Implement func decl which returns struct

This fixes issue rui314#75

parse.c

@@ -1937,12 +1937,24 @@ static Type *read_declarator_array(Type *basety) {
     return make_array_type(t, len);
 }
 
-static Type *read_declarator_func(Type *basety, Vector *param) {
+static Vector *param_types(Vector *params);
+
+static Type *read_declarator_func(Type *basety, Vector *params) {
     if (basety->kind == KIND_FUNC)
         error("function returning a function");
     if (basety->kind == KIND_ARRAY)
         error("function returning an array");
-    return read_func_param_list(param, basety);
+    Type *functype = read_func_param_list(params, basety);
+    if (is_large_struct(functype->rettype)) {
+        Vector *new_params = make_vector();
+        vec_push(new_params,
+                 ast_lvar(make_ptr_type(functype->rettype), make_tempname()));
+        if (params)
+            vec_append(new_params, params);
+        params = new_params;
+        functype->params = param_types(params);
+    }
+    return functype;
 }
 
 static Type *read_declarator_tail(Type *basety, Vector *params) {

test/struct.c

@@ -322,7 +322,7 @@ struct abi_check {
     struct abi_check_nest z;
 };
 
-typedef struct abi_check_small {
+struct abi_check_small {
     int x;
 };
 
@@ -400,6 +400,16 @@ static void struct_call_small(void) {
     expect(43, struct_arg_func_small(1, (struct abi_check_small){ 42 }));
 }
 
+static struct abi_check return_struct_func_decl(struct abi_check s, int x);
+
+static void struct_return_with_decl(void) {
+    expect(42, return_struct_func_decl((struct abi_check){}, 42).x);
+}
+
+static struct abi_check return_struct_func_decl(struct abi_check s, int x) {
+    return (struct abi_check){ x };
+}
+
 void testmain() {
     print("struct");
     t1();
@@ -433,4 +443,5 @@ void testmain() {
     struct_global();
     struct_return();
     struct_call_small();
+    struct_return_with_decl();
 }