On Tue, 17 Sep 2024 09:35:02 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 57 commits: > > - fix CompressedClassPointersEncodingScheme yet again for linux aarch64 > - Fixes post-8340184 > - Merge upstream up to and including 8340184 > - Merge remote-tracking branch 'origin/master' into JDK-8305895-v4 > - Fix > test/hotspot/jtreg/runtime/CompressedOops/CompressedClassPointersEncodingScheme.java > - Fix loop on aarch64 > - clarify obscure assert in metasapce setup > - Rework compressedklass encoding > - remove stray debug output > - Fixes post 8338526 > - ... and 47 more: https://git.openjdk.org/jdk/compare/7849f252...28a26aed Hi, We've gone through the rest of the Metaspace code and looked at the tests. It looks OK to us. Would like to see some style cleanups in the tests, but that can wait as a follow up. test/hotspot/gtest/metaspace/test_clms.cpp line 193: > 191: > 192: { > 193: // Nonclass arena allocation. The style in this source file isn't really up to scratch, especially *these* lines. Anyway, it's in the tests, so I'm OK with this being fixed in a follow up RFE. ------------- PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2309360771 PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1763005291
