Report a "symbol duplication" error for LTO

Fixes #1414

src/passes.cc

@@ -327,9 +327,7 @@ void resolve_symbols(Context<E> &ctx) {
 
     if (!flag)
       return;
-
     clear_symbols(ctx);
-    resolve_symbols(ctx);
   }
 }
 
@@ -346,6 +344,12 @@ void do_lto(Context<E> &ctx) {
   parse_symbol_version(ctx);
   compute_import_export(ctx);
 
+  // If multiple IR object files define the same symbol, the LTO backend
+  // would choose one of them randomly instead of reporting an error.
+  // So we need to check for symbol duplication error before doing an LTO.
+  if (!ctx.arg.allow_multiple_definition)
+    check_duplicate_symbols(ctx);
+
   // Invoke the LTO plugin. This step compiles IR object files into a few
   // big ELF files.
   std::vector<ObjectFile<E> *> lto_objs = run_lto_plugin(ctx);
@@ -1037,7 +1041,7 @@ void check_duplicate_symbols(Context<E> &ctx) {
       Symbol<E> &sym = *file->symbols[i];
 
       // Skip if our symbol is undef or weak
-      if (sym.file == file || sym.file == ctx.internal_obj ||
+      if (!sym.file || sym.file == file || sym.file == ctx.internal_obj ||
           esym.is_undef() || esym.is_common() || (esym.st_bind == STB_WEAK))
         continue;
 

test/duplicate-error-lto.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+nm mold | grep '__tsan_init' && skip
+test_cflags -flto || skip
+
+cat <<EOF | $CC -o $t/a.o -c -xc - -flto
+void foo() {}
+EOF
+
+cat <<EOF | $CC -o $t/b.o -c -xc - -flto
+int main() {}
+EOF
+
+not $CC -B. -o $t/exe1 $t/a.o $t/a.o $t/b.o -flto |&
+  grep 'duplicate symbol.*: foo$'