JDK-8240556 proposes to "abort" concurrent mark right after initial mark if the gc happens to free enough memory to go below the IHOP threshold.

For that it needs to undo any marking work already done in the pause (currently we immediately mark roots from non-Java heap).

The current proposed patch for that undoes the marks in the pause, which is a time consuming process, even if done in parallel.

For this reason it would be nice if the G1ConcurrentMarkThread could be cleaned up a little to do that bitmap clear concurrently.

(Even without that reason, it would be a good cleanup candidate)