RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v15]

[Xiaohong Gong]

On Tue, 15 Jul 2025 12:45:51 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> So the Neon implementation gets kicked in when SVE is not available (UseSVE == 0) whether the vector length is 8 or 16 but we emit Neon instructions for UseSVE ==1 and vector length == 16 only. I am not sure how I can eliminate `UseSVE` here.
>> 
>> When the vector length == 8 with SVE1, we generate the SVE `tbl` instruction (with single input). This is done for `T_INT` and `T_FLOAT` types so that we avoid generating the `mulv`/`addv` instructions for the Neon `tbl` instruction.
>
> But why would the Neon implementation fail if UseSVE ==1? Surely it would still work, and if it still works this comment is wrong.

Yeah, I think this still work when UseSVE >=1.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2209144612