8255400: Shenandoah: C2 failures after JDK-8255000

Reviewed-by: rkennke

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

@@ -2314,12 +2314,12 @@ void MemoryGraphFixer::collect_memory_nodes() {
                   Node* m = _memory_nodes[c->in(k)->_idx];
                   assert(m != NULL, "expect memory state");
                   if (u->in(k) != m) {
-                    phi = NULL;
+                    phi = NodeSentinel;
                   }
                 }
               }
             }
-            if (phi == NULL) {
+            if (phi == NodeSentinel) {
               phi = new PhiNode(c, Type::MEMORY, _phase->C->get_adr_type(_alias));
               for (uint k = 1; k < c->req(); k++) {
                 Node* m = _memory_nodes[c->in(k)->_idx];
@@ -2328,8 +2328,11 @@ void MemoryGraphFixer::collect_memory_nodes() {
               }
             }
           }
-          assert(phi != NULL, "");
-          regions.map(c->_idx, phi);
+          if (phi != NULL) {
+            regions.map(c->_idx, phi);
+          } else {
+            assert(c->unique_ctrl_out()->Opcode() == Op_Halt, "expected memory state");
+          }
         }
         Node* current_region = regions[c->_idx];
         if (current_region != prev_region) {
@@ -2340,7 +2343,7 @@ void MemoryGraphFixer::collect_memory_nodes() {
         }
       } else if (prev_mem == NULL || prev_mem->is_Phi() || ctrl_or_self(prev_mem) != c) {
         Node* m = _memory_nodes[_phase->idom(c)->_idx];
-        assert(m != NULL, "expect memory state");
+        assert(m != NULL || c->Opcode() == Op_Halt, "expect memory state");
         if (m != prev_mem) {
           _memory_nodes.map(c->_idx, m);
           progress = true;
@@ -2364,7 +2367,8 @@ void MemoryGraphFixer::collect_memory_nodes() {
     Node* c = rpo_list.at(i);
     if (c->is_Region() && (_include_lsm || !c->is_OuterStripMinedLoop())) {
       Node* n = regions[c->_idx];
-      if (n->is_Phi() && n->_idx >= last && n->in(0) == c) {
+      assert(n != NULL || c->unique_ctrl_out()->Opcode() == Op_Halt, "expected memory state");
+      if (n != NULL && n->is_Phi() && n->_idx >= last && n->in(0) == c) {
         _phase->register_new_node(n, c);
       }
     }
@@ -2373,10 +2377,12 @@ void MemoryGraphFixer::collect_memory_nodes() {
     Node* c = rpo_list.at(i);
     if (c->is_Region() && (_include_lsm || !c->is_OuterStripMinedLoop())) {
       Node* n = regions[c->_idx];
+      assert(n != NULL || c->unique_ctrl_out()->Opcode() == Op_Halt, "expected memory state");
       for (DUIterator_Fast imax, i = c->fast_outs(imax); i < imax; i++) {
         Node* u = c->fast_out(i);
         if (u->is_Phi() && u->bottom_type() == Type::MEMORY &&
             u != n) {
+          assert(c->unique_ctrl_out()->Opcode() != Op_Halt, "expected memory state");
           if (u->adr_type() == TypePtr::BOTTOM) {
             fix_memory_uses(u, n, n, c);
           } else if (_phase->C->get_alias_index(u->adr_type()) == _alias) {

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

@@ -0,0 +1,75 @@
+/*
+ * 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 8255400
+ * @summary C2 failures after JDK-8255000
+ * @requires vm.gc.Shenandoah
+ * @modules java.base/jdk.internal.misc:+open
+ *
+ * @run main/othervm -XX:-UseOnStackReplacement -XX:-BackgroundCompilation -XX:-TieredCompilation -XX:+UseShenandoahGC TestBarrierExpansionDeadMemPhi
+ *
+ *
+ */
+
+import jdk.internal.misc.Unsafe;
+import java.util.Arrays;
+import java.lang.reflect.Field;
+
+public class TestBarrierExpansionDeadMemPhi {
+
+    static final jdk.internal.misc.Unsafe UNSAFE = Unsafe.getUnsafe();
+
+    static final long F_OFFSET;
+
+    static class A {
+        int f;
+    }
+
+    static {
+        try {
+            Field fField = A.class.getDeclaredField("f");
+            F_OFFSET = UNSAFE.objectFieldOffset(fField);
+        } catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    static int test(Object[] array) {
+        int f = 0;
+        for (int i = 0; i < 100; i++) {
+            f += UNSAFE.getInt(array[i], F_OFFSET);
+        }
+        return f;
+    }
+
+    static public void main(String[] args) {
+        Object[] array = new Object[100];
+        Arrays.fill(array, new A());
+
+        for (int i = 0; i < 20000; i++) {
+            test(array);
+        }
+    }
+}