On Fri, 20 Mar 2026 08:15:01 GMT, Thomas Stuefe <[email protected]> wrote:
>> This change removes the uncompressed Klass pointer mode and, with compressed >> Klass pointers remaining as the only option, the >> `UseCompressedClassPointers` switch. >> >> For motivation, please take a look at CSR associated with the deprecation >> (which we did for JDK 25) and the preparatory discussion we had at the start >> of the year around this topic [2]. >> >> This patch is quite invasive and touches many parts of the JVM, since its >> goal is to remove most traces of the uncompressed Klass path and to take >> advantage of opportunities for simplification. In some cases, I did not take >> opportunities for further simplification to keep the patch somewhat legible; >> it will be onerous enough to review. >> >> ### Implementation Notes >> >> With uncompressed Klass pointers removed, we have three modes of operation >> left (including 32-bit): >> a) 64-bit, COH off - this is the old `+UseCompressedClassPointers` mode. >> This is now the standard mode until we run with COH by default. >> b) 64-bit, COH on >> c) 32-bit - Here, we run with a "fake" narrow Klass pointer mode. We run >> with hardcoded narrowKlass base == NULL and shift = 0, so nKlass == Klass*. >> The difference to (a, b) is that we don't use a class space. Most of this >> was implemented with JDK-8363998 [3] - here, we just add a compile-time >> switch `INCLUDE_CLASSSPACE`, which is true on 32-bit, false on 64-bit. >> >> I ensured *arm32* builds and I performed some rudimentary checks (selected >> metaspace/gc tests, and a simple Spring PetClinic run). Vendors with an >> interest in arm32 will have to step up and do their own, more thorough unit >> testing. Also, I did not see anyone doing follow-up work after JDK-8363998 >> [3] - so some issues may still lurk from that patch as well (but maybe >> JDK-8363998 was just not breaking anything). >> >> I did not check *zero 32-bit*, the only other platform supporting 32-bit. >> Anyone with an interest in 32-bit zero should chip in. >> >> Pre-existing errors: While working on this patch, I stumbled over a few >> occurrences of old but benign bugs. Mostly old code assuming >> CompressedClassPointers and CompressedOops were still tied together >> (example: Arguments::set_heap_size()). These bugs are implicitly fixed with >> this patch. >> >> ### Testing >> >> - tier 1 2 3 locally on Linux x64 >> - SAP ran their whole set of tests for all the platforms they support. >> >> >> [1] https://bugs.openjdk.org/browse/JDK-8350754 >> [2] https://mail.openjdk.org/pipermail/hotspot-dev/2025-February/101023.html >> [3] https://bugs.o... > > Thomas Stuefe has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 69 commits: > > - Merge branch 'master' into JDK-8363996-Obsolete-UseCompressedClassPointers > - removed CompressedKlassPointers::needs_class_space, since we have now > INCLUDE_CLASS_SPACE > - Feedback Roman > - Merge Master > - Merge branch 'master' into JDK-8363996-Obsolete-UseCompressedClassPointers > - Replace Metaspace::using_class_space with define > - Update src/hotspot/cpu/x86/macroAssembler_x86.hpp > > Co-authored-by: David Holmes > <[email protected]> > - Replace Klass::_metadata union with narrowKlass member > - Ivan: fix various instances of ObjLayout::undefined should assert > - Ivan: Update src/hotspot/share/oops/instanceKlass.cpp > > Co-authored-by: Ivan Walulya <[email protected]> > - ... and 59 more: https://git.openjdk.org/jdk/compare/f2550e89...35f672c2 Impressive work. Thank you for doing it. I have only few nitpicks only. src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 765: > 763: #ifdef ASSERT > 764: // TraceBytecodes does not use r12 but saves it over the call, so > don't verify > 765: // if (!TraceBytecodes) verify_heapbase("call_VM_base: heap base > corrupted?"); Only `if()` was commented before. Now you put `verify_heapbase()` under comment too. Even so `verify_heapbase()` is empty on this platform we should not change this. src/hotspot/share/memory/classLoaderMetaspace.hpp line 2: > 1: /* > 2: * Copyright (c) 2011, 2025, Oracle and/or its affiliates. All rights > reserved. 2026. And consider updating year in other files. ------------- PR Review: https://git.openjdk.org/jdk/pull/28366#pullrequestreview-3982669865 PR Review Comment: https://git.openjdk.org/jdk/pull/28366#discussion_r2966774135 PR Review Comment: https://git.openjdk.org/jdk/pull/28366#discussion_r2967008057
