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.