[Bug tree-optimization/94043] [9/10 Regression] ICE in superloop_at_depth, at cfgloop.c:78

["linkw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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);
+         }
     }