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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101009] [12 Regression] wrong code with "-O3
 -fno-tree-sra"
Date: Fri, 11 Jun 2021 07:19:39 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101009

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
So interestingly we do compute a distance vector of zero but we fail to add=
 it,
instead we end up returning true from build_classic_dist_vector_1 without
setting *init_b to true (the access fns are not POLYNOMIAL_CHREC but are
equal).  Then in build_classic_dist_vector we skip

  /* Save the distance vector if we initialized one.  */
  if (init_b)
..

and run into

      /* There is a distance of 1 on all the outer loops: Example:
         there is a dependence of distance 1 on loop_1 for the array A.

         | loop_1
         |   A[5] =3D ...
         | endloop
      */
      add_outer_distances (ddr, dist_v,
                           lambda_vector_first_nz (dist_v,
                                                   DDR_NB_LOOPS (ddr), 0));

which ends up pushing a distance vector (1) as "outer distance".  But in
the skipped if () case we'd only ever do that if DDR_NB_LOOPS > 1.

Both

@@ -5435,7 +5437,7 @@ build_classic_dist_vector (struct
data_dependence_relation
 *ddr,
            save_dist_v (ddr, save_v);
        }
     }
-  else
+  else if (DDR_NB_LOOPS (ddr) > 1)
     {
       /* There is a distance of 1 on all the outer loops: Example:
         there is a dependence of distance 1 on loop_1 for the array A.

and

@@ -5121,6 +5121,8 @@ build_classic_dist_vector_1 (struct
data_dependence_relati
on *ddr,
          non_affine_dependence_relation (ddr);
          return false;
        }
+      else
+       *init_b =3D true;
     }

   return true;

fix the miscompilation.  For the first patch we end up with no distance
vector in ddr->dist_vects and for the second with a single { 0 } distance
vector.  IMHO the second looks more correct to me but maybe it is intended
that non-affine but constant indexes do not get a distance vector.

Anybody with some insights here?=