8279837: C2: assert(is_Loop()) failed: invalid node class: Region

Backport-of: 65eb066b630b892f22492457e0f86165e5ef4765

src/hotspot/share/opto/loopTransform.cpp

@@ -3200,6 +3200,11 @@ bool IdealLoopTree::do_one_iteration_loop(PhaseIdealLoop *phase) {
 //=============================================================================
 //------------------------------iteration_split_impl---------------------------
 bool IdealLoopTree::iteration_split_impl(PhaseIdealLoop *phase, Node_List &old_new) {
+  if (!_head->is_Loop()) {
+    // Head could be a region with a NeverBranch that was added in beautify loops but the region was not
+    // yet transformed into a LoopNode. Bail out and wait until beautify loops turns it into a Loop node.
+    return false;
+  }
   // Compute loop trip count if possible.
   compute_trip_count(phase);
 

test/hotspot/jtreg/compiler/loopopts/TestIterationSplitWithRegionHead.java

@@ -0,0 +1,82 @@
+/*
+ * 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
+ * @requires vm.compiler2.enabled
+ * @bug 8279837
+ * @summary Tests infinite loop with region head in iteration split.
+ * @run main/othervm -Xcomp -XX:-TieredCompilation
+ *      -XX:CompileCommand=compileonly,compiler.loopopts.TestIterationSplitWithRegionHead::test
+ *      -XX:CompileCommand=dontinline,compiler.loopopts.TestIterationSplitWithRegionHead::*
+ *      compiler.loopopts.TestIterationSplitWithRegionHead
+ */
+
+package compiler.loopopts;
+
+public class TestIterationSplitWithRegionHead {
+
+    static boolean flagFalse = false;
+
+    public static void main(String[] args) {
+        test();
+    }
+
+    public static void test() {
+        // 1) The loop tree is built. We find that nested loop N2 is an infinite loop and add a NeverBranch
+        // to the inner loop to make it reachable. But the current loop tree does not have N2, yet. The
+        // resulting loop tree is:
+        //
+        //   Loop: N0/N0  has_call has_sfpt
+        //     Loop: N77/N121  has_call // N1 outer
+        //       Loop: N77/N111  has_call sfpts={ 111 97 } // N1 inner
+        //
+        // 2) beautify_loops() finds that the outer loop head of N1 is shared and thus adds a new region
+        // in merge_many_backedges(). As a result, the loop tree is built again. This time, the NeverBranch
+        // in the inner loop of N2 allows that a loop tree can be built for it:
+        //
+        //   Loop: N0/N0  has_call has_sfpt
+        //     Loop: N216/N213  limit_check profile_predicated predicated has_call sfpts={ 111 97 } // N1 shared loop head
+        //     Loop: N196/N201  sfpts={ 201 } // N2 inner loop now discovered with the new NeverBranch
+        //
+        // However, a LoopNode is only added by beautify_loops() which won't be called until the next iteration of loop opts.
+        // This means that we have a Region node (N196) as head in the loop tree which cannot be handled by iteration_split_impl()
+        // resulting in an assertion failure.
+
+        // Nested loop N1
+        while (flagFalse) {
+            while (dontInlineFalse()) {
+            }
+        }
+        dontInlineFalse();
+
+        // Nested loop N2
+        while (flagFalse) {
+            while (true) ;  // Detected as infinite inner loop by C2 -> NeverBranch added
+        }
+    }
+
+    public static boolean dontInlineFalse() {
+        return false;
+    }
+}