RFR: 8138888: Remove ExtendedOopClosure::apply_to_weak_ref_discovered_field

[Stefan Johansson]

Hi all,

Please review view this patch to address:
https://bugs.openjdk.java.net/browse/JDK-8138888

Webrev:
http://cr.openjdk.java.net/~sjohanss/8138888/hotspot.00/

Summary:
When working on JDK-8138737 I noticed that the handling of the reference 
fields next, discovered and referent is treated differently by depending 
on which closure is being applied. This is controlled by 
ExtendedOopClosure::apply_to_weak_ref_discovered_field. I could not find 
any real good reason for this except that there was a check that looked 
like this:

// Treat discovered as normal oop, if ref is not "active" (next non-NULL)
if (!oopDesc::is_null(next_oop) && contains(disc_addr)) {

Because of this check (that I and some other folks I've discussed this 
with can't understand) some closures need to override 
apply_to_weak_ref_discovered_field to ensure that the closure is applied 
correctly. By removing this check on next, I believe we can get rid of 
apply_to_weak_ref_discovered_field as well. I've read the code and done 
testing without being able to find a case where this won't work.

This change will help when doing JDK-8138737, because the closure in 
that patch need to be applied to all fields.

Testing:
* JPRT
* RBT and Aurora runs with JTREG, tonga and BigApps
* Locally run gc-test-suite with G1

Thanks,
Stefan