[parisc-linux] xchg implementation

[LaMont Jones]

> This is a uniprocessor optimisation of the xchg() case where it's _not_
> exchanging with zero.  What I need is a spare CR register.  As far as I
> can see from the comments, CR28-30 are currently reserved; leaving CRs
> 24, 25 and 31 available.  Unless someone's using one of them without
> documenting it somewhere...?  That really does need to be written down
> somewhere.

IIRC, CR24 and 25 are readable from user space (not writable), and can be
quite useful for thread specific data, thread id, etc...  It might do to
leave them available for such a use.

lamont