BIT_FFS(9freebsd) on Linux
The text 4b46c9dd9826, as Debian 9 (Stretch) shipped it.
BITSET_DEFINE, BITSET_T_INITIALIZER, BITSET_FSET, BIT_CLR, BIT_COPY, BIT_ISSET, BIT_SET, BIT_ZERO, BIT_FILL, BIT_SETOF, BIT_EMPTY, BIT_ISFULLSET, BIT_FFS, BIT_COUNT, BIT_SUBSET, BIT_OVERLAP, BIT_CMP, BIT_OR, BIT_AND, BIT_NAND, BIT_CLR_ATOMIC, BIT_SET_ATOMIC, BIT_SET_ATOMIC_ACQ, BIT_AND_ATOMIC, BIT_OR_ATOMIC, BIT_COPY_STORE_REL – bitset manipulation macros