RFR: 8321014: RISC-V: C2 VectorLoadShuffle [v3]

[Hamlin Li]

On Tue, 23 Apr 2024 01:46:33 GMT, Fei Yang <fyang at openjdk.org> wrote:

>> Hamlin Li has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
>> 
>>  - refine code
>>  - Merge branch 'master' into vector-load-shuffle
>>  - add comment
>>  - Initial commit
>
> src/hotspot/cpu/riscv/riscv_v.ad line 81:
> 
>> 79:       case Op_VectorLoadShuffle:
>> 80:       case Op_VectorRearrange:
>> 81:         // vlen >= 4 is required, because min vector size for byte is 4 on riscv,
> 
> I was not aware of such a constraint before. Is this a constraint at the ISA level or is a performance consideration? I didn't find where it is mentioned in the RVV spec.

No, it's an existing constraint in jdk itself, please check the code in `Matcher::min_vector_size`, I think it's partially for performance consideration. It's the same as aarch64 and x86.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18835#discussion_r1575773427