[gcc r9-9954] tree-optimization/103237 - avoid vectorizing unhandled double reductions

[Richard Biener]

https://gcc.gnu.org/g:68c80396906ab10051a69a2181e114c61dd4ee8a

commit r9-9954-g68c80396906ab10051a69a2181e114c61dd4ee8a
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Nov 15 11:37:56 2021 +0100

    tree-optimization/103237 - avoid vectorizing unhandled double reductions
    
    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.
    
    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.
    
    (cherry picked from commit 220bd61874cf114667b44f9ded76ed0639eb278b)

Diff:
---
 gcc/testsuite/gcc.dg/torture/pr103237.c | 24 ++++++++++++++++++++++++
 gcc/tree-vect-loop.c                    | 11 +++++++++++
 2 files changed, 35 insertions(+)

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 21c53cbb4a7..795ab61a63b 100644
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -2925,6 +2925,17 @@ vect_is_simple_reduction (loop_vec_info loop_info, stmt_vec_info phi_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.  */