Currently, G1 Full GC uses the default reference processing enqueue code that executes the post-barrier code during enqueuing.

This is not necessary, or even desired. G1 full gc throws away all remembered set information later anyway, so generating it in the first place does not make sense.

Also, in the general case, collectors are not always ready to execute the barriers during GC, potentially causing issues in the future. It also prevents tightening use of runtime barrier set (G1BarrierSet), disallowing its use during gc completely.