot/bin/llvm-ar \
       --set target.x86_64-unknown-linux-gnu.ranlib=/rustroot/bin/llvm-ranlib \
+      --set llvm.link-shared=true \
       --set llvm.thin-lto=true \
       --set rust.jemalloc
 ENV SCRIPT python2.7 ../x.py dist --host $HOSTS --target $HOSTS

src/ci/docker/x86_64-gnu-llvm-7/Dockerfile

@@ -21,11 +21,11 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
-# using llvm-link-shared due to libffi issues -- see #34486
+# using llvm.link-shared due to libffi issues -- see #34486
 ENV RUST_CONFIGURE_ARGS \
       --build=x86_64-unknown-linux-gnu \
       --llvm-root=/usr/lib/llvm-7 \
-      --enable-llvm-link-shared \
+      --set llvm.link-shared=true \
       --set rust.thin-lto-import-instr-limit=10
 
 ENV SCRIPT python2.7 ../x.py test src/tools/tidy && python2.7 ../x.py test

src/librustc_llvm/build.rs

@@ -127,6 +127,7 @@ fn main() {
         }
 
         if flag.starts_with("-flto") {
+            // Handled below.
             continue;
         }
 
@@ -153,6 +154,10 @@ fn main() {
         cfg.define("NDEBUG", None);
     }
 
+    if let Ok(kind) = env::var("LLVM_LTO") {
+        cfg.flag(&format!("-flto={}", kind));
+    }
+
     build_helper::rerun_if_changed_anything_in_dir(Path::new("../rustllvm"));
     cfg.file("../rustllvm/PassWrapper.cpp")
         .file("../rustllvm/RustWrapper.cpp")