8271056: C2: "assert(no_dead_loop) failed: dead loop detected" due to cmoving identity

Reviewed-by: kvn, thartmann

src/hotspot/share/opto/cfgnode.cpp

@@ -1366,9 +1366,12 @@ Node* PhiNode::Identity(PhaseGVN* phase) {
   }
 
   int true_path = is_diamond_phi();
-  if (true_path != 0) {
+  // Delay CMove'ing identity if Ideal has not had the chance to handle unsafe cases, yet.
+  if (true_path != 0 && !(phase->is_IterGVN() && wait_for_region_igvn(phase))) {
     Node* id = is_cmove_id(phase, true_path);
-    if (id != NULL)  return id;
+    if (id != NULL) {
+      return id;
+    }
   }
 
   // Looking for phis with identical inputs.  If we find one that has

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

@@ -0,0 +1,82 @@
+/*
+ * Copyright (c) 2021, 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
+ * @key stress randomness
+ * @bug 8271056
+ * @summary A dead data loop is created when applying an unsafe case of Cmov'ing identity.
+ * @run main/othervm -Xcomp -XX:-TieredCompilation -XX:CompileCommand=compileonly,compiler.c2.TestDeadDataLoopCmoveIdentity::*
+ *                   -XX:+UnlockDiagnosticVMOptions -XX:+StressIGVN -XX:StressSeed=359948366 compiler.c2.TestDeadDataLoopCmoveIdentity
+ * @run main/othervm -Xcomp -XX:-TieredCompilation -XX:CompileCommand=compileonly,compiler.c2.TestDeadDataLoopCmoveIdentity::*
+ *                   -XX:+UnlockDiagnosticVMOptions -XX:+StressIGVN compiler.c2.TestDeadDataLoopCmoveIdentity
+ */
+
+package compiler.c2;
+
+public class TestDeadDataLoopCmoveIdentity {
+    static boolean bFld;
+
+    public static void main(String[] strArr) {
+        test();
+        test2();
+    }
+
+    static void test() {
+        int i33 = 51925, iArr3[] = new int[10];
+        if (bFld) {
+            ;
+        } else if (bFld) {
+            for (int i = 0; i < 100; i++) { }
+            do {
+                if (i33 != 0) {
+                }
+                int i34 = 1;
+                do {
+                    switch (0) {
+                        case 122: { }
+                    }
+                } while (i34 < 1);
+                i33 += i33 + 3;
+            } while (i33 < 5);
+        }
+    }
+
+    static void test2() {
+        int i33 = 51925, iArr3[] = new int[10];
+        if (bFld) {
+            ;
+        } else if (bFld) {
+            do {
+                if (i33 != 0) {
+                }
+                int i34 = 1;
+                do {
+                    switch (0) {
+                        case 122: {}
+                    }
+                } while (i34 < 1);
+            } while (++i33 < 5);
+        }
+    }
+}