RFR: 8291600: [vectorapi] vector cast op check is not always needed for vector mask cast

[Xiaohong Gong]

On Tue, 9 Aug 2022 16:36:53 GMT, Quan Anh Mai <duke at openjdk.org> wrote:

> Yes you are right, the code would be mostly the same, which means we can reuse the existing match rules to additionally match `VectorMaskCast` for those cases. For other cases, in particular, narrowing cast to subword types, since avx < 3 does not support truncation cast and only provides saturation cast instructions. We need to truncate the upper bits ourselves. For example, a cast from `int` to `byte` is done as follow
> 
> ```
> vpand dst, src, [external address mask]
> vpackusdw dst, dst
> vpermq dst, dst, 0x08
> vpackuswb dst, dst
> ```
> 
> For vector mask cast, we can get rid of the first masking and use the `vpackss`s instead, which removes the need to reference memory. Thanks.

I see, thanks! So would you like to provide the missing x86 backend implementation for `VectorMaskCast` ? If so we can use 
`VectorMaskCast` for all cases to simply the current codes? Thanks a lot!

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PR: https://git.openjdk.org/jdk/pull/9737