https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173
--- Comment #27 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Given that the builtins exist for 10 years already, I think changing it for them is too late, though they don't seem to take backwards compatibility as seriously. They don't document the [0, 1] restriction and the behavior implemented in GCC is what I saw when trying it. Note, in many cases it isn't that big deal, because if carry_in is in [0, 1] range and compiler can see it from VRP, it can still optimize it. And given that carry_out is always in [0, 1] range, for chained cases worst case the first additions will be less optimized but the chained will be already better.
