How to use the @glimmer/vm.Register.fp function in @glimmer/vm

builder.pushPrimitiveReference(null);
    }

    expr(params[0], builder);

    builder.enter(2);

    builder.putIterator();

    builder.jumpUnless('ELSE');

    builder.pushFrame();

    builder.returnTo('ITER');

    builder.dup(Register.fp, 1);

    builder.enterList('BODY');

    builder.label('ITER');
    builder.iterate('BREAK');

    builder.label('BODY');
    builder.invokeStaticBlock(unwrap(template), 2);
    builder.pop(2);
    builder.exit();
    builder.return();

    builder.label('BREAK');
    builder.exitList();
    builder.popFrame();

body() {
        builder.putIterator();

        builder.jumpUnless('ELSE');

        builder.pushFrame();

        builder.dup(Register.fp, 1);

        builder.returnTo('ITER');
        builder.enterList('BODY');

        builder.label('ITER');
        builder.iterate('BREAK');

        builder.label('BODY');
        builder.invokeStaticBlock(unwrap(template), 2);
        builder.pop(2);
        builder.jump('FINALLY');

        builder.label('BREAK');
        builder.exitList();
        builder.popFrame();