>>>
>>x86-64, interestingly, retains vestigial meaning of the %fs and %gs
>>registers (but no others) to use as a base pointer for this reason
>>alone.
>
> Well, on x86-64 this is purely x86-64 ABI designers decision, they could
> pick one of %r8 - %r15 and use that as thread pointer instead (and were
> recommended to do so).
>
Wasting an architectural register is still bloody expensive, even if 1
among 16 is less than 1 among 8. They'll be using %gs as thread pointer
in userspace and CPU pointer in kernel space.
-hpa
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