RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v16]

[Andrew Haley]

On Thu, 24 Jul 2025 09:29:43 GMT, Bhavana Kilambi <bkilambi at openjdk.org> wrote:

>> src/hotspot/cpu/aarch64/aarch64_vector.ad line 261:
>> 
>>> 259: 
>>> 260:         // Because the SVE2 "tbl" instruction is unpredicated and partial operations cannot be generated
>>> 261:         // using masks, we currently disable this operation on machines where length_in_bytes <
>> 
>> Suggestion:
>> 
>>         // using masks, we disable this operation on machines where length_in_bytes <
>
> Thanks. I use "currently" here because even now we can add support for cases where `length_in_bytes < MaxVectorSize && length_in_bytes > 8` but we currently do not have machines with SVE2 enabled and `length_in_bytes < MaxVectorSize && length_in_bytes > 8` for example - we do not have an SVE2 machine with `MaxVectorSize = 256` to test this operation for `length_in_bytes = 128` but we can add that support in future if such machines become available for testing.

Sure, but "currently" doesn't help the reader to understand that. If you want to say we don't support this because at the time of writing we don't have machines with SVE2 enabled and length_in_bytes >128 so we can't test it, then say so. Explicitly.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2228171020