RFR: 8305895: Implementation: JEP 450: Compact Object Headers (Experimental) [v15]

[Roman Kennke]

On Fri, 12 May 2023 17:18:34 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> 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.

I'm sorry, I think I butchered this PR while trying to merge latest upstream through all the dependent PRs. Let's continue the discussion the new PR #13961. I hope I haven't caused anything breakage (for some reason, this PR now shows as "Merged" which worries me. I believe the Skara bot did that. I wonder where it has been merged to.)

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PR Comment: https://git.openjdk.org/jdk/pull/13844#issuecomment-1546066712