[PATCH] tree-optimization/101445 - fix negative stride SLP vect with gaps

[Richard Biener]

The following fixes the IV adjustment for the gap in a negative
stride SLP vectorization.  The adjustment was in the wrong direction,
now fixes as in the patch.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

2021-07-14  Richard Biener  <rguent...@suse.de>

        PR tree-optimization/101445
        * tree-vect-stmts.c (vectorizable_load): Do the gap adjustment
        of the IV in the correct direction for negative stride
        accesses.

        * gcc.dg/vect/pr101445.c: New testcase.
---
 gcc/testsuite/gcc.dg/vect/pr101445.c | 28 ++++++++++++++++++++++++++++
 gcc/tree-vect-stmts.c                |  6 ++++++
 2 files changed, 34 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/vect/pr101445.c

diff --git a/gcc/testsuite/gcc.dg/vect/pr101445.c 
b/gcc/testsuite/gcc.dg/vect/pr101445.c
new file mode 100644
index 00000000000..f8a6e9ce6f7
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr101445.c
@@ -0,0 +1,28 @@
+/* { dg-do run } */
+
+#include "tree-vect.h"
+
+int a[35] = { 1, 1, 3 };
+
+void __attribute__((noipa))
+foo ()
+{
+  for (int b = 4; b >= 0; b--)
+    {
+      int tem = a[b * 5 + 3 + 1];
+      a[b * 5 + 3] = tem;
+      a[b * 5 + 2] = tem;
+      a[b * 5 + 1] = tem;
+      a[b * 5 + 0] = tem;
+    }
+}
+
+int main()
+{
+  check_vect ();
+  foo ();
+  for (int d = 0; d < 25; d++)
+    if (a[d] != 0)
+      __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
index e590f34d75d..3980f0918b2 100644
--- a/gcc/tree-vect-stmts.c
+++ b/gcc/tree-vect-stmts.c
@@ -9759,6 +9759,9 @@ vectorizable_load (vec_info *vinfo,
                  poly_wide_int bump_val
                    = (wi::to_wide (TYPE_SIZE_UNIT (elem_type))
                       * group_gap_adj);
+                 if (tree_int_cst_sgn
+                       (vect_dr_behavior (vinfo, dr_info)->step) == -1)
+                   bump_val = -bump_val;
                  tree bump = wide_int_to_tree (sizetype, bump_val);
                  dataref_ptr = bump_vector_ptr (vinfo, dataref_ptr, ptr_incr,
                                                 gsi, stmt_info, bump);
@@ -9772,6 +9775,9 @@ vectorizable_load (vec_info *vinfo,
              poly_wide_int bump_val
                = (wi::to_wide (TYPE_SIZE_UNIT (elem_type))
                   * group_gap_adj);
+             if (tree_int_cst_sgn
+                   (vect_dr_behavior (vinfo, dr_info)->step) == -1)
+               bump_val = -bump_val;
              tree bump = wide_int_to_tree (sizetype, bump_val);
              dataref_ptr = bump_vector_ptr (vinfo, dataref_ptr, ptr_incr, gsi,
                                             stmt_info, bump);
-- 
2.26.2