RFR(S): 8188773: PPC64 and s390: Fix UseMembar and enable ShareVtableStubs

[Doerr, Martin]

Hi,

can somebody please review this small PPC64 and s390 change?
http://cr.openjdk.java.net/~mdoerr/8188773_ppc64_s390_UseMembar_ShareVtableStubs/webrev.00/

It fixes UseMembar and enables ShareVtableStubs on these 2 platforms:
On PPC64, we currently generate unnecessary fence instructions which make performance even worse.
On s390, the signal handler needs a fix in order to work with -XX:+UseMember.
In addition, switch on ShareVtableStubs in order to reduce code cache usage. Modern processors don't seem to benefit from having it off it since the predictors have become better.

Best regards,
Martin