RFR: 8264104: Eliminate unnecessary vector mask conversion during VectorUnbox for floating point VectorMask

[Xiaohong Gong]

On Mon, 29 Mar 2021 06:00:46 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

> The Vector API defines different element types for floating point VectorMask. For example, the bitwise related APIs use "`long/int`", while data related APIs use "`double/float`". This makes some optimizations that based on the type checking cannot work well.
> 
> For example, the VectorBox/Unbox elimination like `"VectorUnbox (VectorBox v) ==> v"` requires the types of output and
> input are equal. Normally this is necessary. However, due to the different element type for floating point VectorMask with the same species, the VectorBox/Unbox pattern is optimized to:
>   VectorLoadMask (VectorStoreMask vmask)
> Actually the types can be treated as the same one for such cases. And considering the vector mask representation is the same for
> vectors with the same element size and vector length, it's safe to do the optimization:
>   VectorLoadMask (VectorStoreMask vmask) ==> vmask
> The same issue exists for GVN which is based on the type of a node. Making the mask node's` hash()/cmp()` methods depends on the element size instead of the element type can fix it.

Hi, could anyone please help to look at this PR? Thanks so much!

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PR: https://git.openjdk.java.net/jdk/pull/3238