[Bug tree-optimization/101105] [11/12 Regression] wrong code at -O3 on x86_64-linux-gnu

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101105

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
So what happens is that vect_compile_time_alias fails to perform the offset
adjustment for the negative step DR

#(Data Ref: 
#  bb: 3 
#  stmt: b[g_40][0] = 0;
#  ref: b[g_40][0];
#  base_object: b;
#  Access function 0: 0
#  Access function 1: {3, +, -1}_1

which is done via

  /* For negative step, we need to adjust address range by TYPE_SIZE_UNIT
     bytes, e.g., int a[3] -> a[1] range is [a+4, a+16) instead of
     [a, a+12) */
  if (tree_int_cst_compare (DR_STEP (a->dr), size_zero_node) < 0)
    {
      const_length_a = (-wi::to_poly_wide (segment_length_a)).force_uhwi ();
      offset_a -= const_length_a;
    }

since we zero segment_length_a because of ignore_step_p.  But that adjustment
cannot be ignored.  I suppose we need to track a separate "offset segment
length"
for this purpose?