llvm dev - Aug 2018 - Is it possible to make unmaped register for special purpose?

Dear All,

I want to set register which is not used in nowhere for special purpose.
(like FS segment register..)

I think there is two methods but not sure.

 1. Make extra "Real Registers".
     If that is possible. Custom Machine Register won't used. because there
is no rule for this register in backend.

 2. Make set one register to unmaped in backend code.
     it make lock one register while running backend logic.

Is it possible methods?  if that, Let me know how can i make it.

Many thanks for any response.
Thanks in advance,
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