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

[Bhavana Kilambi]

On Wed, 18 Jun 2025 08:57:45 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>>> Yes, 0-7 for 64-bit Byte but this op is selecting from two vectors so the index should contain values in the range of [0, 2 * VLEN)
>> 
>> Yes, that's the problem. Thanks!
>
>> > 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

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2154058041