RFR: 8338967: Improve performance for MemorySegment::fill [v6]

[Maurizio Cimadamore]

On Wed, 28 Aug 2024 09:28:59 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/ee29fdf0-a7cf-4d5b-bb6b-278b01d97e3c)
>> 
>> 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 incrementally with one additional commit since the last revision:
> 
>   Switch to bit checking instead of switch statement

src/java.base/share/classes/jdk/internal/foreign/AbstractMemorySegmentImpl.java line 59:

> 57: import sun.nio.ch.DirectBuffer;
> 58: 
> 59: import static java.lang.foreign.ValueLayout.*;

Please use direct imports

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PR Review Comment: https://git.openjdk.org/jdk/pull/20712#discussion_r1734408375