https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126973
--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> --- I am 99% the problem is clang does not implement the ABI correctly. ctzg_uN is correct for GCC while for clang, there is an assumption that unsigned short/unsigned _BitInt(16) is zero extended which is not a valid assumption.
