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  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. ------------- Commit messages: - Chage type declarations in benchmark - Clean up and add doc - Add content hash method and tests - Use parameter i tests - Remove debug line - Add an internal contentHash method Changes: https://git.openjdk.org/jdk/pull/22364/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22364&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8294432 Stats: 338 lines in 3 files changed: 338 ins; 0 del; 0 mod Patch: https://git.openjdk.org/jdk/pull/22364.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/22364/head:pull/22364 PR: https://git.openjdk.org/jdk/pull/22364
