Add -warn-common

cmdline.cc

@@ -290,6 +290,10 @@ void parse_nonpositional_args(std::span<std::string_view> args,
     } else if (read_flag(args, "stats")) {
       config.stats = true;
       Counter::enabled = true;
+    } else if (read_flag(args, "warn-common")) {
+      config.warn_common = true;
+    } else if (read_flag(args, "no-warn-common")) {
+      config.warn_common = false;
     } else if (read_z_flag(args, "now")) {
       config.z_now = true;
     } else if (read_z_flag(args, "execstack")) {

mold.h

@@ -89,6 +89,7 @@ struct Config {
   bool strip_all = false;
   bool strip_debug = false;
   bool trace = false;
+  bool warn_common = false;
   bool z_copyreloc = true;
   bool z_defs = false;
   bool z_delete = true;

object_file.cc

@@ -848,8 +848,11 @@ void ObjectFile::convert_common_symbols() {
       continue;
 
     Symbol *sym = symbols[i];
-    if (sym->file != this)
+    if (sym->file != this) {
+      if (config.warn_common)
+        SyncOut() << *this << ": " << "multiple common symbols: " << *sym;
       continue;
+    }
 
     assert(sym->esym->st_value);
 

test/warn-common.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+set -e
+cd $(dirname $0)
+echo -n "Testing $(basename -s .sh $0) ... "
+t=$(pwd)/tmp/$(basename -s .sh $0)
+mkdir -p $t
+
+cat <<EOF | clang -fcommon -c -xc -o $t/a.o -
+int foo;
+EOF
+
+cat <<EOF | clang -fcommon -c -xc -o $t/b.o -
+int foo;
+
+int main() {
+  return 0;
+}
+EOF
+
+clang -fuse-ld=`pwd`/../mold -o $t/exe $t/a.o $t/b.o
+! fgrep -q 'multiple common symbols' $t/log
+
+clang -fuse-ld=`pwd`/../mold -o $t/exe $t/a.o $t/b.o -Wl,-warn-common >& $t/log
+fgrep -q 'multiple common symbols' $t/log
+
+echo OK