[vectorIntrinsics] RFR: 8283709: Add x86 back-end implementation for bit BIT_COUNT operation [v3]

[Xiaohong Gong]

On Mon, 28 Mar 2022 06:03:57 GMT, Ningsheng Jian <njian at openjdk.org> wrote:

>> We can have type homogeneity for PopCountVL across VectorAPI and SLP once we start auto-vectorizing ConvL2I  operation.
>
> I think that makes the code hard to maintain, and I think @merykitty 's unaddressed comment in: https://github.com/openjdk/jdk/pull/6857#discussion_r780917717 is a preferable approach. 
> 
> FYI: @fg1417 has a ConvL2I auto-vectorization support under review: https://github.com/openjdk/jdk/pull/7806.

Yeah, so if the `ConvL2I` is supported to be auto-vectorized after then, the `PopCountL` is better to be auto-vectorized to "PopCountVL + VectorCastL2X" ? Do you have any plan on it? Thanks so much！

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PR: https://git.openjdk.java.net/panama-vector/pull/185