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.")                  \