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

[Emanuel Peter]

On Tue, 4 Nov 2025 08:05:13 GMT, erifan <duke at openjdk.org> wrote:

>> @erifan That looks like a good set of updates to me :)
>> 
>> General question: can `sve2` not use the vectors from `neon` or `sve`, which do support those vector sizes? But that would be a separate issue anyway, we are for now just trying to fix up the test.
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27723#discussion_r2489203476