RFR: 8369456: [TESTBUG] Fix the test failure of TestSelectFromTwoVectorOp.java on sve2 platforms [v2]

[erifan]

On Tue, 4 Nov 2025 08:24:56 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Hi @eme64 machines that support SVE2 will necessarily also support SVE1 and NEON. The situation is like this:
>> 
>> 1. sve1 and sve2 use different implementations. sve1 supports all cases, but sve2 currently does not support partial cases where `vector_length_in_byte > 16B`.
>> 2. From [aarch64_vector.ad](https://github.com/openjdk/jdk/blob/4f9f086847f531ab1791727d74955cfd8ec56811/src/hotspot/cpu/aarch64/aarch64_vector.ad#L273) these cases are not supported on sve2 because there is no such hardware, not because it's impossible to support them. Currently, all available machines on the market that support SVE2 are 128-bit, so even if we implement support for a larger width, it won't be usable.
>> 3. The sve1 implementation can also run on sve2 machines with `-XX:UseSVE=1`
>
> @erifan Ok, thanks for the explanations! I thought it was probably just because the VM implementation is limited at the moment. Once hardware is available, there would probably be a bigger investment in making SVE2 work for larger vector lengths.

Yeah, that's usually the case. Anyway, this only concerns performance and doesn't affect correctness, since there's a default Java implementation.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27723#discussion_r2489227810