[vectorIntrinsics+compress] RFR: 8274971: Add PrefixMask API

[Jatin Bhateja]

On Mon, 11 Oct 2021 22:51:10 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

> @JoshuaZhuwj Yes, let us use mask.compress() name instead. For AVX512, one way is to use the PEXT instruction for mask.compress() implementation:
> 
> * Move the mask from k register to r register
> * PEXT on r register with other register having all 1s.
> * Move from r register to k register
>   Are you planning to add the test cases as well?

Thanks @sviswa7 , I can pick this up once the API definition gets checked in.

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PR: https://git.openjdk.java.net/panama-vector/pull/148