[2/8] Delay swapping data refs in prune_runtime_alias_test_list

[Richard Sandiford]

prune_runtime_alias_test_list swapped dr_as between two dr_with_seg_len
pairs before finally deciding whether to merge them.  Bailing out later
would therefore leave the pairs in an incorrect state.

IMO a better fix would be to split this out into a subroutine that
produces a temporary dr_with_seg_len on success, rather than changing
an existing one in-place.  It would then be easy to merge both the dr_as
and dr_bs if we wanted to, rather than requiring one of them to be equal.
But here I tried to do something that could be backported if necessary.


2019-11-11  Richard Sandiford  <richard.sandif...@arm.com>

gcc/
        * tree-data-ref.c (prune_runtime_alias_test_list): Delay
        swapping the dr_as based on init values until we've decided
        whether to merge them.

Index: gcc/tree-data-ref.c
===================================================================
--- gcc/tree-data-ref.c 2019-11-11 18:30:43.203244558 +0000
+++ gcc/tree-data-ref.c 2019-11-11 18:30:47.199216669 +0000
@@ -1556,13 +1556,6 @@ prune_runtime_alias_test_list (vec<dr_wi
          if (!ordered_p (init_a1, init_a2))
            continue;
 
-         /* Make sure dr_a1 starts left of dr_a2.  */
-         if (maybe_gt (init_a1, init_a2))
-           {
-             std::swap (*dr_a1, *dr_a2);
-             std::swap (init_a1, init_a2);
-           }
-
          /* Work out what the segment length would be if we did combine
             DR_A1 and DR_A2:
 
@@ -1579,7 +1572,10 @@ prune_runtime_alias_test_list (vec<dr_wi
 
             The lengths both have sizetype, so the sign is taken from
             the step instead.  */
-         if (!operand_equal_p (dr_a1->seg_len, dr_a2->seg_len, 0))
+         poly_uint64 new_seg_len = 0;
+         bool new_seg_len_p = !operand_equal_p (dr_a1->seg_len,
+                                                dr_a2->seg_len, 0);
+         if (new_seg_len_p)
            {
              poly_uint64 seg_len_a1, seg_len_a2;
              if (!poly_int_tree_p (dr_a1->seg_len, &seg_len_a1)
@@ -1597,14 +1593,24 @@ prune_runtime_alias_test_list (vec<dr_wi
              int sign_a = tree_int_cst_sgn (indicator_a);
              int sign_b = tree_int_cst_sgn (indicator_b);
 
-             poly_uint64 new_seg_len;
              if (sign_a <= 0 && sign_b <= 0)
                new_seg_len = lower_bound (seg_len_a1, seg_len_a2);
              else if (sign_a >= 0 && sign_b >= 0)
                new_seg_len = upper_bound (seg_len_a1, seg_len_a2);
              else
                continue;
+           }
+         /* At this point we're committed to merging the refs.  */
 
+         /* Make sure dr_a1 starts left of dr_a2.  */
+         if (maybe_gt (init_a1, init_a2))
+           {
+             std::swap (*dr_a1, *dr_a2);
+             std::swap (init_a1, init_a2);
+           }
+
+         if (new_seg_len_p)
+           {
              dr_a1->seg_len = build_int_cst (TREE_TYPE (dr_a1->seg_len),
                                              new_seg_len);
              dr_a1->align = MIN (dr_a1->align, known_alignment (new_seg_len));