RFR: 8309953: Strengthen and optimize oopDesc age methods [v2]

[David Holmes]

On Wed, 14 Jun 2023 10:49:44 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> See the RFE for discussion. Basically, there is little reason to do two loads of mark word, when we can do one. 
>> 
>> Additional testing:
>>  - [x] Eyeballing generated code
>>  - [x] Linux x86_64 fastdebug `tier1 tier2 tier3`
>>  - [x] Linux AArch64 fastdebug `tier1 tier2 tier3`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8309953-oopdesc-age
>  - Revert helper method removals
>  - Fix

The issue is not whether we can construct a benchmark that demonstrates a gain with this kind of micro-optimisation, but whether the micro-optimisation provides sufficient gain in general to trade-off against the reduction in code readability. I'm dubious about the value but will leave it to the GC folk to make the call. Thanks for the updated info.

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PR Comment: https://git.openjdk.org/jdk/pull/14456#issuecomment-1592173561