RFR: 8339531: Improve performance of MemorySegment::mismatch [v9]

[Maurizio Cimadamore]

On Thu, 5 Sep 2024 17:47:16 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR proposes to improve the performance of `MemorySegment::mismatch` by using Java code rather than transitioning to native code.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix errors in a benchmark

src/java.base/share/classes/jdk/internal/foreign/SegmentBulkOperations.java line 175:

> 173:         } else {
> 174:             long i;
> 175:             if (SCOPED_MEMORY_ACCESS.getByte(src.sessionImpl(), src.unsafeGetBase(), src.unsafeGetOffset() + srcFromOffset) !=

I looked at this the other day, and I couldn't immediately tell whether this test is needed or not - shouldn't it be covered by the subsequent loop? Is it a shortcut (but only for the first element) - how much does that buy really?

src/java.base/share/classes/jdk/internal/foreign/SegmentBulkOperations.java line 179:

> 177:                 return 0;
> 178:             }
> 179:             i = AbstractMemorySegmentImpl.vectorizedMismatchLargeForBytes(src.sessionImpl(), dst.sessionImpl(),

We should probably move `vectorizedMismatchLargeForBytes` in this class too, for clarity

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PR Review Comment: https://git.openjdk.org/jdk/pull/20848#discussion_r1745946359
PR Review Comment: https://git.openjdk.org/jdk/pull/20848#discussion_r1745944712