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

[Francesco Nigro]

On Tue, 3 Sep 2024 08:39:02 GMT, Per Minborg <pminborg at openjdk.org> wrote:

>> I found similar small improvements to be had (I wrote about them offline) when replacing the bitwise-based tests (e.g. `foo & 4 != 0`) with a more explicit check for `remainingBytes >=4`. Seems like bitwise operations are not as optimized (or perhaps the assembly instructions for them is overall more convoluted - I haven't checked).
>
> I've tried 
> 
> 
> final long longValue = Byte.toUnsignedLong(value) * 0x0101010101010101L;
> 
> 
> But it had the same performance as explicit bit shifting on M1.

@minborg the ` ELEM_SIZE` is a `Param` field right? Just to be 100% sure of it...

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PR Review Comment: https://git.openjdk.org/jdk/pull/20712#discussion_r1741681612