RFR: 8338023: Support two vector selectFrom API [v7]

[Paul Sandoz]

On Fri, 6 Sep 2024 18:08:09 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> src/jdk.incubator.vector/share/classes/jdk/incubator/vector/Vector.java line 2770:
>> 
>>> 2768: 
>>> 2769:     /**
>>> 2770:      * Rearranges the lane elements of two vectors, selecting lanes
>> 
>> I have a bit of a name concern here. Why are we calling it "select" and not "rearrange"? Because for a single "from" vector we also call it "rearrange", right? Is "select" not often synonymous to "blend", which works also with two "from" vectors, but with a mask and not indexing for "selection/rearranging"?
>
> We already have another flavor of [selectFrom](https://docs.oracle.com/en/java/javase/22/docs/api/jdk.incubator.vector/jdk/incubator/vector/Vector.html#selectFrom(jdk.incubator.vector.Vector)) which permutes single vector, new API extents its semantics to two vector selection, so we kept the nomenclature consistent.

Select operates only on vectors where the `this` vector represents the indexes to *select* elements from the other vectors. Rearrange operates on vectors and a shuffle argument that *rearranges* elements from the other vectors. The former behavior can be specified in terms of the latter behavior, and ideally the equivalent expressions should result in ~same generated sequence of instructions. However, we are not there yet and need to further optimize shuffles to make that happen. But, we can optimize `selectFrom` with the dependent change to wrap indexes instead of throwing when out of bounds.

(Separately there is an annoying issue with select, that we should not address in this PR. Using a Float/Double Vector for indexes is awkward.)

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PR Review Comment: https://git.openjdk.org/jdk/pull/20508#discussion_r1759182233