Summary

The HotSpot command line option -XX:+UseEmptySlotsInSupers, that implemented the pre JDK 15 object layout format should be deprecated.

Problem

This option adds extra cases and code to the HotSpot object layout code which is being modified for JEP 401.

Solution

We should deprecate and remove this option, since it affects something that Java users should be not be able to observe.

Specification

diff --git a/src/hotspot/share/runtime/arguments.cpp b/src/hotspot/share/runtime/arguments.cpp
index 9ff8b878692..2ea044a97df 100644
--- a/src/hotspot/share/runtime/arguments.cpp
+++ b/src/hotspot/share/runtime/arguments.cpp
@@ -502,6 +502,7 @@ static SpecialFlag const special_jvm_flags[] = {
   { "UseSharedSpaces",              JDK_Version::jdk(18), JDK_Version::jdk(19), JDK_Version::undefined() },
   { "RegisterFinalizersAtInit",     JDK_Version::jdk(22), JDK_Version::jdk(23), JDK_Version::jdk(24) },
   { "PreserveAllAnnotations",       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) },
 #if defined(X86)
   { "UseRTMLocking",                JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },
   { "UseRTMDeopt",                  JDK_Version::jdk(23), JDK_Version::jdk(24), JDK_Version::jdk(25) },
diff --git a/src/hotspot/share/runtime/globals.hpp b/src/hotspot/share/runtime/globals.hpp
index 575d9a3de36..6b9e5858fd5 100644
--- a/src/hotspot/share/runtime/globals.hpp
+++ b/src/hotspot/share/runtime/globals.hpp
@@ -1958,7 +1958,8 @@ const int ObjectAlignmentInBytes = 8;
       "Use platform unstable time where supported for timestamps only") \
                                                                             \
   product(bool, UseEmptySlotsInSupers, true,                                \
-                "Allow allocating fields in empty slots of super-classes")  \
+          "(Deprecated) Allow allocating fields in empty slots of "         \
+          "super-classes")                                                  \
                                                                             \
   product(bool, DeoptimizeNMethodBarriersALot, false, DIAGNOSTIC,           \
                 "Make nmethod barriers deoptimise a lot.")                  \