RFR: 8338967: Improve performance for MemorySegment::fill [v5]

[Maurizio Cimadamore]

On Fri, 30 Aug 2024 12:15:36 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> @minborg Hi! I didn't checked the numbers with the benchmark I've written at https://github.com/openjdk/jdk/pull/20712#discussion_r1732802685 which is meant to stress the branch predictor (without enough `samples` i.e. past 128K on my machine) - can you give it a shot with M1 🙏 ?
>
> @franz1981 Here is what I get if I run your performance test on my M1 Mac (unfortunately no -perf data):
> 
> 
> Base
> Benchmark                         (samples)  (shuffle)  Mode  Cnt        Score        Error  Units
> TestBranchFill.heap_segment_fill       1024      false  avgt   30    58597.625 ?   1871.313  ns/op
> TestBranchFill.heap_segment_fill       1024       true  avgt   30    64309.859 ?   1164.360  ns/op
> TestBranchFill.heap_segment_fill     128000      false  avgt   30  7136796.445 ? 152120.060  ns/op
> TestBranchFill.heap_segment_fill     128000       true  avgt   30  7908474.120 ?  49184.950  ns/op
> 
> 
> Patch
> Benchmark                         (samples)  (shuffle)  Mode  Cnt        Score       Error  Units
> TestBranchFill.heap_segment_fill       1024      false  avgt   30     3695.815 ?    24.615  ns/op
> TestBranchFill.heap_segment_fill       1024       true  avgt   30     3938.582 ?   124.510  ns/op
> TestBranchFill.heap_segment_fill     128000      false  avgt   30   420845.301 ?  1605.080  ns/op
> TestBranchFill.heap_segment_fill     128000       true  avgt   30  1778362.506 ? 39250.756  ns/op

> Thanks @minborg to run it, so it seems that 128K, despite the additional call (due to not inlining something), makes nuking the pipeline of M1 a severe affair:

If I understand correctly, this benchmark attempts to call `fill` with different segment sizes (in a loop) - correct? It's understandable that, in this case, we can't optimize as well, because we have different branches which get taken or not in a less predictable fashion. The important question is (for this PR): does the work proposed here cause a _regression_ in the case you have in mind? E.g. is the `setMemory` intrinsics better than the branchy logic we have here?

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PR Comment: https://git.openjdk.org/jdk/pull/20712#issuecomment-2321596145