[lilliput-jdk17u:lilliput] RFR: 8303450: [Lilliput/JDK17] Runtime flag to enable Lilliput [v3]

[Thomas Stuefe]

On Fri, 10 Mar 2023 21:07:35 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This change introduces an experimental runtime flag -XX:[+/-]UseCompactObjectHeaders which can be used to disable Lilliput. The flag guards all the relevant places that directly affect the object layout and the accesses to Klass* etc. The change also restores the previous behaviour when -UseCompactObjectHeaders is selected.
>> 
>> One special difficulty is that CDS archives that are generated with compact object headers cannot currently be read with old-style headers, and vice versa. I work around this by disabling CDS when UseCompactObjectHeaders is set to non-default. This makes a few tests fail, but should otherwise be harmless. (In theory, we could implement parsing archives and loading objects with the correct header layout, but ... life's too short ;-) )
>> 
>> Testing:
>>  - [x] tier1 (x86_64, x86_32, aarch64) +UseCompactObjectHeaders
>>  - [x] tier2 (x86_64, x86_32, aarch64) +UseCompactObjectHeaders
>>  - [x] tier1 (x86_64, x86_32, aarch64) -UseCompactObjectHeaders
>>  - [x] tier2 (x86_64, x86_32, aarch64) -UseCompactObjectHeaders
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove misplaced assert

Gave this an eye-over, and it looks fine.

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Marked as reviewed by stuefe (Reviewer).

PR: https://git.openjdk.org/lilliput-jdk17u/pull/8