RFR: 8370691: Add new HalffloatVector type and enable intrinsification of float16 vector operations

[Paul Sandoz]

On Mon, 27 Oct 2025 15:19:48 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> Add new HalffloatVector type and corresponding concrete vector classes in addition to existing primitive vector types, maintaining operation parity with the FloatVector type.
> - Add necessary inline expander support.
>    - Enable intrinsification for a few vector operations, namely ADD/SUB/MUL/DIV/MAX/MIN/FMA.
> - Use existing Float16 vector IR and backend support.
> - Extended the existing VectorAPI JTREG test suite for the newly added HalffloatVector operations.
>  
> The idea here is to first be at par with Float16 auto-vectorization support before intrinsifying new operations (conversions, reduction, etc).
> 
> The following are the performance numbers for some of the selected HalfflotVector benchmarking kernels compared to equivalent Float16OperationsBenchmark kernels.
> 
> <img width="837" height="650" alt="{A2BA2D85-085A-489F-8DDD-0FCFB5986EA5}" src="https://github.com/user-attachments/assets/2f34138e-80f4-4870-9b5f-f64bf43c1b11" />
> 
> Initial RFP[1] was floated on the panama-dev mailing list.
> 
> Kindly review the draft PR and share your feedback.
> 
> Best Regards,
> Jatin
> 
> [1] https://mail.openjdk.org/pipermail/panama-dev/2025-August/021100.html

Some quick comments.

We should be consistent in the naming, and rename `Halfloat*` to `Float16*`.

When you generate the fallback code for unary/binary etc can you push the carrier type and conversations into the uOp/bOp implementations so you don't have to explicitly operate on the carrier type and do the conversions as you do now e.g.,:

v0.uOp(m, (i, a) -> float16ToShortBits(Float16.valueOf(-(shortBitsToFloat16(($type$)a).floatValue()))));


The transition of intrinsic arguments from `vsp.elementType()` to `vsp.carrierType(), vsp.operType()` is a little unfortunate. Is this because HotSpot cannot directly refer to the `Float16` class from the incubating module? Requiring two arguments means they can get out of sync. Previously the class provided all the information needed, now arguably the type does.

-------------

PR Review: https://git.openjdk.org/jdk/pull/28002#pullrequestreview-3445662107