RFR: 8357531: The `SegmentBulkOperations::fill` method can be improved using overlaps [v2]

[Maurizio Cimadamore]

On Thu, 22 May 2025 08:14:35 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> This PR builds on a concept John Rose told me about some time ago. Instead of combining memory operations of various sizes, a single large and skewed memory operation can be made to clean up the tail of remaining bytes.
>> 
>> This has the effect of simplifying and shortening the code. The number of branches to evaluate is reduced.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Correct typo in comment

src/java.base/share/classes/jdk/internal/foreign/SegmentBulkOperations.java line 93:

> 91:             case 2 -> {
> 92:                 SCOPED_MEMORY_ACCESS.putShortUnaligned(dst.sessionImpl(), dst.unsafeGetBase(), dst.unsafeGetOffset(), (short) longValue, !Architecture.isLittleEndian());
> 93:                 SCOPED_MEMORY_ACCESS.putShortUnaligned(dst.sessionImpl(), dst.unsafeGetBase(), dst.unsafeGetOffset() + len - Short.BYTES, (short) longValue, !Architecture.isLittleEndian());

So that I understand, since this configuration might pick up both length = 2 and length = 3, in 50% of cases there's a redundant store? This is also true for length = 4, 5, 6, 7 -- but in this case the chance of redundant store is reduced to 25%.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25383#discussion_r2102016436