RFR(S): 8233111: Epoch shift synchronization point for Compiler threads

[Markus Gronlund]

Greetings,

Please review the following change set:

Bug: https://bugs.openjdk.java.net/browse/JDK-8233111 
Webrev: http://cr.openjdk.java.net/~mgronlun/8233111/webrev01/
Test: jdk_jfr
Summary: please see bug description

Thanks
Markus