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

[Bhavana Kilambi]

On Tue, 17 Jun 2025 08:54:47 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Bhavana Kilambi has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Addressed review comments and added a JTREG test
>
> src/hotspot/cpu/aarch64/aarch64_vector_ad.m4 line 5155:
> 
>> 5153: // values across function calls and usually used for long-lived values), we can use any two volatile
>> 5154: // registers between V16-V31.
>> 5155: instruct vselect_from_two_vectors_HS_Neon(vReg dst, vReg_V17 src1, vReg_V18 src2,
> 
> I think it's worth replicating this pattern a couple of times with more vector registers. The allocator might prefer some in the set v8-v15.

I tried to add 4 different variants for each match rule and chose the registers at random (two from the volatile and two from the non-volatile set). Please review if this is ok? Thanks!

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2164023985