Add return that doesn't take any value

codegen.c

@@ -433,7 +433,8 @@ static void gen_stmt(Node *node) {
     gen_stmt(node->lhs);
     return;
   case ND_RETURN:
-    gen_expr(node->lhs);
+    if (node->lhs)
+      gen_expr(node->lhs);
     println("  jmp .L.return.%s", current_fn->name);
     return;
   case ND_EXPR_STMT:

parse.c

@@ -1050,7 +1050,7 @@ static bool is_typename(Token *tok) {
   return find_typedef(tok);
 }
 
-// stmt = "return" expr ";"
+// stmt = "return" expr? ";"
 //      | "if" "(" expr ")" stmt ("else" stmt)?
 //      | "switch" "(" expr ")" stmt
 //      | "case" const-expr ":" stmt
@@ -1066,6 +1066,9 @@ static bool is_typename(Token *tok) {
 static Node *stmt(Token **rest, Token *tok) {
   if (equal(tok, "return")) {
     Node *node = new_node(ND_RETURN, tok);
+    if (consume(rest, tok->next, ";"))
+      return node;
+
     Node *exp = expr(&tok, tok->next);
     *rest = skip(tok, ";");
 

test/function.c

@@ -61,6 +61,10 @@ int counter() {
   return i++ + j++;
 }
 
+void ret_none() {
+  return;
+}
+
 int main() {
   ASSERT(3, ret3());
   ASSERT(8, add2(3, 5));
@@ -100,6 +104,8 @@ int main() {
   ASSERT(4, counter());
   ASSERT(6, counter());
 
+  ret_none();
+
   printf("OK\n");
   return 0;
 }