RFR: 8305895: Implementation: JEP 450: Compact Object Headers (Experimental) [v7]

[Roman Kennke]

> This is the main body of the JEP 450: Compact Object Headers (Experimental).
> 
> Main changes:
>  - Introduction of the (experimental) flag UseCompactObjectHeaders. All changes in this PR are protected by this flag.
>  - The compressed Klass* can now be stored in the mark-word of objects. In order to be able to do this, we are building on #10907, #13582 and #13779 to protect the relevant (upper 32) bits of the mark-word. Significant parts of this PR deal with loading the compressed Klass* from the mark-word, and dealing with (monitor-)locked objects. When the object is monitor-locked, we load the displaced mark-word from the monitor, and load the compressed Klass* from there. 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, and/or reach through to the monitor when the object is locked by a monitor.
>  - The identity hash-code is narrowed to 25 bits.
>  - 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 can now store their length at offset 8. Due to alignment restrictions, array elements will still start at offset 16. #11044 will resolve that restriction and allow array elements to start at offset 12 (except for long, double and uncompressed oops, which are still required to start at an element-aligned offset).
>  - 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.
> 
> Testing:
>  (+UseCompactObjectHeaders tests are run with the flag hard-patched into the build, to also catch @flagless tests, and to avoid mismatches with CDS - see above.)
>  - [x] tier1 (x86_64)
>  - [x] tier2 (x86_64)
>  - [x] tier3 (x86_64)
>  - [ ] tier4 (x86_64)
>  - [x] tier1 (aarch64)
>  - [x] tier2 (aarch64)
>  - [x] tier3 (aarch64)
>  - [ ] tier4 (aarch64)
>  - [ ] tier1 (x86_64) +UseCompactObjectHeaders
>  - [ ] tier2 (x86_64) +UseCompactObjectHeaders
>  - [ ] tier3 (x86_64) +UseCompactObjectHeaders
>  - [ ] tier4 (x86_64) +UseCompactObjectHeaders
>  - [ ] tier1 (aarch64) +UseCompactObjectHeaders
>  - [ ] tier2 (aarch64) +UseCompactObjectHeaders
>  - [ ] tier3 (aarch64) +UseCompactObjectHeaders
>  - [ ] tier4 (aarch64) +UseCompactObjectHeaders

Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 28 commits:

 - Merge branch 'JDK-8305898' into JDK-8305895
 - @shipilev review, round 2
 - Fix build
 - @shipilev comments, round 1
 - Allow to resolve mark with LW locking
 - Use new lightweight locking with compact headers
 - Merge branch 'JDK-8305898' into JDK-8305895
 - Imporve GetObjectSizeIntrinsicsTest
 - Some GC fixes
 - Add BaseOffsets test
 - ... and 18 more: https://git.openjdk.org/jdk/compare/39c33727...58046e58

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Changes: https://git.openjdk.org/jdk/pull/13844/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13844&range=06
  Stats: 1183 lines in 81 files changed: 944 ins; 79 del; 160 mod
  Patch: https://git.openjdk.org/jdk/pull/13844.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13844/head:pull/13844

PR: https://git.openjdk.org/jdk/pull/13844