RFR: 8226823: Adjust BarrierSetC2 for C2 runtime calls

[Roman Kennke]

Thanks, Nils!

Roman

> Looks good,
> 
> // Nils
> 
> On 2019-07-01 17:28, Roman Kennke wrote:
>>   In JDK-8226695 we want to emit runtime calls in C2 ideal graph that
>> return an oop. This requires some adjustments in the GC interface for
>> escape analysis in order to work.
>>
>> Specifically, we need:
>> - Access to a bunch of stuff in ConnectionGraph that is currently
>> private. Unfortunately we can't just befriend BarrierSetC2 and
>> transitively its subclasses.
>> - Apply the GC hook to EA earlier such that it can also handle calls.
>>
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8226823
>> Webrev:
>> http://cr.openjdk.java.net/~rkennke/JDK-8226823/webrev.00/
>>
>> Can I please get reviews?
>>
>> Thanks,
>> Roman
>>