RFR: 8342095: Add autovectorizer support for subword vector casts

[Emanuel Peter]

On Tue, 4 Feb 2025 19:40:59 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> test/micro/org/openjdk/bench/vm/compiler/VectorSubword.java line 73:
>> 
>>> 71:         }
>>> 72:     }
>>> 73: }
>> 
>> Ah, these are all casting to smaller types. What about casting to larger types?
>
> Originally I thought that larger type conversions were only available on AVX-512 based on the ad-file, but reading it more carefully now I see that they are indeed supported for AVX! I'll go ahead and generalize the changes further to make casting to larger types supported as well. I think this leaves `char` as the only type not supported, which I can look at in a follow-up RFE.

Well it's ok to leave the patch as is, and just have a follow-up RFE that improves things from here. It could make things easier to review.

@jaskarth

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PR Review Comment: https://git.openjdk.org/jdk/pull/23413#discussion_r1942516088
PR Review Comment: https://git.openjdk.org/jdk/pull/23413#discussion_r1942516705