RFR: 8329331: Intrinsify Unsafe::setMemory [v2]

[David Holmes]

On Mon, 1 Apr 2024 21:30:19 GMT, Scott Gibbons <sgibbons at openjdk.org> wrote:

>> This code makes an intrinsic stub for `Unsafe::setMemory`.  See [this PR](https://github.com/openjdk/jdk/pull/16760) for discussion around this change.
>> 
>> Overall, making this an intrinsic improves overall performance of `Unsafe::setMemory` by up to 4x for all buffer sizes.
>> 
>> Tested with tier-1 (and full CI).  I've added a table of the before and after numbers for the JMH I ran (`MemorySegmentZeroUnsafe`).
>> 
>> [setMemoryBM.txt](https://github.com/openjdk/jdk/files/14808974/setMemoryBM.txt)
>
> Scott Gibbons has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use non-sse fill (old left in)

This looks like it is still a Draft/work-in-progress. There is only code for x64 and it doesn't appear it will build on other platforms. Also there are still a bunch of `if 0` in the code that should not be there.

-------------

PR Review: https://git.openjdk.org/jdk/pull/18555#pullrequestreview-1972492070