Add -fcommon and -fno-common flags

chibicc.h

@@ -397,4 +397,5 @@ int display_width(char *p, int len);
 bool file_exists(char *path);
 
 extern StringArray include_paths;
+extern bool opt_fcommon;
 extern char *base_file;

codegen.c

@@ -1136,7 +1136,7 @@ static void emit_data(Obj *prog) {
       ? MAX(16, var->align) : var->align;
     println("  .align %d", align);
 
-    if (var->is_tentative) {
+    if (opt_fcommon && var->is_tentative) {
       println("  .comm %s, %d, %d", var->name, var->ty->size, align);
       continue;
     }

main.c

@@ -1,6 +1,7 @@
 #include "chibicc.h"
 
 StringArray include_paths;
+bool opt_fcommon = true;
 
 static bool opt_E;
 static bool opt_S;
@@ -87,6 +88,16 @@ static void parse_args(int argc, char **argv) {
       continue;
     }
 
+    if (!strcmp(argv[i], "-fcommon")) {
+      opt_fcommon = true;
+      continue;
+    }
+
+    if (!strcmp(argv[i], "-fno-common")) {
+      opt_fcommon = false;
+      continue;
+    }
+
     if (!strcmp(argv[i], "-c")) {
       opt_c = true;
       continue;

test/driver.sh

@@ -170,4 +170,15 @@ check -idirafter
 echo "#include \"idirafter\"" | $chibicc -idirafter $tmp/dir1 -I$tmp/dir2 -E - | grep -q bar
 check -idirafter
 
+# -fcommon
+echo 'int foo;' | $chibicc -S -o- - | grep -q '\.comm foo'
+check '-fcommon (default)'
+
+echo 'int foo;' | $chibicc -fcommon -S -o- - | grep -q '\.comm foo'
+check '-fcommon'
+
+# -fno-common
+echo 'int foo;' | $chibicc -fno-common -S -o- - | grep -q '^foo:'
+check '-fno-common'
+
 echo OK