tree-optimization/107451 - SLP load vectorization issue

When vectorizing SLP loads with permutations we can access excess
elements when the load vector type is bigger than the group size
and the vectorization factor covers less groups than necessary
to fill it.  Since we know the code will only access up to
group_size * VF elements in the unpermuted vector we can simply
fill the rest of the vector with whatever we want.  For simplicity
this patch chooses to repeat the last group.

	PR tree-optimization/107451
	* tree-vect-stmts.cc (vectorizable_load): Avoid loading
	SLP group members from group numbers in excess of the
	vectorization factor.

	* gcc.dg/torture/pr107451.c: New testcase.

2 files changed, 41 insertions, 6 deletions

diff --git a/gcc/testsuite/gcc.dg/torture/pr107451.c b/gcc/testsuite/gcc.dg/torture/pr107451.c
new file mode 100644
index 0000000..a17574c
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr107451.c
@@ -0,0 +1,27 @@
+/* { dg-do run } */
+/* { dg-additional-options "-ftree-vectorize -fno-vect-cost-model" } */
+/* { dg-additional-options "-mavx2" { target avx2_runtime } } */
+
+double getdot(int n, const double *x, int inc_x, const double *y)
+{
+  int i, ix = 0;
+  double dot[4] = { 0.0, 0.0, 0.0, 0.0 } ;
+
+  for(i = 0; i < n; i++) {
+      dot[0] += x[ix]   * y[ix]   ;
+      dot[1] += x[ix+1] * y[ix+1] ;
+      dot[2] += x[ix]   * y[ix+1] ;
+      dot[3] += x[ix+1] * y[ix]   ;
+      ix += inc_x ;
+  }
+
+  return dot[0] + dot[1] + dot[2] + dot[3];
+}
+
+int main()
+{
+  double x[2] = {0, 0}, y[2] = {0, 0};
+  if (getdot(1, x, 4096*4096, y) != 0.)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index 5485da5..8f8deaf 100644
--- a/gcc/tree-vect-stmts.cc
+++ b/gcc/tree-vect-stmts.cc
@@ -9235,6 +9235,7 @@ vectorizable_load (vec_info *vinfo,
       unsigned int group_el = 0;
       unsigned HOST_WIDE_INT
 	elsz = tree_to_uhwi (TYPE_SIZE_UNIT (TREE_TYPE (vectype)));
+      unsigned int n_groups = 0;
       for (j = 0; j < ncopies; j++)
 	{
 	  if (nloads > 1)
@@ -9256,12 +9257,19 @@ vectorizable_load (vec_info *vinfo,
 	      if (! slp
 		  || group_el == group_size)
 		{
-		  tree newoff = copy_ssa_name (running_off);
-		  gimple *incr = gimple_build_assign (newoff, POINTER_PLUS_EXPR,
-						      running_off, stride_step);
-		  vect_finish_stmt_generation (vinfo, stmt_info, incr, gsi);
-
-		  running_off = newoff;
+		  n_groups++;
+		  /* When doing SLP make sure to not load elements from
+		     the next vector iteration, those will not be accessed
+		     so just use the last element again.  See PR107451.  */
+		  if (!slp || known_lt (n_groups, vf))
+		    {
+		      tree newoff = copy_ssa_name (running_off);
+		      gimple *incr
+			= gimple_build_assign (newoff, POINTER_PLUS_EXPR,
+					       running_off, stride_step);
+		      vect_finish_stmt_generation (vinfo, stmt_info, incr, gsi);
+		      running_off = newoff;
+		    }
 		  group_el = 0;
 		}
 	    }