Inefficient bytecode for constructor-less ES6 child classes

[bnoordhuis]

Refs: #750

Ex.

class Foo extends Bar { /* no construct() */ }

Emits:

        /* super */
        emit_op(s, OP_scope_get_var);
        emit_atom(s, JS_ATOM_this_active_func);
        emit_u16(s, 0);

        emit_op(s, OP_get_super);

        emit_op(s, OP_scope_get_var);
        emit_atom(s, JS_ATOM_new_target);
        emit_u16(s, 0);

        emit_op(s, OP_array_from);
        emit_u16(s, 0);
        emit_op(s, OP_push_i32);
        emit_u32(s, 0);

        /* arguments */
        emit_op(s, OP_scope_get_var);
        emit_atom(s, JS_ATOM_arguments);
        emit_u16(s, 0);

        emit_op(s, OP_append);
        /* drop the index */
        emit_op(s, OP_drop);

        emit_op(s, OP_apply);
        emit_u16(s, 1);
        /* set the 'this' value */
        emit_op(s, OP_dup);
        emit_op(s, OP_scope_put_var_init);
        emit_atom(s, JS_ATOM_this);
        emit_u16(s, 0);
        emit_class_field_init(s);

https://github.com/quickjs-ng/quickjs/blob/4b9d0ebdba3ace6b7f6de21c3e3806ab4055bd7e/quickjs.c#L21898-L21938

Pretty bulky and inefficient (splatting arguments, doing js_function_apply, etc.) Probably better handled through a dedicated opcode.

Affects WBT benchmarks like babylon that create tons of such objects.

[saghul]

Hum, how come has_super doesn't work there? Then it would skip a bunch of that, wouldn't it?

Also, looks like this was changed in a8a5ecb any chance it was made somehow worse?

[bnoordhuis]

how come has_super doesn't work there?

It does - ⬆️ is the bytecode for when has_super == true.

looks like this was changed in a8a5ecb any chance it was made somehow worse?

With 20/20 hindsight it would've probably been better to add and emit a specialized opcode but 🤷, Divy's commit at least improved spec conformance.

[saghul]

Ah hold on. You mean all of that happens even when there is no constructor which calls super?!

[bnoordhuis]

Yep, that's right. It essentially synthesizes a constructor because of this:

class A { constructor(x) { print(x) } }
class B extends A { /* no constructor */ }
new B(42) // prints 42