KC_BITS_OFF
KASSERT((uint8_t *)kc + KC_BITS_OFF == (uint8_t *)&kc->kc_field);
#define KC_GETSTRUCT(b) ((kcpuset_impl_t *)((char *)(b) - KC_BITS_OFF))
#define KC_GETCSTRUCT(b) ((const kcpuset_impl_t *)((const char *)(b) - KC_BITS_OFF))