8373524: C2: no reachable node should have no use

Reviewed-by: chagedorn, mhaessig

Author: rwestrel

src/hotspot/share/opto/cfgnode.cpp

@@ -2642,7 +2642,7 @@ Node *PhiNode::Ideal(PhaseGVN *phase, bool can_reshape) {
               }
               Node* phi = mms.memory();
               assert(made_new_phi || phi->in(i) == n, "replace the i-th merge by a slice");
-              phi->set_req(i, mms.memory2());
+              phi->set_req_X(i, mms.memory2(), phase);
             }
           }
         }
@@ -2651,7 +2651,9 @@ Node *PhiNode::Ideal(PhaseGVN *phase, bool can_reshape) {
           for (MergeMemStream mms(result); mms.next_non_empty(); ) {
             Node* phi = mms.memory();
             for (uint i = 1; i < req(); ++i) {
-              if (phi->in(i) == this)  phi->set_req(i, phi);
+              if (phi->in(i) == this) {
+                phi->set_req_X(i, phi, phase);
+              }
             }
           }
         }
@@ -2673,7 +2675,7 @@ Node *PhiNode::Ideal(PhaseGVN *phase, bool can_reshape) {
       Node *ii = in(i);
       Node *new_in = MemNode::optimize_memory_chain(ii, at, nullptr, phase);
       if (ii != new_in ) {
-        set_req(i, new_in);
+        set_req_X(i, new_in, phase);
         progress = this;
       }
     }

test/hotspot/jtreg/compiler/igvn/TestNodeWithNoUseAfterPhiIdeal.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright (c) 2025 IBM Corporation. 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 8373524
+ * @summary C2: no reachable node should have no use
+ * @run main/othervm -Xbatch ${test.main.class}
+ * @run main ${test.main.class}
+ */
+
+package compiler.igvn;
+
+public class TestNodeWithNoUseAfterPhiIdeal {
+    volatile boolean _mutatorToggle;
+
+    boolean _mutatorFlip() {
+        synchronized (TestNodeWithNoUseAfterPhiIdeal.class) {
+            _mutatorToggle = new MyBoolean(!_mutatorToggle).v;
+            return _mutatorToggle;
+        }
+    }
+
+    class MyBoolean {
+        boolean v;
+        MyBoolean(boolean v) {
+            int N = 32;
+            for (int i = 0; i < N; i++) {
+                this.v = v;
+            }
+        }
+    }
+
+    int[] arr;
+    void test() {
+        int limit = 2;
+        boolean flag1 = _mutatorFlip();
+        for (; limit < 4; limit *= 2) {
+            if (flag1) {
+                break;
+            }
+        }
+        int zero = 34;
+        for (int peel = 2; peel < limit; peel++) {
+            synchronized (TestNodeWithNoUseAfterPhiIdeal.class) {
+                zero = 0;
+            }
+        }
+        if (zero == 0) {
+            arr = new int[8];
+        } else {
+            int M = 4;
+            for (int i = 0; i < M; i++) {
+                boolean flag2 = _mutatorFlip();
+                boolean flag3 = _mutatorFlip();
+                if (flag2) {
+                    break;
+                }
+            }
+        }
+    }
+
+    public static void main(String[] args) {
+        TestNodeWithNoUseAfterPhiIdeal t = new TestNodeWithNoUseAfterPhiIdeal();
+        for (int i = 0; i < 10_000; i++) {
+            t.test();
+        }
+    }
+}