[master] RFR: Trim identity-hashcode to 25bit [v2]

[Aleksey Shipilev]

On Mon, 10 May 2021 18:58:27 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> I'd like to trim the identity hashcode to 25 bits to make room in the upper 32bits for the compressed class-pointer.
>> 
>> This could in theory regress performance on workloads that make heavy use of i-hashes, but I could not show this in practice (by specjvm workloads like compiler which uses lots of ihashes).
>> 
>> Next step would be to move the Klass* into the upper 32bit part. (If you look closely, this would be the exact header layout of 32bit JVMs.) And then get rid of the dedicated Klass* field. :-D Eventually I'd make ihash-bits and klass* bits configurable to trade one for the other.
>> 
>> Testing:
>>  - [x] some manual benchmarks
>>  - [x] tier1
>>  - [ ] tier2
>
> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Fix docs about new header layout
>  - Adjust testcase to reflect new header layout

Ok, good, here is a provisional approval, while you correct the comments.

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Marked as reviewed by shade (Committer).

PR: https://git.openjdk.java.net/lilliput/pull/3