8371464: C2: assert(no_dead_loop) failed: dead loop detected

Reviewed-by: chagedorn, dfenacci

src/hotspot/share/opto/cfgnode.cpp

@@ -326,7 +326,7 @@ bool RegionNode::is_unreachable_region(const PhaseGVN* phase) {
 
   // First, cut the simple case of fallthrough region when NONE of
   // region's phis references itself directly or through a data node.
-  if (is_possible_unsafe_loop(phase)) {
+  if (is_possible_unsafe_loop()) {
     // If we have a possible unsafe loop, check if the region node is actually unreachable from root.
     if (is_unreachable_from_root(phase)) {
       _is_unreachable_region = true;
@@ -336,7 +336,7 @@ bool RegionNode::is_unreachable_region(const PhaseGVN* phase) {
   return false;
 }
 
-bool RegionNode::is_possible_unsafe_loop(const PhaseGVN* phase) const {
+bool RegionNode::is_possible_unsafe_loop() const {
   uint max = outcnt();
   uint i;
   for (i = 0; i < max; i++) {
@@ -634,8 +634,8 @@ Node *RegionNode::Ideal(PhaseGVN *phase, bool can_reshape) {
     }
   } else if (can_reshape && cnt == 1) {
     // Is it dead loop?
-    // If it is LoopNopde it had 2 (+1 itself) inputs and
-    // one of them was cut. The loop is dead if it was EntryContol.
+    // If it is LoopNode it had 2 (+1 itself) inputs and
+    // one of them was cut. The loop is dead if it was EntryControl.
     // Loop node may have only one input because entry path
     // is removed in PhaseIdealLoop::Dominators().
     assert(!this->is_Loop() || cnt_orig <= 3, "Loop node should have 3 or less inputs");
@@ -1392,7 +1392,7 @@ bool PhiNode::try_clean_memory_phi(PhaseIterGVN* igvn) {
   }
   assert(is_diamond_phi() > 0, "sanity");
   assert(req() == 3, "same as region");
-  const Node* region = in(0);
+  RegionNode* region = in(0)->as_Region();
   for (uint i = 1; i < 3; i++) {
     Node* phi_input = in(i);
     if (phi_input != nullptr && phi_input->is_MergeMem() && region->in(i)->outcnt() == 1) {
@@ -1400,8 +1400,9 @@ bool PhiNode::try_clean_memory_phi(PhaseIterGVN* igvn) {
       MergeMemNode* merge_mem = phi_input->as_MergeMem();
       uint j = 3 - i;
       Node* other_phi_input = in(j);
-      if (other_phi_input != nullptr && other_phi_input == merge_mem->base_memory()) {
+      if (other_phi_input != nullptr && other_phi_input == merge_mem->base_memory() && !is_data_loop(region, phi_input, igvn)) {
         // merge_mem is a successor memory to other_phi_input, and is not pinned inside the diamond, so push it out.
+        // Only proceed if the transformation doesn't create a data loop
         // This will allow the diamond to collapse completely if there are no other phis left.
         igvn->replace_node(this, merge_mem);
         return true;

src/hotspot/share/opto/cfgnode.hpp

@@ -84,7 +84,7 @@ private:
   bool _is_unreachable_region;
   LoopStatus _loop_status;
 
-  bool is_possible_unsafe_loop(const PhaseGVN* phase) const;
+  bool is_possible_unsafe_loop() const;
   bool is_unreachable_from_root(const PhaseGVN* phase) const;
 public:
   // Node layout (parallels PhiNode):

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

@@ -0,0 +1,83 @@
+/*
+ * 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 8371464
+ * @summary C2: assert(no_dead_loop) failed: dead loop detected
+ * @run main/othervm -Xcomp -XX:CompileOnly=TestDeadLoopAtMergeMem::test TestDeadLoopAtMergeMem
+ * @run main TestDeadLoopAtMergeMem
+ */
+
+public class TestDeadLoopAtMergeMem {
+    static final int N = 400;
+    static long instanceCount;
+    boolean bFld;
+    float fArrFld[];
+    static int iArrFld[] = new int[N];
+    long vMeth_check_sum;
+
+    public static void main(String[] strArr) {
+        TestDeadLoopAtMergeMem r = new TestDeadLoopAtMergeMem();
+        for (int i = 0; i < 1000; i++) {
+            r.test((short) 0, instanceCount);
+        }
+    }
+
+    void test(short s, long l) {
+        int i11 = 6, i12, i13 = 6, i14 = 2;
+        byte byArr2[] = new byte[N];
+        init(byArr2, (byte) 4);
+        helper(66.118169, i11);
+        for (i12 = 3; i12 < 23; i12++) {
+            if (bFld) {
+                instanceCount = 5;
+            } else if (bFld) {
+                fArrFld[i12] = s;
+                do {
+                    try {
+                        i11 = i13 / i12 % i12;
+                    } catch (ArithmeticException a_e) {
+                    }
+                } while (i14 < 8);
+            }
+        }
+        for (int i15 : iArrFld) {
+            try {
+                i11 = 1 / i15;
+            } catch (ArithmeticException a_e) {
+            }
+        }
+        vMeth_check_sum += i11;
+    }
+
+    void helper(double d, int i) {
+        int i1[] = new int[N];
+    }
+
+    public static void init(byte[] a, byte seed) {
+        for (int j = 0; j < a.length; j++) {
+            a[j] = (byte) ((j % 2 == 0) ? seed + j : seed - j);
+        }
+    }
+}