RFR(XS): 8151030: PPC64: AllocatePrefetchStyle=4 is out of range

[Doerr, Martin]

Hi,

I'd like to resolve a very old PPC64 issue:
https://bugs.openjdk.java.net/browse/JDK-8151030

There's code for AllocatePrefetchStyle=4 which is not an accepted option. It was used for a special experimental prefetch mode using dcbz instructions to combine prefetching and zeroing in the TLABs.
However, this code was never contributed and there are no plans to work on it. So I'd like to simply remove this small part of it.

In addition to that, AllocatePrefetchLines is currently set to 3 by default which doesn't make sense to me. PPC64 has an automatic prefetch engine and executing several prefetch instructions for succeeding cache lines doesn't seem to be beneficial at all.
So I'm setting it to 1 by default. I couldn't observe regressions on Power7, Power8 and Power9.

Webrev:
http://cr.openjdk.java.net/~mdoerr/8151030_ppc_prefetch/webrev.00/

Please review.

If somebody from IBM would like to check performance impact of changing the AllocatePrefetchLines + Distance, I'll be glad to receive feedback.

Best regards,
Martin