RFR: 8283091: Support type conversion between different data sizes in SLP [v3]

[Vladimir Kozlov]

On Fri, 8 Apr 2022 07:15:03 GMT, Fei Gao <fgao at openjdk.org> wrote:

>>> costing more than scalar instructions, as we know that there are only two elements for VectorCastD2I on 128-bit NEON machine.
>> 
>> So shall we disable `vcvt2Dto2I` for NEON?
>
>> So shall we disable `vcvt2Dto2I` for NEON?
> 
> I'm afraid we can't. We still need to support it in VectorAPI.

@fg1417  Thank you for suggesting this optimization. I see that it was not updated for some time. Do you still intend to work on it?

Please update to latest JDK (Loom was integrated) and run performance again. Also include % of changes.

I have the same concern as @DamonFool about regression when vectorizing some conversions. May be we should have additional `Matcher` property we could consult when trying to **auto-vectorize**. I understand that we need `vcvt2Dto2I` when VectorAPI specifically asking to generate it but we should not enforce auto-generation.

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PR: https://git.openjdk.java.net/jdk/pull/7806