tree-optimization/112859 - bogus loop distribution

When we get a zero distance vector we still have to check for the
situation of a common inner loop with zero distance.  But we can
still allow a zero distance for the loop we distribute
(gcc.dg/tree-ssa/ldist-33.c is such a case).  This is because
zero distances in non-outermost loops are a misrepresentation
of dependence by dependence analysis.

Note that test coverage of loop distribution of loop nests is
very low.

	PR tree-optimization/112859
	PR tree-optimization/115347
	* tree-loop-distribution.cc
	(loop_distribution::pg_add_dependence_edges): For a zero
	distance vector still make sure to not have an inner
	loop with zero distance.

	* gcc.dg/torture/pr112859.c: New testcase.
	* gcc.dg/torture/pr115347.c: Likewise.

3 files changed, 61 insertions, 11 deletions

diff --git a/gcc/testsuite/gcc.dg/torture/pr112859.c b/gcc/testsuite/gcc.dg/torture/pr112859.c
new file mode 100644
index 0000000..18f5bf4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr112859.c
@@ -0,0 +1,24 @@
+/* { dg-do run } */
+/* { dg-additional-options "-ftree-loop-distribution" } */
+
+struct a {
+  char b;
+  int c;
+} f, *i = &f;
+static struct a e[4];
+int *d, **g = &d;
+static int h, j;
+int main()
+{
+  for (; h < 1; h++) {
+    struct a k = {1, 1};
+    for (j = 0; j < 2; j++) {
+      *i = e[h];
+      e[h] = k;
+    }
+    *g = 0;
+  }
+  if (f.c != 1)
+    __builtin_abort();
+  return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/torture/pr115347.c b/gcc/testsuite/gcc.dg/torture/pr115347.c
new file mode 100644
index 0000000..2299495
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr115347.c
@@ -0,0 +1,21 @@
+/* { dg-do run } */
+/* { dg-additional-options "-ftree-loop-distribution" } */
+
+struct a {
+  int b;
+  int c;
+} d, e[2];
+int f, g, h;
+int main()
+{
+  for (; f < 1; f++) {
+    for (h = 0; h < 2; h++) {
+      d = e[f];
+      g = e[1].c;
+      e[f].c = 1;
+    }
+  }
+  if (d.c != 1)
+    __builtin_abort();
+  return 0;
+}
diff --git a/gcc/tree-loop-distribution.cc b/gcc/tree-loop-distribution.cc
index 7c0e4a8..9d9d2ae 100644
--- a/gcc/tree-loop-distribution.cc
+++ b/gcc/tree-loop-distribution.cc
@@ -2178,25 +2178,30 @@ loop_distribution::pg_add_dependence_edges (struct graph *rdg, int dir,
 		 gcc.dg/tree-ssa/pr94969.c.  */
 	      if (DDR_NUM_DIST_VECTS (ddr) != 1)
 		this_dir = 2;
-	      /* If the overlap is exact preserve stmt order.  */
-	      else if (lambda_vector_zerop (DDR_DIST_VECT (ddr, 0),
-					    DDR_NB_LOOPS (ddr)))
-		;
-	      /* Else as the distance vector is lexicographic positive swap
-		 the dependence direction.  */
 	      else
 		{
-		  if (DDR_REVERSED_P (ddr))
-		    this_dir = -this_dir;
-		  this_dir = -this_dir;
-
+		  /* If the overlap is exact preserve stmt order.  */
+		  if (lambda_vector_zerop (DDR_DIST_VECT (ddr, 0),
+					   DDR_NB_LOOPS (ddr)))
+		    ;
+		  /* Else as the distance vector is lexicographic positive swap
+		     the dependence direction.  */
+		  else
+		    {
+		      if (DDR_REVERSED_P (ddr))
+			this_dir = -this_dir;
+		      this_dir = -this_dir;
+		    }
 		  /* When then dependence distance of the innermost common
 		     loop of the DRs is zero we have a conflict.  */
 		  auto l1 = gimple_bb (DR_STMT (dr1))->loop_father;
 		  auto l2 = gimple_bb (DR_STMT (dr2))->loop_father;
 		  int idx = index_in_loop_nest (find_common_loop (l1, l2)->num,
 						DDR_LOOP_NEST (ddr));
-		  if (DDR_DIST_VECT (ddr, 0)[idx] == 0)
+		  if (DDR_DIST_VECT (ddr, 0)[idx] == 0
+		      /* Unless it is the outermost loop which is the one
+			 we eventually distribute.  */
+		      && idx != 0)
 		    this_dir = 2;
 		}
 	    }