RFR: 8357531: The `SegmentBulkOperations::fill` method can be improved using overlaps [v11]

[Per Minborg]

On Tue, 5 Aug 2025 14:04:26 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.
>> 
>> It should be noted that the performance of the fill operation affects the allocation of new segments (as they are zeroed out before being returned to the client code).
>> 
>> This PR passes tier1, tier2, and tier3 on multiple platforms.
>
> Per Minborg has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve performance

The new performance improvements gave some 8% better performance for smaller segments.

Before improvements


Benchmark                              (ELEM_SIZE)  Mode  Cnt  Score   Error  Units
SegmentBulkFill.nativeSegmentFillJava            2  avgt   30  1.322 ± 0.020  ns/op
SegmentBulkFill.nativeSegmentFillJava            3  avgt   30  1.313 ± 0.009  ns/op
SegmentBulkFill.nativeSegmentFillJava            4  avgt   30  1.323 ± 0.023  ns/op
SegmentBulkFill.nativeSegmentFillJava            5  avgt   30  1.309 ± 0.006  ns/op
SegmentBulkFill.nativeSegmentFillJava            6  avgt   30  1.310 ± 0.017  ns/op
SegmentBulkFill.nativeSegmentFillJava            7  avgt   30  1.308 ± 0.004  ns/op
SegmentBulkFill.nativeSegmentFillJava            8  avgt   30  1.312 ± 0.008  ns/op
SegmentBulkFill.nativeSegmentFillJava           12  avgt   30  1.316 ± 0.025  ns/op


After improvements


Benchmark                              (ELEM_SIZE)  Mode  Cnt  Score   Error  Units
SegmentBulkFill.nativeSegmentFillJava            2  avgt   30  1.230 ± 0.034  ns/op
SegmentBulkFill.nativeSegmentFillJava            3  avgt   30  1.228 ± 0.037  ns/op
SegmentBulkFill.nativeSegmentFillJava            4  avgt   30  1.243 ± 0.034  ns/op
SegmentBulkFill.nativeSegmentFillJava            5  avgt   30  1.230 ± 0.026  ns/op
SegmentBulkFill.nativeSegmentFillJava            6  avgt   30  1.224 ± 0.025  ns/op
SegmentBulkFill.nativeSegmentFillJava            7  avgt   30  1.232 ± 0.039  ns/op
SegmentBulkFill.nativeSegmentFillJava            8  avgt   30  1.208 ± 0.009  ns/op
SegmentBulkFill.nativeSegmentFillJava           12  avgt   30  1.242 ± 0.030  ns/op

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PR Comment: https://git.openjdk.org/jdk/pull/25383#issuecomment-3155415358