Fix wrong code generated by unroll-and-jam pass

There is a loophole in the unroll-and-jam pass that can quickly result in
wrong code generation.  The code reads:

    if (!compute_data_dependences_for_loop (outer, true, &loop_nest,
                                &datarefs, &dependences))
        {
          if (dump_file && (dump_flags & TDF_DETAILS))
            fprintf (dump_file, "Cannot analyze data dependencies\n");
          free_data_refs (datarefs);
          free_dependence_relations (dependences);
          continue;
        }

but compute_data_dependences_for_loop may return true even if the analysis
is reported as failing by compute_affine_dependence for a dependence pair:

(compute_affine_dependence
  ref_a: data[_14], stmt_a: data[_14] = i_59;
  ref_b: data[_14], stmt_b: data[_14] = i_59;
Data ref a:
Data ref b:
affine dependence test not usable: access function not affine or constant.
) -> dependence analysis failed

Note that this is a self-dependence pair and the code for them reads:

          /* Nothing interesting for the self dependencies. */
          if (dra == drb)
            continue;

This means that the pass may reorder "complex" accesses to the same memory
location in successive iterations, which is OK for reads but not for writes.

gcc/
	* gimple-loop-jam.cc (tree_loop_unroll_and_jam): Bail out for a self
	dependency that is a write-after-write if the access function is not
	affine or constant.

gcc/testsuite/
	* gcc.c-torture/execute/20221006-1.c: New test.

0 files changed, 0 insertions, 0 deletions