[ELF][LTO] Export symbols from DSOs correctly

With this change, mold can now build Clang with LTO and all Clang tests pass.

elf/lto.cc

@@ -486,6 +486,7 @@ ObjectFile<E> *read_lto_object(Context<E> &ctx, MappedFile<Context<E>> *mf) {
 
   // Create mold's object instance
   ObjectFile<E> *obj = new ObjectFile<E>;
+  obj->filename = mf->name;
   obj->symbols.push_back(new Symbol<E>);
   obj->first_global = 1;
   obj->is_lto_obj = true;
@@ -563,6 +564,12 @@ void do_lto(Context<E> &ctx) {
       file->is_alive = false;
 
   std::erase_if(ctx.objs, [](ObjectFile<E> *file) { return file->is_lto_obj; });
+
+  // Re-compute symbol versions and import/export information
+  // because `resolve_symbols` may have overwrite them.
+  apply_version_script(ctx);
+  parse_symbol_version(ctx);
+  compute_import_export(ctx);
 }
 
 template <typename E>

elf/main.cc

@@ -469,6 +469,15 @@ static int elf_main(int argc, char **argv) {
   // included to the final output.
   resolve_symbols(ctx);
 
+  // Apply version scripts.
+  apply_version_script(ctx);
+
+  // Parse symbol version suffixes (e.g. "foo@ver1").
+  parse_symbol_version(ctx);
+
+  // Set is_import and is_export bits for each symbol.
+  compute_import_export(ctx);
+
   // Do LTO
   if (ctx.has_lto_object)
     do_lto(ctx);
@@ -482,15 +491,6 @@ static int elf_main(int argc, char **argv) {
   // Create .bss sections for common symbols.
   convert_common_symbols(ctx);
 
-  // Apply version scripts.
-  apply_version_script(ctx);
-
-  // Parse symbol version suffixes (e.g. "foo@ver1").
-  parse_symbol_version(ctx);
-
-  // Set is_import and is_export bits for each symbol.
-  compute_import_export(ctx);
-
   // Garbage-collect unreachable sections.
   if (ctx.arg.gc_sections)
     gc_sections(ctx);

test/elf/lto-dso.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+export LANG=
+set -e
+CC="${CC:-cc}"
+CXX="${CXX:-c++}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+mold="$(pwd)/mold"
+t=out/test/elf/$testname
+mkdir -p $t
+
+cat <<EOF | $CC -flto -c -fPIC -o $t/a.o -xc -
+void foo() {}
+EOF
+
+$CC -B. -shared -o $t/b.so -flto $t/a.o
+nm -D $t/b.so | grep -q 'T foo'
+
+echo OK

test/elf/lto-version-script.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+export LANG=
+set -e
+CC="${CC:-cc}"
+CXX="${CXX:-c++}"
+testname=$(basename "$0" .sh)
+echo -n "Testing $testname ... "
+cd "$(dirname "$0")"/../..
+mold="$(pwd)/mold"
+t=out/test/elf/$testname
+mkdir -p $t
+
+cat <<EOF | $CC -flto -c -fPIC -o $t/a.o -xc -
+void foo() {}
+void bar() {}
+EOF
+
+cat <<EOF > $t/b.script
+{
+  global: foo;
+  local: *;
+};
+EOF
+
+$CC -B. -shared -o $t/c.so -flto $t/a.o -Wl,-version-script=$t/b.script
+nm -D $t/c.so | grep -q 'T foo'
+! nm -D $t/c.so | grep -q 'T bar' || false
+
+echo OK