RFR: 8075930: AARCH64: Use FP Register in C2

[Edward Nevill]

On Thu, 2015-03-26 at 09:49 +0100, Zoltán Majó wrote:

> As I mentioned earlier, I'm currently working on a patch that intends to 
> disable saving SP into FP on method handle invokes on x86 (JDK-8068945). 
> Maybe the same could be done for aarch64 as well and then you could use 
> both fp and r19 as "general purpose" registers. I plan to send out the 
> RFR this week or early next week and will let you know.

Thanks. This is a nastiness that needs cleaning up. I will wait for your
patch before doing anything else on this.

FWIW: The benefit on aarch64 is not so much that we get an extra
register but that we get an extra callee saved register for free (ie
there is no cost to saving and restoring it in the callee).

On entry/exit aarch64 does 'stp x29, x30, [sp, #N]' and 'ldp x29, x30,
[sp, #N]' (where x29==frame pointer and x30==return address), however
x29 is unsed so is saved and restored redundantly. We could just save
and restore x30, but there is no additional cost to saving x29.

All the best,
Ed.