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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101801] New: vect_worthwhile_without_simd_p
 is broken
Date: Fri, 06 Aug 2021 10:26:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101801

            Bug ID: 101801
           Summary: vect_worthwhile_without_simd_p is broken
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

vect_worthwhile_without_simd_p is currently

bool
vect_worthwhile_without_simd_p (vec_info *vinfo, tree_code code)
{
  loop_vec_info loop_vinfo =3D dyn_cast <loop_vec_info> (vinfo);
  unsigned HOST_WIDE_INT value;
  return (loop_vinfo
          && LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant (&value)
          && value >=3D vect_min_worthwhile_factor (code));
}

which means it's never worthwhile to BB vectorize.  Also the VF check
doesn't honor SLP so that a fully SLPed loop with VF =3D=3D 1 is never
considered worthwhile to vectorize.

I ran into this beast when looking at vectorization of mask condition
operations like cond_mask1 & cond_mask2 which, for AVX512, have
integer mode but vectorizable_operation does

  /* Worthwhile without SIMD support?  Check only during analysis.  */
  if (!VECTOR_MODE_P (vec_mode)
      && !vec_stmt
      && !vect_worthwhile_without_simd_p (vinfo, code))
    {
      if (dump_enabled_p ())
        dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
                         "not worthwhile without SIMD support.\n");
      return false;
    }

and in my case with SLP the VF was indeed one and vectorization failed.
I think the code should not look at the vectorization factor but instead
at the vector type (and its number of components).=