On Wed, 19 Nov 2025 20:22:19 GMT, ExE Boss <[email protected]> wrote: >> _This patch is not intended for JDK 26_. >> >> I'm posting it now to collect feedback and, barring any objections, plan to >> push it once JDK 27 opens. >> >> 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. This was >> implemented with JDK-8363998 [3] - for more details, please see that issue >> and its PR. >> >> 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... > > src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 4998: > >> 4996: ldrw(tmp1, Address(obj1, oopDesc::klass_offset_in_bytes())); >> 4997: ldrw(tmp2, Address(obj2, oopDesc::klass_offset_in_bytes())); >> 4998: cmpw(tmp1, tmp2); > > Same here, but with `cmpw(tmp1, tmp2)`.
I did your suggested changes, but will hold on doing similar changes (moving instructions out of now common branches) for now; I feel that this makes the diff less clear, and this PR will be annoying to review as it is; lots of onerous work. Let's hear what others think once I undrafted this. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/28366#discussion_r2581326249
