RFR: 8329331: Intrinsify Unsafe::setMemory [v24]

[Scott Gibbons]

On Fri, 19 Apr 2024 22:08:52 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:
> 
>   Long to short jmp; other cleanup

The SIGBUS was due to improper scoping of the UnsafeCopyMemoryMark.  The change is:

`   {`
`     // Add set memory mark to protect against unsafe accesses faulting`
`-    UnsafeCopyMemoryMark(this, ((t == T_BYTE) && !aligned), true);`
`+    UnsafeCopyMemoryMark usmm(this, ((t == T_BYTE) && !aligned), true);`
`     __ generate_fill(t, aligned, to, value, r11, rax, xmm0);`
`   }`

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PR Comment: https://git.openjdk.org/jdk/pull/18555#issuecomment-2067758164