[pcwalton]

Do you have a reference on G1 read barriers? I believe you, but I can't find any information about that from a Google search.

I don't immediately see why you'd need a read barrier for snapshot-at-the-beginning. You only need to trace references from gray objects to white objects that were deleted, which is a write.

[vitalyd]

Sorry, that was my mistake - it has pre-write barriers for SATB but not actual read barriers.

However, ZGC will have read barriers and if Shenandoah ever gets integrated into Hotspot, it has them too.