RFR: Check for NULL in is-alive-closures

[Roman Kennke]

Am 02.02.2018 um 15:03 schrieb Aleksey Shipilev:
> On 02/02/2018 01:55 PM, Roman Kennke wrote:
>>> http://cr.openjdk.java.net/~rkennke/fixisalivecl/webrev.00/
>>
>> The situation might arise only with traversal GC, because it can mark through the Reference object
>> right after it is created, but fields not yet initialized. In this case, referent is still NULL, but
>> it can already be visible to the GC. A subsequent putfield to store the referent will do the right
>> thing (i.e. put the referent in the traversal work queue, and mark it live). Returning 'is-alive'
>> for the NULL is still the best we can do I think.
> 
> I guess it is then safer to add to do this:
> 
>     if (_heap->concurrent_traversal_in_progress() && oopDesc::is_null(obj)) {
>       // (add the comment from above here)
>       return true;
>     }
> 
> -Aleksey
> 

HmmmHmm. Ok:

http://cr.openjdk.java.net/~rkennke/fixisalivecl/webrev.01/

Good?

Roman