RFR: 8329331: Intrinsify Unsafe::setMemory [v4]

[Dean Long]

On Wed, 3 Apr 2024 15:15:24 GMT, Scott Gibbons <sgibbons at openjdk.org> wrote:

>> This code makes an intrinsic stub for `Unsafe::setMemory` for x86_64.  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:
> 
>   Fix Windows

I think the right approach is to turn it into a loop in the IR, which I think is what Doug was implying.  That way C2 can do all its usual optimizations, like unrolling, vectorization, and redundant store elimination (if it is an on-heap primitive array that was just allocated, then there is no need to zero the parts that are being "set").

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Changes requested by dlong (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18555#pullrequestreview-1981533209