RFR: 8166188: G1 Needs pre barrier on dereference of weak JNI handles

[Kim Barrett]

> On Jan 19, 2017, at 11:28 AM, Mikael Gerdin <mikael.gerdin at oracle.com> wrote:
> 
> Hi Kim,
> 
> On 2017-01-19 06:31, Kim Barrett wrote:
>> Please review this change to jweak to support G1.
>> 
>> […]
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8166188
>> 
>> Webrev:
>> http://cr.openjdk.java.net/~kbarrett/8166188/hotspot.02/
> 
> I thought a bit about this issue today and unfortunately I've uncovered yet another location where a weak JNI handle needs to have its weak tag cleared.
> Consider the following JNI code snippet
> 
> […]
> I've created a test to provoke this situation and put it at:
> http://cr.openjdk.java.net/~mgerdin/8166188-pass-arguments/webrev/
> run the test with
> $ make test-hotspot-jtreg-native
> 
> Sorry for not coming up with this sooner, I just realized this problem today.

Well, I’m happy you found it before this went out the door.  Despite looking for this
sort of thing, I completely missed this one.  I need to re-review casts to oop*.