[vectorIntrinsics+compress] RFR: 8274664: Add support for compress/expand api methods

[Paul Sandoz]

On Fri, 1 Oct 2021 22:33:43 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

> Add support for the cross lane operations to compress/expand the lane elements of a vector selecting lanes under the control of a mask. The remaining lanes of the destination vector could be either set to zero or get the value from corresponding elements of the background vector.
> The following methods are added:
>  public abstract Vector<E> compress(VectorMask<E> m);
>  public abstract Vector<E> compress(VectorMask<E> m, Vector<E> v);
>  public abstract Vector<E> expand(VectorMask<E> m);
>  public abstract Vector<E> expand(VectorMask<E> m, Vector<E> v);

This is an great start. I think it best to get the code in and iterate.

It should be possible to reduce some code by composition e..g. compress(m) === compress(m. zeroVector), at least from a test assertion perspective. Unsure from an intrinsic perspective.

Also i wonder for `compress(m, v)` whether values from `v` should drawn starting from zero rather than popcount of `m`, thus allowing easier partitioning:

  pFalse = v.compress(m.not());
  p = v.compress(m, pFalse);

Again, i am unsure from an intrinsic perspective.

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Marked as reviewed by psandoz (Committer).

PR: https://git.openjdk.java.net/panama-vector/pull/143