https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94043
--- Comment #4 from Kewen Lin <linkw at gcc dot gnu.org> --- This was just exposed from my commit, it can also be reproduced without my commit but with -fno-vect-cost-model. Some loops we have for this case: ;; Loop 1 ;; header 3, latch 10 ;; depth 1, outer 0 ;; nodes: 3 10 8 23 25 34 35 26 29 32 33 38 4 11 37 31 ;; Loop 2 ;; header 4, latch 11 ;; depth 2, outer 1 ;; nodes: 4 11 ;; Loop 4 ;; header 26, latch 29 ;; depth 2, outer 1 ;; nodes: 26 29 When we are doing versioning for loop4 required for aliasing check, the related scalar_loop_iters is based on e2.2_31, which is defined in BB 4, that is: <bb 4> [local count: 4343773762]: # e2.2_31 = PHI <_15(11), 1(37)> # ivtmp_14 = PHI <ivtmp_23(11), 4(37)> For the codes: if ((def_bb = gimple_bb (SSA_NAME_DEF_STMT (USE_FROM_PTR (use_p)))) && flow_bb_inside_loop_p (outermost, def_bb)) outermost = superloop_at_depth (loop, bb_loop_depth (def_bb) + 1) bb_loop_depth is 2, the +1 make the assertion in superloop_at_depth fail since the current loop 4 only has the depth 2. I think the existing code has the assumption that all operands in stmts of cond_expr_stmt_list are defined in some outer loop of current, but the assumption breaks in this case. I guess the current scalar_loop_iters is valid? Then the fix can be: --- a/gcc/tree-vect-loop-manip.c +++ b/gcc/tree-vect-loop-manip.c @@ -3312,7 +3312,13 @@ vect_loop_versioning (loop_vec_info loop_vinfo, FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE) if ((def_bb = gimple_bb (SSA_NAME_DEF_STMT (USE_FROM_PTR (use_p)))) && flow_bb_inside_loop_p (outermost, def_bb)) - outermost = superloop_at_depth (loop, loop_depth (outermost) + 1); + { + /* Def block can be in either one outer loop of loop_to_version or + one sibling of outer loop of loop_to_version. */ + class loop *common_loop + = find_common_loop (def_bb->loop_father, loop); + outermost = superloop_at_depth (loop, loop_depth (common_loop) + 1); + } }