Fix the behavior of local array initializer

After observing the implementation of cfront, it appears that the local
array initializer may not generate correct instructions to initialize
all elements.

For example, consider the following code:

    int main() {
        int a[5] = {5, 10};
        return a[0] + a[1] + a[2] + a[3] + a[4]
    }

If the number of elements in the initializer-list is less than the size
of array, the generated instructions only initialize the first elements.
The remaining elements are left uninitialized, which may cause the
return value in the above example to differ from the expected 15.

When the initializer-list is empty, all elements remain uninitialized.

Therefore, this commit improves the cfront to generate more instructions
to initialize all elements of an array exactly.

Author: DrXiao

src/parser.c

@@ -1255,6 +1255,49 @@ void parse_array_init(var_t *var,
                 break;
         }
     }
+
+    if (parent != GLOBAL_BLOCK && emit_code && !is_implicit) {
+        /* e.g.:
+         *
+         * 1.
+         *      int main() {
+         *          int a[5] = {};
+         *          return a[0] + a[1] + a[2] + a[3] + a[4];
+         *      }
+         *
+         * 2.
+         *      int main() {
+         *          int a[5] = {5, 10}
+         *          return a[0] + a[1] + a[2] + a[3] + a[4];
+         *      }
+         *
+         * The initializer should set the value of the first elements, and
+         * initialize other elements without explicit assignments to 0.
+         *
+         * Therefore, the first and second cases return 0 and 15, respectively.
+         * */
+        for (; count < var->array_size; count++) {
+            var_t *val = require_var(parent);
+            gen_name_to(val->var_name);
+            val->init_val = 0;
+            add_insn(parent, *bb, OP_load_constant, val, NULL, NULL, 0, NULL);
+
+            var_t target = {0};
+            target.type = var->type;
+            target.ptr_level = 0;
+            var_t *v = resize_var(parent, bb, val, &target);
+
+            var_t *elem_addr = compute_element_address(parent, bb, base_addr,
+                                                       count, elem_size);
+
+            if (elem_size <= 4) {
+                add_insn(parent, *bb, OP_write, NULL, elem_addr, v, elem_size,
+                         NULL);
+            } else {
+                fatal("Unsupported: struct assignment > 4 bytes in array");
+            }
+        }
+    }
     lex_expect(T_close_curly);
 
     if (is_implicit) {