When is RBX used for base pointer?

[H.J. Lu]

Hi,

We need a callee-saved register for stack alignment. In 64bit, our
choices are rbx, and
r12-r15. r12-r15 need the REX byte and r12 also needs the SIB byte. So
I'd like to use rbx.
x86-64 psABI says rbx is callee-saved and optionally used as base
pointer. However,
I couldn't find an example where rbx is used as base pointer. Is this
a carryover from
i386 psABI where ebx is used to hold GOT?

Thanks.

H.J.