RFR: 8338023: Support two vector selectFrom API

[Jatin Bhateja]

On Mon, 12 Aug 2024 22:03:44 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

> The results look promising. I can provide guidance on the specification e.g., we can specify the behavior in terms of rearrange, with the addition of throwing on out of bounds indexes.
> 
> Regarding the throwing of exceptions, some wider context will help to know where we are heading before we finalize the specification. I believe we are considering changing the default throwing behavior for index out of bounds to wrapping, thereby we can avoid bounds checks. If that is the case we should wait until that is done then update rather than submitting a CSR just yet?
> 
> I see you created a specific intrinsic, which will avoid the cost of shuffle creation. Should we apply the same approach (in a subsequent PR) to the single argument shuffle? Or perhaps if we manage to optimize shuffles and change the default wrapping we don't require a specific intrinsic and can just use defer to rearrange?

Hi @PaulSandoz ,
Thanks for your comments. With this new API we intend to enforce stricter specification w.r.t to index values to emit a lean instruction sequence preventing any cycles spent on massaging inputs to a consumable form, thus preventing redundant wrapping and unwrapping operations.

Existing [two vector rearrange API](https://docs.oracle.com/en/java/javase/22/docs/api/jdk.incubator.vector/jdk/incubator/vector/Vector.html#rearrange(jdk.incubator.vector.VectorShuffle,jdk.incubator.vector.Vector)) has a flexible specification which allows wrapping out of bounds shuffle indexes into exceptional index with a -ve value.

Even if we optimize existing two vector rearrange implementation we will still need to emit additional instructions to generate an indexes which lie within two vector range [0, 2*VLEN). I see this as a specialized API like vector compress/expand which cater to targets like x86-AVX512+ and aarch64-SVE which offers direct instruction for two vector lookups.

May be the API nomenclature can be refined to better reflect its semantics i.e. from selectFrom to twoVectorLookup ?

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PR Comment: https://git.openjdk.org/jdk/pull/20508#issuecomment-2288062038