8341471: Reversed field layout caused by unstable sorting

Reviewed-by: jwaters, jsjolen

src/hotspot/share/classfile/fieldLayoutBuilder.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2020, 2024, 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
@@ -101,17 +101,13 @@ class LayoutRawBlock : public ResourceObj {
   // sort fields in decreasing order.
   // Note: with line types, the comparison should include alignment constraint if sizes are equals
   static int compare_size_inverted(LayoutRawBlock** x, LayoutRawBlock** y)  {
-#ifdef _WINDOWS
-    // qsort() on Windows reverse the order of fields with the same size
-    // the extension of the comparison function below preserves this order
     int diff = (*y)->size() - (*x)->size();
+    // qsort() may reverse the order of fields with the same size.
+    // The extension is to ensure stable sort.
     if (diff == 0) {
       diff = (*x)->field_index() - (*y)->field_index();
     }
     return diff;
-#else
-    return (*y)->size() - (*x)->size();
-#endif // _WINDOWS
   }
 
 };

test/hotspot/jtreg/runtime/FieldLayout/TestFieldLayout.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2024, Arm Limited. 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.
+ */
+
+import java.lang.reflect.Field;
+import jdk.internal.misc.Unsafe;
+
+/*
+ * @test
+ * @bug 8341471
+ * @summary Reversed field layout caused by unstable sorting
+ * @modules java.base/jdk.internal.misc
+ * @run main/othervm TestFieldLayout
+ */
+
+public class TestFieldLayout {
+
+    private static final Unsafe U = Unsafe.getUnsafe();
+
+    public static void main(String[] args) throws Exception {
+
+        boolean endResult = true;
+        long previous = 0;
+
+        for (Field f : Test.class.getDeclaredFields()) {
+            long current = U.objectFieldOffset(f);
+            if (current < previous) {
+                System.out.printf("FAILED: field %s offset %d previous %d\n",
+                                  f.getName(), current, previous);
+                endResult = false;
+            }
+            previous = current;
+        }
+
+        System.out.println(endResult ? "Test PASSES" : "Test FAILS");
+        if (!endResult) {
+            throw new Error("Test failed");
+        }
+    }
+
+    public class Test {
+        char a000;
+        char a001;
+        char a002;
+        char a003;
+        char a004;
+        char a005;
+        char a006;
+        char a007;
+        char a008;
+        char a009;
+        char a00a;
+        char a00b;
+    }
+
+}
+