RFR: 8305895: Implementation: JEP 450: Compact Object Headers (Experimental) [v10]

[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 incrementally with three additional commits since the last revision:

 - Merge remote-tracking branch 'origin/JDK-8305895' into JDK-8305895
 - Move compact klass offset into C2
 - Merge branch 'JDK-8305898' into JDK-8305895

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13844/files
  - new: https://git.openjdk.org/jdk/pull/13844/files/698384ec..e7a0f67c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13844&range=09
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13844&range=08-09

  Stats: 75 lines in 17 files changed: 37 ins; 9 del; 29 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