8330699: Obsolete -XX:+UseEmptySlotsInSupers

Reviewed-by: shade, fparain, dholmes

src/hotspot/share/classfile/fieldLayoutBuilder.cpp

@@ -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
@@ -137,7 +137,7 @@ void FieldLayout::initialize_instance_layout(const InstanceKlass* super_klass) {
   } else {
     bool has_fields = reconstruct_layout(super_klass);
     fill_holes(super_klass);
-    if ((UseEmptySlotsInSupers && !super_klass->has_contended_annotations()) || !has_fields) {
+    if (!super_klass->has_contended_annotations() || !has_fields) {
       _start = _blocks;  // start allocating fields from the first empty block
     } else {
       _start = _last;    // append fields at the end of the reconstructed layout
@@ -364,20 +364,6 @@ void FieldLayout::fill_holes(const InstanceKlass* super_klass) {
     b = p;
   }
 
-  if (!UseEmptySlotsInSupers) {
-    // Add an empty slots to align fields of the subclass on a heapOopSize boundary
-    // in order to emulate the behavior of the previous algorithm
-    int align = (b->offset() + b->size()) % heapOopSize;
-    if (align != 0) {
-      int sz = heapOopSize - align;
-      LayoutRawBlock* p = new LayoutRawBlock(LayoutRawBlock::EMPTY, sz);
-      p->set_offset(b->offset() + b->size());
-      b->set_next_block(p);
-      p->set_prev_block(b);
-      b = p;
-    }
-  }
-
   LayoutRawBlock* last = new LayoutRawBlock(LayoutRawBlock::EMPTY, INT_MAX);
   last->set_offset(b->offset() + b->size());
   assert(last->offset() > 0, "Sanity check");

src/hotspot/share/runtime/arguments.cpp

@@ -505,7 +505,6 @@ static SpecialFlag const special_jvm_flags[] = {
   { "DontYieldALot",                JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },
   { "PreserveAllAnnotations",       JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },
   { "UseNotificationThread",        JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },
-  { "UseEmptySlotsInSupers",        JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },
   // --- Deprecated alias flags (see also aliased_jvm_flags) - sorted by obsolete_in then expired_in:
   { "CreateMinidumpOnCrash",        JDK_Version::jdk(9),  JDK_Version::undefined(), JDK_Version::undefined() },
 
@@ -513,6 +512,7 @@ static SpecialFlag const special_jvm_flags[] = {
 
   { "MetaspaceReclaimPolicy",       JDK_Version::undefined(), JDK_Version::jdk(21), JDK_Version::undefined() },
 
+  { "UseEmptySlotsInSupers",        JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },
   { "OldSize",                      JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },
 #if defined(X86)
   { "UseRTMLocking",                JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },

src/hotspot/share/runtime/globals.hpp

@@ -1948,10 +1948,6 @@ const int ObjectAlignmentInBytes = 8;
   product(bool, UseFastUnorderedTimeStamps, false, EXPERIMENTAL,            \
           "Use platform unstable time where supported for timestamps only") \
                                                                             \
-  product(bool, UseEmptySlotsInSupers, true,                                \
-          "(Deprecated) Allow allocating fields in empty slots of "         \
-          "super-classes")                                                  \
-                                                                            \
   product(bool, DeoptimizeNMethodBarriersALot, false, DIAGNOSTIC,           \
                 "Make nmethod barriers deoptimise a lot.")                  \
                                                                             \

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

@@ -1,79 +0,0 @@
-/*
- * Copyright (c) 2020, 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 8239014
- * @summary -XX:-UseEmptySlotsInSupers sometime fails to reproduce the layout of the old code
- * @library /test/lib
- * @modules java.base/jdk.internal.misc
- *          java.management
- * @requires vm.bits == "64" & vm.opt.final.UseCompressedOops == true & vm.gc != "Z"
- * @run main/othervm -XX:+UseCompressedClassPointers -XX:-UseEmptySlotsInSupers OldLayoutCheck
- */
-
-/*
- * @test
- * @requires vm.bits == "32"
- * @library /test/lib
- * @modules java.base/jdk.internal.misc
- *          java.management
- * @run main/othervm -XX:-UseEmptySlotsInSupers OldLayoutCheck
- */
-
-import java.lang.reflect.Field;
-import java.util.Arrays;
-import java.util.Comparator;
-import jdk.internal.misc.Unsafe;
-
-import jdk.test.lib.Asserts;
-import jdk.test.lib.Platform;
-
-public class OldLayoutCheck {
-
-    static class LIClass {
-        public long l;
-        public int i;
-    }
-
-    // 32-bit VMs: @0:  8 byte header,  @8: long field, @16:  int field
-    // 64-bit VMs: @0: 12 byte header, @12:  int field, @16: long field
-    static final long INT_OFFSET  = Platform.is64bit() ? 12L : 16L;
-    static final long LONG_OFFSET = Platform.is64bit() ? 16L :  8L;
-
-    static public void main(String[] args) {
-        Unsafe unsafe = Unsafe.getUnsafe();
-        Class c = LIClass.class;
-        Field[] fields = c.getFields();
-        for (int i = 0; i < fields.length; i++) {
-            long offset = unsafe.objectFieldOffset(fields[i]);
-            if (fields[i].getType() == int.class) {
-                Asserts.assertEquals(offset, INT_OFFSET, "Misplaced int field");
-            } else if (fields[i].getType() == long.class) {
-                Asserts.assertEquals(offset, LONG_OFFSET, "Misplaced long field");
-            } else {
-                Asserts.fail("Unexpected field type");
-            }
-        }
-    }
-}