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.

diff --git a/gcc/gimple-loop-jam.cc b/gcc/gimple-loop-jam.cc
index a8a57d3d3840c7fc77b392e9408583309255c8f0..4f7a6e5bbae587c9b715818ac30802df2daddb02 100644 (file)
--- a/gcc/gimple-loop-jam.cc
+++ b/gcc/gimple-loop-jam.cc
@@ -545,11 +545,25 @@ tree_loop_unroll_and_jam (void)
          /* If the refs are independend there's nothing to do.  */
          if (DDR_ARE_DEPENDENT (ddr) == chrec_known)
            continue;
+
          dra = DDR_A (ddr);
          drb = DDR_B (ddr);
-         /* Nothing interesting for the self dependencies.  */
+
+         /* Nothing interesting for the self dependencies, except for WAW if
+            the access function is not affine or constant because we may end
+            up reordering writes to the same location.  */
          if (dra == drb)
-           continue;
+           {
+             if (DR_IS_WRITE (dra)
+                 && !DR_ACCESS_FNS (dra).is_empty ()
+                 && DDR_ARE_DEPENDENT (ddr) == chrec_dont_know)
+               {
+                 unroll_factor = 0;
+                 break;
+               }
+             else
+               continue;
+           }
 
          /* Now check the distance vector, for determining a sensible
             outer unroll factor, and for validity of merging the inner

diff --git a/gcc/testsuite/gcc.c-torture/execute/20221006-1.c b/gcc/testsuite/gcc.c-torture/execute/20221006-1.c
new file mode 100644 (file)
index 0000000..80deb3a
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/20221006-1.c
@@ -0,0 +1,29 @@
+#include <stdlib.h>
+
+int
+main (int argc, char** argv)
+{
+  const int len = argc == 2 ? atoi(argv[1]) : 4;
+
+  int count;
+  int data[64];
+  int M1[len][len];
+  int M2[len][len];
+
+  for (int i = 0; i < len; i++)
+    for (int j = 0 ; j < len ; j++)
+      M1[i][j] = M2[i][j] = i*len + j;
+
+  M2[1][0] = M2[0][1];
+
+  /* This writes successively 0 and 1 into data[M2[0][1]].  */
+  for (int i = 0; i < len - 1; i++)
+    for (int j = 0 ; j < len ; j++)
+      if (M1[i+1][j] > M1[i][j]) 
+        data[M2[i][j]] = i;
+
+  if (data [M2[0][1]] != 1)
+    abort ();
+
+  return 0;
+}