8286177: C2: "failed: non-reduction loop contains reduction nodes" assert failure

Reviewed-by: chagedorn, kvn

src/hotspot/share/opto/superword.cpp

@@ -110,8 +110,6 @@ bool SuperWord::transform_loop(IdealLoopTree* lpt, bool do_optimization) {
     return false; // skip malformed counted loop
   }
 
-  assert(!lpt->has_reduction_nodes() || cl->is_reduction_loop(),
-         "non-reduction loop contains reduction nodes");
   if (cl->is_rce_post_loop() && cl->is_reduction_loop()) {
     // Post loop vectorization doesn't support reductions
     return false;
@@ -2399,6 +2397,11 @@ bool SuperWord::output() {
     return false;
   }
 
+  // Check that the loop to be vectorized does not have inconsistent reduction
+  // information, which would likely lead to a miscompilation.
+  assert(!lpt()->has_reduction_nodes() || cl->is_reduction_loop(),
+         "non-reduction loop contains reduction nodes");
+
 #ifndef PRODUCT
   if (TraceLoopOpts) {
     tty->print("SuperWord::output    ");

test/hotspot/jtreg/compiler/loopopts/superword/TestHoistedReductionNode.java

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/**
+ * @test
+ * @bug 8286177
+ * @summary Test that inconsistent reduction node-loop state does not trigger
+ *          assertion failures when the inconsistency does not lead to a
+ *          miscompilation.
+ * @run main/othervm -Xbatch compiler.loopopts.superword.TestHoistedReductionNode
+ */
+package compiler.loopopts.superword;
+
+public class TestHoistedReductionNode {
+
+    static boolean b = true;
+
+    static int test() {
+        int acc = 0;
+        int i = 0;
+        do {
+            int j = 0;
+            do {
+                if (b) {
+                    acc += j;
+                }
+                j++;
+            } while (j < 5);
+            i++;
+        } while (i < 100);
+        return acc;
+
+    }
+
+    public static void main(String[] args) {
+        for (int i = 0; i < 10_000; i++) {
+            test();
+        }
+    }
+}