RFR: 8363989: AArch64: Add missing backend support of VectorAPI expand operation

[Emanuel Peter]

On Tue, 9 Sep 2025 09:26:31 GMT, erifan <duke at openjdk.org> wrote:

>> The algorithm description here is great. Please paste all of it from "Since there are" to "but with different instructions where appropriate." into this PR, before the vector expand implementation.
>
> @theRealAph @e1iu could you help take another look of this PR, thanks !

@erifan I'm seeing `gtest/GTestWrapper.java` fail on `aarch64` machines.

Looks like this:

[ RUN      ] AssemblerAArch64.validate_vm
[12.324s][warning][os] Loading hsdis library failed
.../test/hotspot/gtest/aarch64/test_assembler_aarch64.cpp:49: Failure
Expected equality of these values:
  insns[i]
    Which is: 335545527
  insns1[i]
    Which is: 335545526
Ours:

Loading hsdis library failed, undisassembled code is shown in MachCode section
[MachCode]
  0x0000ffff97c20548: b604 0014 
[/MachCode]
Theirs:

Loading hsdis library failed, undisassembled code is shown in MachCode section
[MachCode]
  0x0000ffff853eb0a8: b704 0014 
[/MachCode]

Could this be related?

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PR Comment: https://git.openjdk.org/jdk/pull/26740#issuecomment-3296237445