8347718: Unexpected NullPointerException in C2 compiled code due to ReduceAllocationMerges

Reviewed-by: kvn, thartmann

Author: Cesar Soares Lucas

src/hotspot/share/opto/escape.cpp

@@ -981,18 +981,24 @@ void ConnectionGraph::reduce_phi_on_cmp(Node* cmp) {
 
   Node* other = cmp->in(1)->is_Con() ? cmp->in(1) : cmp->in(2);
   Node* zero = _igvn->intcon(0);
+  Node* one = _igvn->intcon(1);
   BoolTest::mask mask = cmp->unique_out()->as_Bool()->_test._test;
 
   // This Phi will merge the result of the Cmps split through the Phi
-  Node* res_phi  = _igvn->transform(PhiNode::make(ophi->in(0), zero, TypeInt::INT));
+  Node* res_phi = PhiNode::make(ophi->in(0), zero, TypeInt::INT);
 
   for (uint i=1; i<ophi->req(); i++) {
     Node* ophi_input = ophi->in(i);
     Node* res_phi_input = nullptr;
 
     const TypeInt* tcmp = optimize_ptr_compare(ophi_input, other);
     if (tcmp->singleton()) {
-      res_phi_input = _igvn->makecon(tcmp);
+      if ((mask == BoolTest::mask::eq && tcmp == TypeInt::CC_EQ) ||
+          (mask == BoolTest::mask::ne && tcmp == TypeInt::CC_GT)) {
+        res_phi_input = one;
+      } else {
+        res_phi_input = zero;
+      }
     } else {
       Node* ncmp = _igvn->transform(cmp->clone());
       ncmp->set_req(1, ophi_input);
@@ -1004,7 +1010,8 @@ void ConnectionGraph::reduce_phi_on_cmp(Node* cmp) {
     res_phi->set_req(i, res_phi_input);
   }
 
-  Node* new_cmp = _igvn->transform(new CmpINode(res_phi, zero));
+  // This CMP always compares whether the output of "res_phi" is TRUE as far as the "mask".
+  Node* new_cmp = _igvn->transform(new CmpINode(_igvn->transform(res_phi), (mask == BoolTest::mask::eq) ? one : zero));
   _igvn->replace_node(cmp, new_cmp);
 }
 

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

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2025, 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 8347718
+ * @summary Check that Reduce Allocation Merges correctly handle "NE" pointer comparisons.
+ * @requires vm.flagless & vm.compiler2.enabled & vm.opt.final.EliminateAllocations
+ * @run main/othervm -XX:CompileCommand=compileonly,*TestReduceAllocationAndPointerComparisons*::*
+ *                   -XX:CompileCommand=dontinline,*TestReduceAllocationAndPointerComparisons*::*
+ *                   -XX:-TieredCompilation -Xcomp -server
+ *                   compiler.c2.TestReduceAllocationAndPointerComparisons
+ * @run main compiler.c2.TestReduceAllocationAndPointerComparisons
+ */
+
+package compiler.c2;
+
+public class TestReduceAllocationAndPointerComparisons {
+    public static void main(String[] args) {
+        for (int i=0; i<50000; i++) {
+            if (test(true) == false) {
+                throw new RuntimeException("Unexpected result.");
+            }
+        }
+    }
+
+    public static boolean test(boolean b) {
+        MyClass obj = new MyClass();
+
+        for (int i = 0; i < 100_000; ++i) { }
+
+        obj = b ? obj : new MyClass();
+        obj = b ? obj : new MyClass();
+
+        if (obj == null) {
+            return false;
+        }
+
+        return b;
+    }
+
+    static class MyClass {
+    }
+}