RFR(M): 8195103: Refactor ReduceInitialCardMarks to not assume all GCs use card marks

[Thomas Schatzl]

Hi,

On Tue, 2018-01-16 at 10:42 +0100, Erik Österlund wrote:

  ^^ sorry for being a bit late...

> Hi,
> 
> The current interface between the compilers and GC regarding the 
> ReduceInitialCardMarks optimization lives in the CollectedHeap.
> However, the optimization is relevant only for collectors with a card
> mark barrier set (CardTableModRefBS). Therefore, this interface ought
> to be moved into CardTableModRef so that code gets less messy when a
> collector does not use card marking. In the process, the 
> CollectedHeap::pre_initialize member function was removed (as it was 
> only used for initializing ReduceInitialCardMarks).
> 
> The optimization needs to check if an object is in young or not.
> This question is now asked to the barrier set rather than the heap.
> For all collectors except G1, this has been implemented by forwarding
> the question to the corresponding heap (inlined member function),
> which is  what was done before. For G1, I chose to instead look at
> the card value  and see if it is a young card, which should give the
> same answer.

Marking the cards young is done concurrently to the application. So you
could get false answers here. However it seems that this is benign,
i.e. at most too many objects are pushed into the deferred card mark
from what I can see.

However the assert in
CardTableModRefBs::flush_deferred_card_mark_barrier() may complain...
i.e. at the time when the object is deferred, the result of is_young()
may be false, but at the time the deferred card mark is flushed,
is_young() will return true.

Note that while this occurrence is not very common, it does happen.

I think this needs to be fixed. Either the mentioned assert, or the
is_young() check. The region type is always good btw.

> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8195103
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8195103/webrev.00/
> 
> Testing: mach5 hs-tier1-5

  looks good to me otherwise.

Thanks,
  Thomas