RFR: 8348678: [PPC64] C2: unaligned vector load/store is ok [v2]

[Martin Doerr]

On Wed, 29 Jan 2025 15:51:03 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> This pr changes `Matcher::misaligned_vectors_ok`  to return `true` on PPC64 for better vectorization during `SuperWord`.
>> IR checking of the corresponding test `TestCastX2NotProcessedIGVN.java` is also enabled.
>> 
>> Tested with `TestCastX2NotProcessedIGVN.java`
>> 
>> The change passed our CI testing:
>> Tier 1-4 of hotspot and jdk. All of langtools and jaxp. Renaissance Suite and SAP specific tests.
>> Testing was done on the main platforms and also on Linux/PPC64le and AIX.
>
> Richard Reingruber has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use -AlignVector by default for POWER8 and later

Should `AlignVector` be disabled regardless of the Power Architecture version? `SuperwordUseVSX` already requires Power8. The 8 Byte load/store instructions which are used otherwise have no alignment requirements, either. But, Power7 is basically dead and the support will probably be removed, soon (https://bugs.openjdk.org/browse/JDK-8331859). So, I'm ok with it.

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PR Comment: https://git.openjdk.org/jdk/pull/23318#issuecomment-2622042634