On Thu, 22 Aug 2024 13:35:08 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 > _coh variants of CDS archiv... This pull request has now been integrated. Changeset: 44ec501a Author: Roman Kennke <rken...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/44ec501a41f4794259dd03cd168838e79334890e Stats: 5212 lines in 218 files changed: 3585 ins; 864 del; 763 mod 8305895: Implement JEP 450: Compact Object Headers (Experimental) Co-authored-by: Sandhya Viswanathan <sviswanat...@openjdk.org> Co-authored-by: Martin Doerr <mdo...@openjdk.org> Co-authored-by: Hamlin Li <m...@openjdk.org> Co-authored-by: Thomas Stuefe <stu...@openjdk.org> Co-authored-by: Amit Kumar <amitku...@openjdk.org> Co-authored-by: Stefan Karlsson <stef...@openjdk.org> Co-authored-by: Coleen Phillimore <cole...@openjdk.org> Co-authored-by: Axel Boldt-Christmas <abold...@openjdk.org> Reviewed-by: coleenp, stefank, stuefe, phh, ihse, lmesnik, tschatzl, matsaave, rcastanedalo, vpaprotski, yzheng, egahlin ------------- PR: https://git.openjdk.org/jdk/pull/20677
