Since I compiled the swift stdlib with -function-sections and -data-sections(I added these two options to my swift compiler). The final ELF would just contain those codes it need. So it might be much smaller than the whole libswiftCore.a
Your information are very useful. I think there's something wrong with my linking script and I'm trying to figure it out. I'll give updated information here if I make some progress.
Thanks for your help!