The following commit:

https://github.com/openjdk/shenandoah/pull/193/commits/ca96f5232c0107856a811b27b6b33794a60f7083

in our initial implementation of block_start() had a bug that was never found or fixed, and was later hidden by #undef'ing the code which was left in place for a potential future where object_start arrays may not be updated atomically wrt card scans. If the code were ever to be not undef'd out there would be problems. Either that piece of unused forward walk code should be removed entirely, or the code in the commented out section should be fixed.

The bug was identified by Kelvin Nilsen.