RFR: 8329331: Intrinsify Unsafe::setMemory [v6]

[Dean Long]

On Sat, 6 Apr 2024 00:13:26 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:
> 
>   Oops

I went ahead and tried a pure-Java implementation, and it is faster for small sizes (up to 8) and only about 1.5x slower for larger sizes, so that might make for an interesting fallback if there is no customized assembler implementation available or if the size is known to me small.

Ideally, I think we would want C2 to be more aware of setMemory stores, so that it can remove redundant stores, like it does with InitializeNode.

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PR Comment: https://git.openjdk.org/jdk/pull/18555#issuecomment-2041271472