RFR: 8234662: Sweeper should keep current nmethod alive before yielding for ICStub refills

[erik.osterlund at oracle.com]

Hi,

Today, when GCs scan the stack, all nmethods roots found from threads 
are nmethods that have gone through an nmethod entry barrier (in the 
case of concurrent class unloading), except the super special sweeper 
nmethod that is currently being processed when it runs out of ICStubs 
and need to safepoint to refill ICStubs, which with the fantastic 
safepoint coalescing optimization that we all know and love, can get 
fused into a GC safepoint, which will find the sweeper nmethod and be 
confused why it has not gone through an entry barrier before it yielded 
to the safepoint like everyone else.

This causes some headache, and I would like to harmonize this better by 
simply calling the nmethod entry barrier on that nmethod in the sweeper, 
before yielding to the safepoint. Then there is no need to treat it as a 
special case.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8234662

Webrev:
http://cr.openjdk.java.net/~eosterlund/8234662/webrev.00/

Thanks.
/Erik