RFR: Replace risky SBS::need_update_refs_barrier with straightforward check

[Aleksey Shipilev]

On 06/18/2018 06:38 PM, Roman Kennke wrote:
> Am 18.06.2018 um 18:34 schrieb Aleksey Shipilev:
>> http://cr.openjdk.java.net/~shade/shenandoah/sbs-risky-check/webrev.01/
>>
>> Heuristics has the automatic switch between separate-UR and CM-with-UR modes. Current check relies
>> on heuristics()->update_refs() to figure out if we should check ur-in-progress or mark-in-progress
>> when updating the references. This is risky, because heuristics may have stale idea about which mode
>> to operate in. It is safer to check all potential GC states at once.
>>
> 
> Good idea!
> 
> Seems like some of it can be collapsed into a single mask check:
> 
> +         _heap->is_update_refs_in_progress() ||
> +         _heap->is_concurrent_traversal_in_progress() ||
> +         (_heap->is_concurrent_mark_in_progress() &&
> _heap->has_forwarded_objects());
> 
> Probably not worth though. Your call.

I thought so too, but it requires two things: checking that has_forwarded bit set, and *either* of
other three bits set. We can compare with "> has_forwarded", which implies this, but it brings in
endianness that I don't want to deal with.

-Aleksey