[9] RFR (XS): 8157560: Reserve space for allocation prefetch only in builds that support allocation prefetching

[Edward Nevill]

Hi Zoltán, 

On Mon, 2016-05-23 at 13:00 +0200, Zoltán Majó wrote:
> Hi,
> 
> 
> please review the path for 8157560.
> 
> https://bugs.openjdk.java.net/browse/JDK-8157560
> 
> Problem: Every thread-local allocation buffer (TLAB) contains a reserved 
> area to accommodate allocation prefetches (i.e., to make sure that 
> instructions performing allocation prefetch do not cause faults). The 
> reserved area is at the end of the TLAB.

On certain architectures (eg aarch64) prefetch will not cause faults. On
these allocating the extra space is redundant.

Is it possible to have this predicated on a pd_ variable (eg
AllocateExtraTLAB which can be true for those arches that require it and
false for those that don't) rather than having it just conditionalised
on COMPILER2.

All the best,
Ed.