There are a bunch of switch statements in various places that are dispatching on the barrier set kind. They usually (or perhaps always) have a case for BarrierSet::ModRef, with some sort of "default" behavior. However, there are no classes with that barrier set kind. So we're pretending we know what code should be there, and taking up code space for it. It would be better to leave out that case and break in some obvious way (such as hitting a ShouldNotReachHere() default: case, as is usually done in these switch statements) if we ever introduce a class that directly uses that barrier set kind. (Although I personally think we should never directly use that kind.) If we break in an obvious way we'll know what needs to be done, rather than unintentionally covering a case incorrectly.