8380060: C2: Wrong execution with COH and arraycopy

src/hotspot/share/opto/escape.cpp

@@ -4539,7 +4539,8 @@ void ConnectionGraph::split_unique_types(GrowableArray<Node *>  &alloc_worklist,
           const TypePtr* adr_type = proj->adr_type();
           const TypePtr* new_adr_type = tinst->add_offset(adr_type->offset());
           if (adr_type != new_adr_type && !init->already_has_narrow_mem_proj_with_adr_type(new_adr_type)) {
-            DEBUG_ONLY( uint alias_idx = _compile->get_alias_index(new_adr_type); )
+            // Do NOT remove the next line: ensure a new alias index is allocated for the instance type.
+            uint alias_idx = _compile->get_alias_index(new_adr_type);
             assert(_compile->get_general_index(alias_idx) == _compile->get_alias_index(adr_type), "new adr type should be narrowed down from existing adr type");
             NarrowMemProjNode* new_proj = new NarrowMemProjNode(init, new_adr_type);
             igvn->set_type(new_proj, new_proj->bottom_type());

test/hotspot/jtreg/compiler/arraycopy/TestBlockArrayCopyMisaligned.java

@@ -0,0 +1,104 @@
+/*
+ * Copyright (c) 2026, 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 8380060
+ * @summary C2 block arraycopy initial-word pick-off folds load to zero
+ *
+ * @run main/othervm -Xbatch -XX:+UseCompactObjectHeaders
+ *                   -XX:CompileCommand=compileonly,compiler.arraycopy.TestBlockArrayCopyMisaligned::test*
+ *                   compiler.arraycopy.TestBlockArrayCopyMisaligned
+ * @run main/othervm -Xbatch -XX:-UseCompactObjectHeaders
+ *                   -XX:CompileCommand=compileonly,compiler.arraycopy.TestBlockArrayCopyMisaligned::test*
+ *                   compiler.arraycopy.TestBlockArrayCopyMisaligned
+ */
+
+package compiler.arraycopy;
+
+import java.util.Arrays;
+
+public class TestBlockArrayCopyMisaligned {
+
+    // Byte array with enough data to exercise the block arraycopy path.
+    // The clone + substring pattern triggers a tightly-coupled allocation
+    // where the source is initialized via a raw arraycopy (clone) and the
+    // destination is a nozero allocation filled by a block arraycopy.
+    static final byte[] BYTES = new byte[] {
+            108, 77, 120, 120, 100, 77, 105, 113,
+             83, 97,  71, 108, 116, 107,  90,  71,
+             72, 107,  73,  85,  54,  65,  61,  61
+    };
+
+    // Test via String(byte[],hibyte,off,count) + substring.
+    // The deprecated String constructor clones the byte array via
+    // Arrays.copyOfRange; substring then does another copyOfRange
+    // on the clone, triggering the block arraycopy optimization.
+    static String testStringSubstring() {
+        String s = new String(BYTES, 0, 0, 24);
+        return s.substring(0, 23);
+    }
+
+    // Test via explicit clone + Arrays.copyOfRange.
+    static byte[] testCloneCopyOfRange() {
+        byte[] clone = BYTES.clone();
+        return Arrays.copyOfRange(clone, 0, 23);
+    }
+
+    // Test via explicit clone + Arrays.copyOf.
+    static byte[] testCloneCopyOf() {
+        byte[] clone = BYTES.clone();
+        return Arrays.copyOf(clone, 20);
+    }
+
+    public static void main(String[] args) {
+        String golden = new String(BYTES, 0, 0, 23);
+
+        for (int i = 0; i < 10_000; i++) {
+            // Test 1: String path
+            String result1 = testStringSubstring();
+            if (!result1.equals(golden)) {
+                throw new RuntimeException("testStringSubstring: expected "
+                    + golden + ", got " + result1);
+            }
+
+            // Test 2: clone + copyOfRange
+            byte[] result2 = testCloneCopyOfRange();
+            for (int j = 0; j < 23; j++) {
+                if (result2[j] != BYTES[j]) {
+                    throw new RuntimeException("testCloneCopyOfRange: mismatch at index " + j
+                        + ", expected " + BYTES[j] + ", got " + result2[j]);
+                }
+            }
+
+            // Test 3: clone + copyOf
+            byte[] result3 = testCloneCopyOf();
+            for (int j = 0; j < 20; j++) {
+                if (result3[j] != BYTES[j]) {
+                    throw new RuntimeException("testCloneCopyOf: mismatch at index " + j
+                        + ", expected " + BYTES[j] + ", got " + result3[j]);
+                }
+            }
+        }
+    }
+}