Integrated: 8305895: Implement JEP 450: Compact Object Headers (Experimental)

[Roman Kennke]

On Thu, 22 Aug 2024 13:35:08 GMT, Roman Kennke <rkennke at 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 <rkennke at 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 <sviswanathan at openjdk.org>
Co-authored-by: Martin Doerr <mdoerr at openjdk.org>
Co-authored-by: Hamlin Li <mli at openjdk.org>
Co-authored-by: Thomas Stuefe <stuefe at openjdk.org>
Co-authored-by: Amit Kumar <amitkumar at openjdk.org>
Co-authored-by: Stefan Karlsson <stefank at openjdk.org>
Co-authored-by: Coleen Phillimore <coleenp at openjdk.org>
Co-authored-by: Axel Boldt-Christmas <aboldtch at openjdk.org>
Reviewed-by: coleenp, stefank, stuefe, phh, ihse, lmesnik, tschatzl, matsaave, rcastanedalo, vpaprotski, yzheng, egahlin

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PR: https://git.openjdk.org/jdk/pull/20677