On Tue, 22 Oct 2024 11:19:19 GMT, Roman Kennke <rken...@openjdk.org> wrote:
>> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 97 commits: > > - Merge tag 'jdk-24+20' into JDK-8305895-v4 > > Added tag jdk-24+20 for changeset 7a64fbbb > - Fix needle copying in indexOf intrinsic for smaller headers > - Compact header riscv (#3) > > Implement compact headers on RISCV > --------- > > Co-authored-by: hamlin <ham...@rivosinc.com> > - Remove extra sanity check > - Problem-list SharedBaseAddress tests on aarch64 > - Address comments by @vpaprotsk > - Fix aarch64.ad > - Merge tag 'jdk-24+19' into JDK-8305895-v4 > > Added tag jdk-24+19 for changeset e7c5bf45 > - PPC64 implementation of Compact Object Headers (JEP 450) > - Increase compiler code stubs size for indexOf intrinsic > - ... and 87 more: https://git.openjdk.org/jdk/compare/7a64fbbb...e324d95b We've identified another failure in our testing: java -XX:+UnlockExperimentalVMOptions -XX:+UseCompactObjectHeaders -XX:TieredStopAtLevel=2 -XX:TLABSize=1 -XX:MinTLABSize=1 ~/tests/HelloWorld.java # # A fatal error has been detected by the Java Runtime Environment: # # Internal Error (src/hotspot/share/jfr/support/jfrObjectAllocationSample.cpp:50), pid=775231, tid=775232 # assert(desired_tlab_size_bytes > alignment_reserve_bytes) failed: invariant ... V [libjvm.so+0xf4ec11] JfrObjectAllocationSample::send_event(Klass const*, unsigned long, bool, Thread*)+0x2d1 (jfrObjectAllocationSample.cpp:50) V [libjvm.so+0x5d7899] AllocTracer::send_allocation_outside_tlab(Klass*, HeapWordImpl**, unsigned long, JavaThread*)+0x39 (allocTracer.cpp:35) V [libjvm.so+0x139d6c5] MemAllocator::Allocation::notify_allocation_jfr_sampler()+0x225 (memAllocator.cpp:214) V [libjvm.so+0x139f928] MemAllocator::allocate() const+0x2a8 (memAllocator.cpp:235) V [libjvm.so+0x18379bd] TypeArrayKlass::allocate_common(int, bool, JavaThread*)+0x13d (collectedHeap.inline.hpp:41) V [libjvm.so+0x14bc5c8] oopFactory::new_typeArray(BasicType, int, JavaThread*)+0x38 (typeArrayKlass.hpp:68) V [libjvm.so+0x8327f1] Runtime1::new_type_array(JavaThread*, Klass*, int)+0xa1 (c1_Runtime1.cpp:388) ------------- PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2429286279
