https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78458
--- Comment #5 from Peter Bergner <bergner at gcc dot gnu.org> --- gdb show we don't like: (gdb) frame 1 #1 0x0000000010a7d328 in lra_set_insn_recog_data (insn=0x3fffb5584340) at /home/bergner/gcc/gcc-fsf-mainline-reg-move_costs-base/gcc/lra.c:963 963 gcc_assert (GET_CODE (PATTERN (insn)) == USE (gdb) pr insn (insn 281 280 129 9 (set (subreg:SI (reg/v:TF 173 [ p1 ]) 12) (subreg:SI (reg:IF 325) 12)) "pr78458.i":17 -1 (nil)) Looking at the -mno-lra output, it's interesting that we don't have any use of IFmode anywhere.