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

[erifan]

On Tue, 4 Nov 2025 07:05:42 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> I added a rule for cases where the `MaxVectorSize` > `16/32/64`, so that these cases will also be tested once they are supported.
>> @eme64  Would you mind take another look, thanks!
>
> @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`

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PR Review Comment: https://git.openjdk.org/jdk/pull/27723#discussion_r2489103183