The BarrierSet::{pre,post}_barrier functions take a "control" value as the second (ctl) argument. But for the G1 implementations at least (I've not looked at others) this argument is unused, with `__ ctrl()` being used instead. It seems like either the ctl argument isn't needed or it should be used instead of reaccessing. (I'm assuming the two expressions always provide the same value, else bad things can probably happen.)