Integrated: 8338967: Improve performance for MemorySegment::fill

[Per Minborg]

On Mon, 26 Aug 2024 11:47:11 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/87e837b6-30e4-4299-a9a1-14776e575825)
> 
> 
> 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.

This pull request has now been integrated.

Changeset: 7a418fc0
Author:    Per Minborg <pminborg at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/7a418fc07464fe359a0b45b6d797c65c573770cb
Stats:     277 lines in 3 files changed: 274 ins; 0 del; 3 mod

8338967: Improve performance for MemorySegment::fill

Reviewed-by: mcimadamore, psandoz

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PR: https://git.openjdk.org/jdk/pull/20712