[PATCH] tree-optimization/103237 - avoid vectorizing unhandled double reductions

[PATCH] tree-optimization/103237 - avoid vectorizing unhandled double reductions

[Richard Biener]

Double reductions which have multiple LC PHIs in the inner loop
are not handled correctly during transformation since those PHIs
are not properly classified as reduction.  The following disables
vectorizing them.

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

2021-11-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103237
	* tree-vect-loop.c (vect_is_simple_reduction): Fail for
	double reductions with multiple inner loop LC PHI nodes.

	* gcc.dg/torture/pr103237.c: New testcase.
---
 gcc/testsuite/gcc.dg/torture/pr103237.c | 24 ++++++++++++++++++++++++
 gcc/tree-vect-loop.c                    | 11 +++++++++++
 2 files changed, 35 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/torture/pr103237.c

diff --git a/gcc/testsuite/gcc.dg/torture/pr103237.c b/gcc/testsuite/gcc.dg/torture/pr103237.c
new file mode 100644
index 00000000000..f2399f9586e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr103237.c
@@ -0,0 +1,24 @@
+/* { dg-do run } */
+/* { dg-additional-options "-ftree-vectorize" } */
+
+int g1;
+unsigned int g2 = -1U;
+static void __attribute__((noipa))
+func_1()
+{
+  int *l_1 = &g1;
+  for (int g3a = 0; g3a != 4; g3a++)
+    for (int l_2 = 0; l_2 <= 3; l_2++)
+      {
+        unsigned int *l_3 = &g2;
+        *l_1 = *l_3 ^= 1;
+      }
+}
+int
+main()
+{
+  func_1();
+  if (g1 != -1)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
index 1cd5dbcb6f7..73efdb96bad 100644
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -3567,6 +3567,17 @@ vect_is_simple_reduction (loop_vec_info loop_info, stmt_vec_info phi_info,
       return def_stmt_info;
     }
 
+  /* When the inner loop of a double reduction ends up with more than
+     one loop-closed PHI we have failed to classify alternate such
+     PHIs as double reduction, leading to wrong code.  See PR103237.  */
+  if (inner_loop_of_double_reduc && lcphis.length () != 1)
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+			 "unhandle double reduction\n");
+      return NULL;
+    }
+
   /* If this isn't a nested cycle or if the nested cycle reduction value
      is used ouside of the inner loop we cannot handle uses of the reduction
      value.  */
-- 
2.31.1