On Tue, 10 Mar 2026 05:49:00 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 65 commits: > > - 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]> > - David: reduce diff in ObjectCountEventVerifier.java > - David: minimize change in GetObjectSizeIntrinsicsTest.java > - David: minimize diffs in TestZGCWithCDS.java > - David: minimize diffs for > runtime/ErrorHandling/TestVMConfigInHsErrFile.java > - ... and 55 more: https://git.openjdk.org/jdk/compare/9a26b4af...be3a902b More partial review from me. src/hotspot/share/memory/metaspace.cpp line 552: > 550: bool Metaspace::initialized() { > 551: return metaspace::MetaspaceContext::context_nonclass() != nullptr > 552: && CLASS_SPACE_ONLY(Metaspace::class_space_is_initialized()) I think this could be simplified to CLASS_SPACE_ONLY(&& Metaspace::class_space_is_initialized()) without the NOT_CLASS_SPACE(true) part. src/hotspot/share/memory/metaspace.hpp line 168: > 166: > 167: static bool is_class_space_allocation(MetadataType mdType) { > 168: return CLASS_SPACE_ONLY(mdType) NOT_CLASS_SPACE(false); This is confusing me. Is ClassType the only non-zero MetadataType? Might be better to write that out as before, otherwise it's a weird implicit conversion from MetadataType to bool. src/hotspot/share/memory/metaspace/metaspaceReporter.cpp line 277: > 275: > 276: out->cr(); > 277: out->print_cr("Chunk freelist%s:", Metaspace::using_class_space() ? > "s" : ""); So this is not needed anymore? ------------- PR Review: https://git.openjdk.org/jdk/pull/28366#pullrequestreview-3937468350 PR Review Comment: https://git.openjdk.org/jdk/pull/28366#discussion_r2925414312 PR Review Comment: https://git.openjdk.org/jdk/pull/28366#discussion_r2925482743 PR Review Comment: https://git.openjdk.org/jdk/pull/28366#discussion_r2925375686
