mask the shift by __n with NBBY*sizeof(uintmax-t)-1
to suppress the following diagnostic from clang:
error: shift count >= width of type [-Werror,-Wshift-count-overflow]
PAR_ATTR = __BITS(63,56),// F=0 memory attributes
^~~~~~~~~~~~~
src/sys/sys/cdefs.h:554:4: note: expanded from macro '__BITS'
((__BIT(MAX((__m), (__n)) + 1) - 1) ^ (__BIT(MIN((__m), (__n))) - 1))
^~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/sys/sys/cdefs.h:550:73: note: expanded from macro '__BIT'
(((uintmax_t)(__n) >= NBBY * sizeof(uintmax_t)) ? 0 : ((uintmax_t)1 << (uintmax_t)(__n)))
^ ~~~~~~~~~~~~~~~~