RFR: Simplify and optimize ShenandoahHeap::requires_marking()

[Aleksey Shipilev]

On 01/10/2018 11:04 PM, Roman Kennke wrote:
> When I implemented partial-GC, I complicated ShenandoahHeap::requires_marking(). This method
> basically decides which oops on SATB queues to keep and which to discard when filtering SATB queues
> (before processing them). The idea behind this was that we need all oops on the queues for
> partial-GC, but only not-yet-marked oops during concurrent marking.
> 
> Work on traversal GC lead to a similar problem, and then it occurred to me that we don't need to
> complicate that code at all: during partial GC, we never use the bitmap. Simply returning
> !is_marked_next(obj) does the same as return true, and is probably. This should restore a little bit
> of performance of the regular Shenandoah mode, at the potential cost of a little bit of performance
> for partial GC. However, I am not even sure that this code is performance critical at all. I
> couldn't see any performance changes.
> 
> http://cr.openjdk.java.net/~rkennke/req-marking/webrev.00/
> 
> Testing: hotspot_gc_shenandoah
> 
> Ok to push?

That makes sense. Looks good.

Thanks,
-Aleksey