RFR: 8212921: Relax code cache oop verification in ZGC

[Per Liden]

On 10/25/2018 03:18 PM, Erik Österlund wrote:
> Hi,
> 
> With concurrent class unloading, the normal state of affairs for ZGC is 
> that nmethods have messed up oops in all GC safepoints, and fix them up 
> during concurrent phases. Therefore, Universe::verify needs to relax the 
> assumption that oops are okay for ZGC.
> 
> Note that during Universe::verify, the oops in the nmethods were 
> redundantly verified. Once by walking the oops in the code cache, and 
> once when calling nmethod::verify(), which did the same verification 
> again. I removed the latter.
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8212921/webrev.00/

I discussed this with Erik off-line. Instead of adjusting the 
verification code to do what ZGC needs, we could instead move ZGC's 
verification to after resurrection is unblocked. At this time, all oops 
we find will be good and no special nmethod verification will be needed.

Here's a webrev to adjust to that model:

http://cr.openjdk.java.net/~pliden/8212921/webrev.0

I also noticed that we could remove some boilerplate code by fusing 
ZVerifyRootOopClosure and ZVerifyHeapOopClosure into a single 
ZVerifyOopClosure, which I did.

I still agree that the current double verification of nmethod oops seems 
completely unnecessary, but it's harmless so that can be handled by a 
separate RFE.

cheers,
Per

> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8212921
> 
> Thanks,
> /Erik