In G1 we can scan cards both during and outside a STW GC. We typically don't want to scan objects that have been copied to a region during a GC. So, we use the save_marks() mechanism to checkpoint top before we start copying objects into the region so that we bound any card scanning by the checkpointed top.

To avoid calling save_marks() on all regions before a GC we rely on a timestamp mechanism. When we call save_marks() on a region we will start allocating to during the GC we tag said region with the current GC timestamp. So, the card scanning is bound by either top_at_save_marks(), if the region's timestamp is current, or top() if it's not. The timestamp is incremented once at the start of a GC which essentially invalidates all the previous save_marks() information.

When we scan cards outside GC we always want to use top and ignore save_marks(). So the card scanning code looks like this:

  if (G1CollectedHeap::heap()->is_gc_active()) {
    mr = mr.intersection(used_region_at_save_marks());
  } else {
    mr = mr.intersection(used_region());
  }

On a separate changeset (7039627) we are changing the code to also increment the timestamp at the end of a GC. This essentially invalidates the save_marks() information on all regions. So we have an opportunity to remove the above check and always rely on the used_region_at_save_marks() returning the right value whether we are during a pause or outside one.