modelsim仿真的时候某些模块被优化，无法在sim的instance中显示出来，但是实际上又没有报warning，这个时候只要在vsim中添加参数-voptargs="+acc"即可解决
如上图所示，不加这个option可能tx以及rx都不会显示，根据modelsim的`ModelSim® SE Command Reference
Manual`文档中关于此参数的描述
-voptargs="<args>"
(optional) Specifies arguments that vsim should pass to vopt when running vopt
automatically. Any arguments must be separated by spaces, just as you would on the vopt
command line.
vsim
The primary purpose of this argument is to pass +acc arguments to vopt; the vopt -o
<name> option is not permitted.
通过以上的描述，可以知道-voptargs是将参数传递给vopt，那么vopt又是什么呢？
The vopt command performs global optimizations on designs after they have been compiled
with vcom or vlog. For detailed usage information on optimization, refer to the chapter titled
“Optimizing Designs with vopt” in the User’s Manual.
在文档中有以下描述：
It is suggested that you specify this option with -voptargs=+acc to enable as much visibility
as possible to allow for functional correctness of your design.