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 --git a/gcc/testsuite/gcc.dg/torture/pr103237.c b/gcc/testsuite/gcc.dg/torture/pr103237.c
new file mode 100644 (file)
index 0000000..f2399f9
--- /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 d9e7d04d9eacdf2831c32d884cdf5204b374664b..3e3f0c00457639e761e869d4c49fe306b4856617 100644 (file)
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -3236,6 +3236,17 @@ 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.  */