RFR: 8338967: Improve performance for MemorySegment::fill [v5]

[Paul Sandoz]

On Tue, 27 Aug 2024 10:38:46 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> The performance of the `MemorySegment::fil` can be improved by replacing the `checkAccess()` method call with calling `checkReadOnly()` instead (as the bounds of the segment itself do not need to be checked).
>> 
>> Also, smaller segments can be handled directly by Java code rather than transitioning to native code.
>> 
>> Here is how the `MemorySegment::fill` performance is improved by this PR:
>> 
>> ![image](https://github.com/user-attachments/assets/92a0bcf2-f5b0-4a91-9c02-39423f870209)
>> 
>> Operations involving 8 or more bytes are delegated to native code whereas smaller segments are handled via a switch rake.
>> 
>> It should be noted that `Arena::allocate` is using `MemorySegment::fil`. Hence, this PR will also have a positive effect on memory allocation performance.
>
> Per Minborg 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 six additional commits since the last revision:
> 
>  - Merge branch 'master' into fill-performance
>  - Fix typo
>  - Add a comment about the old switch type
>  - Remove unused import
>  - Reduce kick-in size and add test
>  - Initial implementation

How fast do we need to be here given we are measuring in a few nanoseconds per operation? 

What if the goal is not to regress from say explicitly filling in a small sized segment or a comparable array (e.g., < 8 bytes) then maybe a loop suffices and the code is simple?

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PR Comment: https://git.openjdk.org/jdk/pull/20712#issuecomment-2313446118