RFR: 8338591: Improve performance of MemorySegment::copy

[Maurizio Cimadamore]

On Tue, 3 Sep 2024 07:52:44 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to handle smaller FFM copy operations with Java code rather than transitioning to native code. This will improve performance. In this PR, copy operations involving zero to 63 bytes will be handled by Java code.
> 
> Here is what it looks like for Windows x64:
> 
> ![image](https://github.com/user-attachments/assets/6b31206e-3b24-4b34-bf38-a1be393186d3)
> 
> Here is another chart for Linux a64:
> 
> ![image](https://github.com/user-attachments/assets/b679bfac-670a-42a5-802b-2b17adf5ec79)
> 
> Other platforms exhibit similar behavior. It should be noted that the gain with this PR is pronounced for certain common sizes that are more likely to appear in code (e.g. 8, 16, 24, and 32)
> 
> It would be possible to use the same code path for the 7arg  `MemorySegment::copy` method if it is similar to:
> 
> 
> MemorySegment.copy(heapSrcSegment, JAVA_BYTE, 0, heapDstSegment, JAVA_BYTE, 0, ELEM_SIZE);
> 
> 
> This could be added in a separate PR.
> 
> This PR has been tested with tier1-3 and passed.

Marked as reviewed by mcimadamore (Reviewer).

The changes look good, I've tested on my machine and also got a nice bump for small size (which is the goal here).

Thanks also for updating the tests to make sure the new code is covered.

src/java.base/share/classes/jdk/internal/foreign/AbstractMemorySegmentImpl.java line 625:

> 623:         if (size <= 0) {
> 624:             // Do nothing
> 625:         } else if (size < COPY_NATIVE_THRESHOLD && !src.overlaps(dst)) {

This is basically a conservative test - correct? E.g. we might end up cosidering two segments as overlapping even when they are not (because of offsets) - but that's ok, I actually think it's a good pragmatic solution.

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PR Review: https://git.openjdk.org/jdk/pull/20829#pullrequestreview-2277168561
PR Comment: https://git.openjdk.org/jdk/pull/20829#issuecomment-2326288310
PR Review Comment: https://git.openjdk.org/jdk/pull/20829#discussion_r1741898334