8249605: C2: assert(no_dead_loop) failed: dead loop detected

Fixed dead loop detection in PhiNode::Ideal() to additionally account for dead MergeMemNodes

Reviewed-by: kvn, thartmann

src/hotspot/share/opto/cfgnode.cpp

@@ -2229,23 +2229,30 @@ Node *PhiNode::Ideal(PhaseGVN *phase, bool can_reshape) {
       } else {
         // We know that at least one MergeMem->base_memory() == this
         // (saw_self == true). If all other inputs also references this phi
-        // (directly or through data nodes) - it is dead loop.
+        // (directly or through data nodes) - it is a dead loop.
         bool saw_safe_input = false;
         for (uint j = 1; j < req(); ++j) {
-          Node *n = in(j);
-          if (n->is_MergeMem() && n->as_MergeMem()->base_memory() == this)
-            continue;              // skip known cases
+          Node* n = in(j);
+          if (n->is_MergeMem()) {
+            MergeMemNode* mm = n->as_MergeMem();
+            if (mm->base_memory() == this || mm->base_memory() == mm->empty_memory()) {
+              // Skip this input if it references back to this phi or if the memory path is dead
+              continue;
+            }
+          }
           if (!is_unsafe_data_reference(n)) {
             saw_safe_input = true; // found safe input
             break;
           }
         }
-        if (!saw_safe_input)
-          return top; // all inputs reference back to this phi - dead loop
+        if (!saw_safe_input) {
+          // There is a dead loop: All inputs are either dead or reference back to this phi
+          return top;
+        }
 
         // Phi(...MergeMem(m0, m1:AT1, m2:AT2)...) into
         //     MergeMem(Phi(...m0...), Phi:AT1(...m1...), Phi:AT2(...m2...))
-        PhaseIterGVN *igvn = phase->is_IterGVN();
+        PhaseIterGVN* igvn = phase->is_IterGVN();
         Node* hook = new Node(1);
         PhiNode* new_base = (PhiNode*) clone();
         // Must eagerly register phis, since they participate in loops.

test/hotspot/jtreg/compiler/c2/TestDeadPhiMergeMemLoop.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright (c) 2020, 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 8249605
+ * @summary A dead memory loop is detected when replacing an actually dead memory phi node in PhiNode::Ideal()
+ *          by a dead MergeMemNode which builds a cycle with one of its slices.
+ *
+ * @run main/othervm -Xcomp -XX:-TieredCompilation
+ *                   -XX:CompileCommand=compileonly,compiler.c2.TestDeadPhiMergeMemLoop::main
+ *                   -XX:CompileCommand=dontinline,compiler.c2.TestDeadPhiMergeMemLoop::dontInline
+ *                   compiler.c2.TestDeadPhiMergeMemLoop
+ */
+
+package compiler.c2;
+
+public class TestDeadPhiMergeMemLoop {
+
+    public static boolean bFld = false;
+    public static double dArrFld[] = new double[400];
+
+    public static void main(String[] strArr) {
+        int x = 1;
+        int i = 0;
+        int iArr[] = new int[400];
+        dontInline();
+
+        if (bFld) {
+            x += x;
+        } else if (bFld) {
+            float f = 1;
+            while (++f < 132) {
+                if (bFld) {
+                    dArrFld[5] = 3;
+                    for (i = (int)(f); i < 12; i++) {
+                    }
+                }
+            }
+        }
+    }
+
+    // Not inlined
+    public static void dontInline() {
+    }
+}