RFR(S) 8142436: [JVMCI] fix management of nmethod::_installed_code field

[Tom Rodriguez]

> Additionally, you can run with +G1VerifyHeapRegionCodeRoots to enable verification of the heap region to nmethod mapping in order to detect other issues which don't cause a crash right away.

I didn’t know that existed.  That probably would have told me exactly what I wanted to know.  Why isn’t that on by default?  Too expensive?  I was getting crashes and VerifyBeforeGC didn’t find anything so I added verify logic that checked that no zombie methods are in the code root set, which helped me track down the bug.  http://hg.openjdk.java.net/graal/graal-jvmci-8/rev/2d21c13d7829 <http://hg.openjdk.java.net/graal/graal-jvmci-8/rev/2d21c13d7829> Having at least minimal verification enabled all the time would be nicer, at least it would have been for me.  Failing this verify could suggest enabling G1VerifyHeapRegionCodeRoots.

tom

> 
> /Mikael
> 
>> 
>> tom
> 

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