Schedule reduction partition in the last.

If reduction partition's SCC is broken by runtime alias checks, force
a negative post order to it so that it will be scheduled in the last.

2020-07-09  Bin Cheng  <bin.cheng@linux.alibaba.com>

gcc/
	PR tree-optimization/95804
	* tree-loop-distribution.c (break_alias_scc_partitions): Force
	negative post order to reduction partition.

gcc/testsuite/
	PR tree-optimization/95804
	* gcc.dg/tree-ssa/pr95804.c: New test.

(cherry picked from commit dd21b03900085c4d60bf03207ad28bcbfbc86a4b)

gcc/testsuite/gcc.dg/tree-ssa/pr95804.c

+/* PR tree-optimization/95804 */
+/* { dg-do compile } */
+/* { dg-options "-O3" } */
+
+int a, b, c, d, e, f;
+void g() {
+  short *h = (short*)&d;
+  char *i = (char*)&b;
+  for (; e; e++) {
+    for (; f; f++) {
+      b = 3;
+      if ((c = 8) >= *i)
+        a = 5 ? *h : 0;
+      h = (short*)g;
+    }
+    i = (char*)&c;
+  }
+}

gcc/tree-loop-distribution.c

@@ -2509,10 +2509,25 @@ loop_distribution::break_alias_scc_partitions (struct graph *rdg,
 	    }
 	}
       /* Restore the postorder information if it's corrupted in finding SCC
-	 with alias dependence edges skipped.  */
+	 with alias dependence edges skipped.  If reduction partition's SCC is
+	 broken by runtime alias checks, we force a negative post order to it
+	 making sure it will be scheduled in the last.  */
       if (num_sccs_no_alias > 0)
+	{
+	  j = -1;
 	  for (i = 0; i < pg->n_vertices; ++i)
+	    {
 	      pg->vertices[i].post = cbdata.vertices_post[i];
+	      struct pg_vdata *data = (struct pg_vdata *)pg->vertices[i].data;
+	      if (data->partition && partition_reduction_p (data->partition))
+		{
+		  gcc_assert (j == -1);
+		  j = i;
+		}
+	    }
+	  if (j >= 0)
+	    pg->vertices[j].post = -1;
+	}
 
       free (cbdata.vertices_component);
       free (cbdata.vertices_post);