https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77421
--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh one more I missed. alter_output_for_subst_insn in gensupport.c has:
if (alt < 2 || *insn_out == '*' || *insn_out != '@')
return insn_out;
The second condition is redundant, since if it's == '*' then it's also != '@'
The fix isn't obvious.
