https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81578
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> --- #1 0x0000000000db5299 in omp_reduction_init_op (loc=2147483656, op=COND_EXPR, type=<integer_type 0x7ffff68a95e8 int>) at /space/rguenther/src/svn/trunk/gcc/omp-low.c:3417 3417 gcc_unreachable (); (gdb) l 3412 gcc_assert (INTEGRAL_TYPE_P (type)); 3413 return TYPE_MAX_VALUE (type); 3414 } 3415 3416 default: 3417 gcc_unreachable (); the thing to note is that autopar re-uses vectorizable_reduction but as that gets enhanced you'll run into non-handled cases in autopar. Need to untie those. In this case cond reduction is not handled by autopar. GCC 7 seems to work though.