Integrated: 8294432: Add provisions to calculate hash values from MemorySegments

[Per Minborg]

On Mon, 25 Nov 2024 15:27:13 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes adding a _JDK-internal_ method for calculating hash codes for content in a `MemorySegment`.
> 
> The internal method uses a polynomial 32-bit hash function equivalent to `Arrays::hashCode`. The new method is almost two times faster than naïvely iterating over individual bytes for larger regions. Also, it is more lean on inlining space compared to a naïve loop.
> 
> 
> 
> Benchmark                          (ELEM_SIZE)  Mode  Cnt   Score   Error  Units
> SegmentBulkHash.array                        8  avgt   30   2.645 ? 0.078  ns/op
> SegmentBulkHash.array                       64  avgt   30   6.062 ? 0.171  ns/op
> SegmentBulkHash.heapSegment                  8  avgt   30   4.181 ? 0.145  ns/op
> SegmentBulkHash.heapSegment                 64  avgt   30  25.716 ? 1.043  ns/op
> SegmentBulkHash.nativeSegment                8  avgt   30   3.939 ? 0.150  ns/op
> SegmentBulkHash.nativeSegment               64  avgt   30  23.262 ? 0.694  ns/op
> SegmentBulkHash.nativeSegmentJava            8  avgt   30   5.219 ? 0.183  ns/op    <- Naïve iteration
> SegmentBulkHash.nativeSegmentJava           64  avgt   30  39.668 ? 1.040  ns/op    <- Naïve iteration
> 
> 
> ![image](https://github.com/user-attachments/assets/5646cf21-b202-4dce-9555-e460f9df4cb6)
> 
> 
> If internal JDK code uses this method, it will automatically benefit from future performance improvements that can be implemented once the Vector API becomes available.

This pull request has now been integrated.

Changeset: a1473ec3
Author:    Per Minborg <pminborg at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/a1473ec302f1e9d22ebb2417fff3bee21134d6f5
Stats:     338 lines in 3 files changed: 338 ins; 0 del; 0 mod

8294432: Add provisions to calculate hash values from MemorySegments

Reviewed-by: mcimadamore

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PR: https://git.openjdk.org/jdk/pull/22364