8247824: CTW: C2 (Shenandoah) compilation fails with SEGV in SBC2Support::pin_and_expand

Reviewed-by: rkennke, thartmann

src/hotspot/share/gc/shenandoah/c2/shenandoahSupport.cpp

@@ -1083,11 +1083,12 @@ void ShenandoahBarrierC2Support::pin_and_expand(PhaseIdealLoop* phase) {
     Node* barrier = state->enqueue_barrier(i);
     Node* ctrl = phase->get_ctrl(barrier);
     IdealLoopTree* loop = phase->get_loop(ctrl);
-    if (loop->_head->is_OuterStripMinedLoop()) {
+    Node* head = loop->head();
+    if (head->is_OuterStripMinedLoop()) {
       // Expanding a barrier here will break loop strip mining
       // verification. Transform the loop so the loop nest doesn't
       // appear as strip mined.
-      OuterStripMinedLoopNode* outer = loop->_head->as_OuterStripMinedLoop();
+      OuterStripMinedLoopNode* outer = head->as_OuterStripMinedLoop();
       hide_strip_mined_loop(outer, outer->unique_ctrl_out()->as_CountedLoop(), phase);
     }
   }
@@ -1289,11 +1290,12 @@ void ShenandoahBarrierC2Support::pin_and_expand(PhaseIdealLoop* phase) {
     ShenandoahLoadReferenceBarrierNode* lrb = state->load_reference_barrier(i);
     Node* ctrl = phase->get_ctrl(lrb);
     IdealLoopTree* loop = phase->get_loop(ctrl);
-    if (loop->_head->is_OuterStripMinedLoop()) {
+    Node* head = loop->head();
+    if (head->is_OuterStripMinedLoop()) {
       // Expanding a barrier here will break loop strip mining
       // verification. Transform the loop so the loop nest doesn't
       // appear as strip mined.
-      OuterStripMinedLoopNode* outer = loop->_head->as_OuterStripMinedLoop();
+      OuterStripMinedLoopNode* outer = head->as_OuterStripMinedLoop();
       hide_strip_mined_loop(outer, outer->unique_ctrl_out()->as_CountedLoop(), phase);
     }
   }

src/hotspot/share/opto/loopnode.hpp

@@ -485,6 +485,7 @@ public:
   Node *_head;                  // Head of loop
   Node *_tail;                  // Tail of loop
   inline Node *tail();          // Handle lazy update of _tail field
+  inline Node *head();          // Handle lazy update of _head field
   PhaseIdealLoop* _phase;
   int _local_loop_unroll_limit;
   int _local_loop_unroll_factor;
@@ -1579,6 +1580,13 @@ inline Node* IdealLoopTree::tail() {
   return _tail;
 }
 
+inline Node* IdealLoopTree::head() {
+  // Handle lazy update of _head field.
+  if (_head->in(0) == NULL) {
+    _head = _phase->get_ctrl(_head);
+  }
+  return _head;
+}
 
 // Iterate over the loop tree using a preorder, left-to-right traversal.
 //

test/hotspot/jtreg/gc/shenandoah/compiler/TestShenandoahLRBInOuterStripMinedLoop.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2020, Red Hat, Inc. 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 8247824
+ * @summary CTW: C2 (Shenandoah) compilation fails with SEGV in SBC2Support::pin_and_expand
+ * @requires vm.flavor == "server"
+ * @requires vm.gc.Shenandoah & !vm.graal.enabled
+ *
+ * @run main/othervm -XX:-BackgroundCompilation -XX:+UseShenandoahGC -XX:LoopMaxUnroll=0 TestShenandoahLRBInOuterStripMinedLoop
+ *
+ */
+
+import java.util.Arrays;
+
+public class TestShenandoahLRBInOuterStripMinedLoop {
+    public static void main(String[] args) {
+        A[] array = new A[4000];
+        Arrays.fill(array, new A());
+        for (int i = 0; i < 20_0000; i++) {
+            test(array);
+        }
+    }
+
+    private static int test(A[] array) {
+        A a = null;
+        int v = 1;
+        A b = null;
+        for (int i = 0; i < 2000; i++) {
+            a = array[i];
+            b = array[2*i];
+            v *= 2;
+        }
+        return a.f + b.f + v;
+    }
+
+    private static class A {
+        public int f;
+    }
+}