https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78037
--- Comment #6 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Jakub Jelinek from comment #5)
> That is because _tzcnt_u64 is expanded as __builtin_ctzll, which is
> explicitly UB with zero argument. So, either we need to expand it like
> x ? __builtin_ctzll (x) : 64 and make sure at RTL level we optimize this into
> just tzcnt instruction if available (though, that is questionable, because
> if you run tzcntq instruction on non-BMI CPUs, it will just be BSF with
> undefined result for 0), or add a new builtin, or unconditionally expand to
> the conditional.
A new UNSPEC insn pattern should be added and expanded from __tzcnt_u{16,32,64}
builtin.
