You could keep incrementing from the current value until the population count matches again, but in the worst case - probably from 2^62 to 2^63 with a single se

You could keep incrementing from the current value until the population count matches again, but in the worst case - probably from 2^62 to 2^63 with a single set bit or wrapping around from 2^63 to 2^0 - that would still take pretty much 2^64 steps.