Stale forwarding bits in nursery GC in StickyImmix #1118
Description
Currently in StickyImmix, we clear both on-the-side mark bits and on-the-side forwarding bits (the Ruby binding currently uses on-the-side forwarding bits) in the Prepare stage of major GC, and we clear neither of them in nursery GC. For Immix and non-moving StickyImmix, this works fine because (1) every GC is a major GC for Immix, and (2) we never forward anything if StickyImmix never moves object.
However, if we use StickyImmix and enable object movement, the forwarding bits set in a full-heap GC will remain set until the next full-heap GC. Some of them are set for from-space objects that have already been moved. If a nursery GC happens in between, and there is an object happened to be allocated at the place where a stale forwarding bits is set, the nursery GC will erroneously consider the object as moved. When this error manifests, weak reference processors will see a non-moved object have a forwarding pointer, and its value is whatever currently stored in the object at the offset for storing forwarding reference.