RFR: 8291600: [vectorapi] vector cast op check is not always needed for vector mask cast [v3]

[Jie Fu]

On Thu, 25 Aug 2022 03:20:16 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

> For a vector mask unbox operation, if the input is a `VectorBox`, and the boxed value type is not equal to the unbox type, we need to do the type casting. Originally, we separate the type casting for two different cases:
> 
> 1. if the vector element size in bytes is equal (e.g. casting between `int` and `float`), the conversion is:
> 
> ```
>   VectorUnbox (VectorBox vmask) ==> VectorMaskCast (vmask)
> ```
> 
> This doesn't need to emit any instructions.
> 
> 2. otherwise (e.g. casting from `short` to `int`), the conversion is:
> 
> ```
>   VectorUnbox (VectorBox vmask) ==> VectorLoadMask (VectorStoreMask vmask)
> ```
> 
> This means we need to convert the short mask vector back to a boolean vector, and then casting the boolean vector to a int vector (i.e. `S->B->I`).
> 
> With the new changes in this PR, `VectorMaskCast` supports the vector mask casting for all cases and all platforms (originally it only supports the first case for most platforms), we can implement the second cases simply with `VectorMaskCast`. That means we can optimize `S->B->I` to `S->I`. This saves the unnecessary narrowing and extending instructions due to the temporary boolean vector result.



So there are at least two optimizations in this pr:
1) Opt for FIRST_NONZERO
2) Opt for vector mask unbox operation

How about splitting them and doing the second one in a new PR？

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