https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93449
--- Comment #7 from Bill Schmidt <wschmidt at gcc dot gnu.org> --- The ELFv2 ABI Appendix B calls for a bcd data type defined as: typedef bcd vector unsigned char; and then defines a bunch of potential functions that can be built around it. The BCD functions (such as __builtin_bcdadd), earlier in the appendix, are defined in terms of vector unsigned char. GCC has just never gotten around to implementing these, due to a combination of user disinterest and resource constraints. We'll have to step up to these, hopefully in GCC 11 (though our plate is really full there already).
