Our most frequent error on x86 is back to being a bad object reference being traced by the GC. The number of places we create object references are limited. We either load them or create them from objects. In the compiler we can create checks that ensure that an object reference that is loaded or converted from an object is well formed. A well formed object reference should be a nullReference or: 1) have a non-null TIB 2) the TIB should be of type TIB 3) have a TIB where we can read the type of the object it would be nice to have non-null allowing variants of the creation routines for places where the GC assumes a reference cannot be null. If there are other places we can insert extra checks then please let me know. This process comes from the thought that if MMTk manipulated objects that represented the state being manipulated then we could check when the state of such objects becomes corrupt. Obviously this technique is intended for non-production use.