[gcc r13-8680] tree-optimization/114733 - neg induction fails for 1 element vectors

[Richard Biener <rguenth@gcc.gnu.org>]

https://gcc.gnu.org/g:b3f9f10e03c570074a517dcfe9df8d3eeddd6aca

commit r13-8680-gb3f9f10e03c570074a517dcfe9df8d3eeddd6aca
Author: Richard Biener <rguenther@suse.de>
Date:   Tue Apr 16 10:46:03 2024 +0200

    tree-optimization/114733 - neg induction fails for 1 element vectors
    
    The neg induction vectorization code isn't prepared to deal with
    single element vectors.
    
            PR tree-optimization/114733
            * tree-vect-loop.cc (vectorizable_nonlinear_induction): Reject
            neg induction vectorization of single element vectors.
    
            * gcc.dg/vect/pr114733.c: New testcase.
    
    (cherry picked from commit 45a41ace55d0ffb1097e374868242329788ec82a)

Diff:
---
 gcc/testsuite/gcc.dg/vect/pr114733.c | 20 ++++++++++++++++++++
 gcc/tree-vect-loop.cc                |  2 ++
 2 files changed, 22 insertions(+)

diff --git a/gcc/testsuite/gcc.dg/vect/pr114733.c b/gcc/testsuite/gcc.dg/vect/pr114733.c
new file mode 100644
index 00000000000..219cbf20469
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr114733.c
@@ -0,0 +1,20 @@
+/* { dg-additional-options "-O3" } */
+
+#include "tree-vect.h"
+
+long b = 1;
+signed char c;
+int d[25];
+
+int main()
+{
+  check_vect ();
+  for (signed char g = 0; g < 8; g += 1)
+    for (short h = 0; h < 25; h += 2) {
+      b *= -1;
+      c ^= d[h];
+    }
+  if (b != 1)
+    abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 31ced32126e..73caeb283e3 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -8944,6 +8944,8 @@ vectorizable_nonlinear_induction (loop_vec_info loop_vinfo,
   switch (induction_type)
     {
     case vect_step_op_neg:
+      if (maybe_eq (TYPE_VECTOR_SUBPARTS (vectype), 1u))
+	return false;
       if (TREE_CODE (init_expr) != INTEGER_CST
 	  && TREE_CODE (init_expr) != REAL_CST)
 	{