adjust libdir for windows, use lld during enzyme build when needed

Author: ZuseZ4

src/bootstrap/src/core/build_steps/llvm.rs

@@ -23,7 +23,7 @@ use crate::core::config::{Config, TargetSelection};
 use crate::utils::build_stamp::{BuildStamp, generate_smart_stamp_hash};
 use crate::utils::exec::command;
 use crate::utils::helpers::{
-    self, exe, get_clang_cl_resource_dir, t, unhashed_basename, up_to_date,
+    self, exe, get_clang_cl_resource_dir, libdir, t, unhashed_basename, up_to_date,
 };
 use crate::{CLang, GitRepo, Kind, trace};
 
@@ -561,7 +561,6 @@ impl Step for Llvm {
             }
         };
 
-        // FIXME(ZuseZ4): Do we need that for Enzyme too?
         // When building LLVM with LLVM_LINK_LLVM_DYLIB for macOS, an unversioned
         // libLLVM.dylib will be built. However, llvm-config will still look
         // for a versioned path like libLLVM-14.dylib. Manually create a symbolic
@@ -1159,7 +1158,7 @@ impl Step for Enzyme {
         let llvm_version_major = llvm::get_llvm_version_major(builder, &host_llvm_config);
         let lib_ext = std::env::consts::DLL_EXTENSION;
         let libenzyme = format!("libEnzyme-{llvm_version_major}");
-        let build_dir = out_dir.join("lib");
+        let build_dir = out_dir.join(libdir(target));
         let dylib = build_dir.join(&libenzyme).with_extension(lib_ext);
 
         trace!("checking build stamp to see if we need to rebuild enzyme artifacts");
@@ -1197,7 +1196,16 @@ impl Step for Enzyme {
         // hard to spot more relevant issues.
         let mut cflags = CcFlags::default();
         cflags.push_all("-Wno-deprecated");
-        configure_cmake(builder, target, &mut cfg, true, LdFlags::default(), cflags, &[]);
+
+        // Logic copied from `configure_llvm`
+        // ThinLTO is only available when building with LLVM, enabling LLD is required.
+        // Apple's linker ld64 supports ThinLTO out of the box though, so don't use LLD on Darwin.
+        let mut ldflags = LdFlags::default();
+        if builder.config.llvm_thin_lto && !target.contains("apple") {
+            ldflags.push_all("-fuse-ld=lld");
+        }
+
+        configure_cmake(builder, target, &mut cfg, true, ldflags, cflags, &[]);
 
         // Re-use the same flags as llvm to control the level of debug information
         // generated by Enzyme.