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

[Per Minborg]

On Wed, 27 Nov 2024 11:21:32 GMT, Maurizio Cimadamore <mcimadamore 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.
>
> test/jdk/java/foreign/TestSegmentBulkOperationsContentHash.java line 49:
> 
>> 47: import static org.junit.jupiter.api.Assertions.*;
>> 48: 
>> 49: final class TestSegmentBulkOperationsContentHash {
> 
> Should we add a test which checks that the result of the content hash is the same as the polynomial described in the javadoc?

We test it is the same as `Arrays.hashCode(byte[])` which implements this scheme.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/22364#discussion_r1860660343