[lld/mac] Implement support for searching dylibs with @loader_path/ in install name

[int3]

Thanks!

lld/MachO/InputFiles.cpp

 // the first document storing child pointers to the rest of them. When we are
 // processing a given TBD file, we store that top-level document in
 // currentTopLevelTapi. When processing re-exports, we search its children for
 // potentially matching documents in the same TBD file. Note that the children
 // themselves don't point to further documents, i.e. this is a two-level tree.
 //
 // Re-exports can either refer to on-disk files, or to documents within .tbd
 // files.
-DylibFile *findDylib(StringRef path, DylibFile *umbrella,
+static DylibFile *findDylib(StringRef path, DylibFile *umbrella,
                             const InterfaceFile *currentTopLevelTapi) {
   if (path::is_absolute(path, path::Style::posix))
     for (StringRef root : config->systemLibraryRoots)
       if (Optional<std::string> dylibPath =
               resolveDylibPath((root + path).str()))
         return loadDylib(*dylibPath, umbrella);
 
-  // TODO: Expand @loader_path, @rpath etc, handle -dylib_file
+  // TODO: Expand @rpath, handle -dylib_file
   SmallString<128> newPath;
   if (config->outputType == MH_EXECUTE &&
       path.consume_front("@executable_path/")) {
     // ld64 allows overriding this with the undocumented flag -executable_path.
     // lld doesn't currently implement that flag.
     path::append(newPath, sys::path::parent_path(config->outputFile), path);
     path = newPath;
+  } else if (path.consume_front("@loader_path/")) {
+    path::append(newPath, sys::path::parent_path(umbrella->getName()), path);
+    path = newPath;
   }
 
   if (currentTopLevelTapi) {
     for (InterfaceFile &child :
          make_pointee_range(currentTopLevelTapi->documents())) {
       assert(child.documents().empty());
       if (path == child.getInstallName()) {
         auto file = make<DylibFile>(child, umbrella);
static bool isImplicitlyLinked(StringRef path) {
   if (path.consume_front("/System/Library/Frameworks/")) {
     StringRef frameworkName = path.take_until([](char c) { return c == '.'; });
     return path::filename(path) == frameworkName;
   }
 
   return false;
 }
 
-void loadReexport(StringRef path, DylibFile *umbrella,
+static void loadReexport(StringRef path, DylibFile *umbrella,
                          const InterfaceFile *currentTopLevelTapi) {
   DylibFile *reexport = findDylib(path, umbrella, currentTopLevelTapi);
   if (!reexport)
     error("unable to locate re-export with install name " + path);
   else if (isImplicitlyLinked(path))
     inputFiles.insert(reexport);
 }
 
 DylibFile::DylibFile(MemoryBufferRef mb, DylibFile *umbrella,

lld/test/MachO/link-search-at-loader-path.s

+# REQUIRES: x86
+
+# RUN: rm -rf %t; split-file %s %t
+# RUN: mkdir %t/subdir
+
+# RUN: llvm-mc -filetype obj -triple x86_64-apple-darwin %t/foo.s -o %t/foo.o
+# RUN: llvm-mc -filetype obj -triple x86_64-apple-darwin %t/bar.s -o %t/bar.o
+# RUN: llvm-mc -filetype obj -triple x86_64-apple-darwin %t/main.s -o %t/main.o
+
+# RUN: %lld -dylib -install_name @loader_path/libfoo.dylib %t/foo.o -o %t/subdir/libfoo.dylib
+
+## Test that @loader_path is replaced by the actual path, not by install_name.
+# RUN: %lld -dylib -reexport_library %t/subdir/libfoo.dylib -install_name /tmp/libbar.dylib %t/bar.o -o %t/subdir/libbar.dylib
+
+# RUN: %lld -lSystem %t/main.o %t/subdir/libbar.dylib -o %t/test
+# RUN: %lld -dylib -lSystem %t/main.o %t/subdir/libbar.dylib -o %t/libtest.dylib
+
+#--- foo.s
+.globl _foo
+_foo:
+  retq
+
+#--- bar.s
+.globl _bar
+_bar:
+  retq
+
+#--- main.s
+.section __TEXT,__text
+.global _main
+_main:
+  callq _foo
+  callq _bar
+  ret