8359121: C2: Region added by vectorizedMismatch intrinsic can survive as a dead node after IGVN

src/hotspot/share/opto/library_call.cpp

@@ -6580,6 +6580,10 @@ bool LibraryCallKit::inline_vectorizedMismatch() {
   memory_phi = _gvn.transform(memory_phi);
   result_phi = _gvn.transform(result_phi);
 
+  record_for_igvn(exit_block);
+  record_for_igvn(memory_phi);
+  record_for_igvn(result_phi);
+
   set_control(exit_block);
   set_all_memory(memory_phi);
   set_result(result_phi);

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

@@ -0,0 +1,54 @@
+/*
+ * 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 8359121
+ * @summary Region node introduced by ArraysSupport.mismatch must be disconnected,
+ *          and not just put aside as dead: when simplifying
+ *          Proj -> Region -> If -> ...
+ *          into
+ *          Proj -> If -> ...
+ *               -> Region
+ *          the dead Region node must be removed from Proj's outputs.
+ * @modules java.base/jdk.internal.util
+ * @run main/othervm -Xcomp
+ *                   -XX:CompileCommand=compileonly,compiler.igvn.RemoveDeadRegionFromVectorizedMismatchIntrinsic::test
+ *                   compiler.igvn.RemoveDeadRegionFromVectorizedMismatchIntrinsic
+ * @run main compiler.igvn.RemoveDeadRegionFromVectorizedMismatchIntrinsic
+ */
+package compiler.igvn;
+
+import jdk.internal.util.ArraysSupport;
+
+public class RemoveDeadRegionFromVectorizedMismatchIntrinsic {
+    public static void main(String[] args) {
+        ArraysSupport.mismatch(new int[0], new int[0], 0);  // loads ArraysSupport
+        test(new byte[0], new byte[0]);
+    }
+
+    public static int test(byte[] a, byte[] b) {
+        int i = ArraysSupport.vectorizedMismatch(a, 0, b, 0, 0, 0);
+        return i >= 0 ? i : 0;
+    }
+}