RFR: 8351623: VectorAPI: Add SVE implementation of subword gather load operation [v5]

[Xiaohong Gong]

On Fri, 5 Sep 2025 10:49:44 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Xiaohong Gong has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:
>> 
>>  - Merge 'jdk:master' into JDK-8351623-sve
>>  - Address review comments
>>  - Refine IR pattern and clean backend rules
>>  - Fix indentation issue and move the helper matcher method to header files
>>  - Merge branch jdk:master into JDK-8351623-sve
>>  - 8351623: VectorAPI: Add SVE implementation of subword gather load operation
>
> Looks very interesting. I have a first series of questions / comments :)
> 
> There is definitively a tradeoff between complexity in the backend and in the C2 IR. So I'm yet trying to wrap my head around that decision. I'm just afraid that adding more very specific C2 IR nodes makes things more complicated to do optimizations in the C2 IR.

Hi @eme64 , I just push a commit which added more comments and assertion in the code. This is just a simple fixing to part of your comments. Regarding to the IR refinement, I need more time taking a look. So could you please take another look at the changes relative to method_rename/comment/assertion? Thanks a lot in advance!

> src/hotspot/cpu/aarch64/aarch64_vector.ad line 6008:
> 
>> 6006: // predicate and place in elements of twice their size within
>> 6007: // the destination predicate.
>> 6008: 
> 
> Suggestion:
> 
> 
> unnecessary empty line

This empty line is auto-generated by the m4 file. I tried some methods to clean it, but all fails. So I have to keep it as it is.

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PR Comment: https://git.openjdk.org/jdk/pull/26236#issuecomment-3301972528
PR Review Comment: https://git.openjdk.org/jdk/pull/26236#discussion_r2354773033