Callee-saved registers

[D.Sturm]

> Again: No, HotSpot does not usually follow the documented native ABI, and
you want do not want to change the HotSpot ABI (even if you think it is
ridiculously inefficient).
Didn't want to make that sound so negative :-) Also since we're so
aggressively inlining there shouldn't be many really small functions around
where it would make a big difference.

> There are ways to make callee save registers more efficient, but don't
worry about that until everything else works. It is an optimization, not an
essential feature. And yes, I have some ideas on how to implement it and
I'm happy to discuss it with you when the time comes.
Absolutely, I already implemented the purely caller-saved version which
should work fine for now. When I get everything working, that would be
interesting to see if such a change to the calling convention would
actually make any measurable difference or not. But you're right that it
shouldn't have a high priority for me.

-Daniel