RFR: 8277617: Optimize array copy and clear on x86_64

[David Holmes]

On Tue, 23 Nov 2021 01:23:04 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

> Currently 32-byte instructions are used for small array copy and clear. 
> This can be optimized by using 64-byte instructions.
> 
> Please review.
> 
> Best Regards,
> Sandhya

This isn't my area but I'm a bit perplexed by the changes. AFAICS this patch does 2 things:

1. It changes all use of `AVX3Threshold` to `VM_Version::avx3_threshold()`
2. It defines `VM_Version::avx3_threshold()` as:
```static int avx3_threshold() { return (supports_serialize() ? 0: AVX3Threshold); }```

but I am at a loss to understand what `supports_serialize()` has to do with using 64-byte instructions for array copy and clear. ??

Thanks,
David

Plus some performance numbers would be useful. Thanks

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PR: https://git.openjdk.java.net/jdk/pull/6512