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

[Per Minborg]

On Tue, 26 Nov 2024 13:55:10 GMT, Viktor Klang <vklang 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.
>
> src/java.base/share/classes/jdk/internal/foreign/SegmentBulkOperations.java line 208:
> 
>> 206:         final long limit = fromOffset + longBytes;
>> 207:         for (; fromOffset < limit; fromOffset += 8) {
>> 208:             long val = SCOPED_MEMORY_ACCESS.getLongUnaligned(segment.sessionImpl(), segment.unsafeGetBase(), segment.unsafeGetOffset() + fromOffset, !Architecture.isLittleEndian());
> 
> @minborg Does it help to hoist the calls to `segment.sessionImpl()` + `segment.unsafeGetBase()` + `segment.unsafeGetOffset()` + `!Architecture.isLittleEndian()` and reusing those throughout the different strides?

I thought that initially as well, but keeping it like this helps the compiler select the correct unsafe code path.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22364#discussion_r1858596703