How do you clear the branch predictor cache in corex-m7?

BPIALL is documented as not implemented, but the branch predictor is documented and it is easy to see in testing that it is implemented. The BTAC bits in ACTLR work as described, you can basically disable it with one bit, and you can prevent new branches from being cached with another. But so far I can't figure out how to clear the BPI cache. (clearing the command cache doesn't help).