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

[Bhavana Kilambi]

On Wed, 18 Jun 2025 08:55:31 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?
>
>> The valid index for a 64-bit byte should be 0-8, right? From the API level of SelectFromTwoVector, it seems the values inside of the index vector is in range of VLEN or 2 * VLEN ?
> 
> 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)

> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2154051232