RFR: 8338967: Improve performance for MemorySegment::fill [v10]

[Maurizio Cimadamore]

On Fri, 30 Aug 2024 22:04:39 GMT, Francesco Nigro <duke at openjdk.org> wrote:

> All of these strategies are better than what we have now, probably because the existing instrinsics still perform some poor decision, but I haven't dug yet into perfasm out to see what it does wrong; maybe is something which could be fixed in the intrinsic itself?

I'm no intrinsics expert, but if I had to guess I'd say that the intrinsics we have do not specialize for small sizes. Also, the use of vector instructions typically comes with additional alignment constraints - meaning that we need a pre-loop (and sometimes a post-loop). This logic, while faster for bigger sizes, has some drawbacks for smaller sizes.

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PR Comment: https://git.openjdk.org/jdk/pull/20712#issuecomment-2324276883