RFR: 8257436: [aarch64] Regressions in ArrayCopyUnalignedDst.testByte/testChar for 65-78 bytes when UseSIMDForMemoryOps is on

[Evgeny Astigeevich]

On Wed, 2 Dec 2020 14:04:42 GMT, Evgeny Astigeevich <github.com+42899633+eastig at openjdk.org> wrote:

>>> > @theRealAph could please check if it causes any issues on ThunderX 2?
>>> 
>>> Is there some script you use that generates the comparison? Otherwise it'll take some time.
>> 
>> Unfortunately, no. I have scripts that do runs. Then I pull all results in Excel.
>> BTW, I attached data comparing SIMD_OFF vs SIMD_ON(patch). JDK-8257436 has data showing regressions with SIMD three ldpq instructions. I'll attach data comparing SIMD_no_fix vs SIMD_fix later.
>
> I updated the attached data. It now includes the following:
> - UseSIMDForMemoryOps is off.
> - UseSIMDForMemoryOps is on.
> - UseSIMDForMemoryOps + fix is on.
> 
> The fix is narrowed to be used for byte and char arraycopies. They have the most problems with unaligned accesses. Int arraycopies are at least 4 byte aligned. Long arraycopies are always 8 byte aligned.

I am attaching performance data for int arraycopy to show the impact of the patch on it:
[test_int_results.txt](https://github.com/openjdk/jdk/files/5631318/test_int_results.txt)

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PR: https://git.openjdk.java.net/jdk/pull/1558