RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v3]

[Andrew Haley]

On Wed, 18 Jun 2025 08:59:25 GMT, Bhavana Kilambi <bkilambi at openjdk.org> wrote:

>>> > Or, maybe we can just disable/unsupport the cases for this op that the vector size is smaller than the max vector size and the max vector size >128-bits. And keep the 64bits as it is. We should add comment and revisit this part once 256bits SVE2 machine is supported in future. WDYT?
>>> 
>>> This sounds better than adding complicated code which may not likely be executed in the near future. We can revisit when adequate support is added.
>> 
>> Agree.
>
> I will add the necessary code to enable/disable these scenarios. Thanks for your valuable insights into this @XiaohongGong

> > Or, maybe we can just disable/unsupport the cases for this op that the vector size is smaller than the max vector size and the max vector size >128-bits. And keep the 64bits as it is. We should add comment and revisit this part once 256bits SVE2 machine is supported in future. WDYT?
> 
> This sounds better than adding complicated code which may not likely be executed in the near future. We can revisit when adequate support is added.

I agree. Please treat it s a general rule that we do not ship code that cannot be tested.

Also, quoth Dijkstra: “Simplicity is a great virtue but it requires hard work to achieve it and education to appreciate it. And to make matters worse: complexity sells better.” It's not worth adding complexity to the compiler when the advantage is so rare (and the effect so small) that no one will ever significantly benefit from it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2154080977