RFR: 8342095: Add autovectorizer support for subword vector casts [v3]

[Emanuel Peter]

On Mon, 17 Feb 2025 15:00:51 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> @jaskarth just ping me whenever I should have a look again!
>
> @eme64 I think it should be good for another look over! I've addressed your review comments in the last commit.
> 
> About the potential for performance degradation, I think it would be unlikely since the code generated by the cast is quite small (as it only needs to truncate or sign-extend) and the patch increases the amount of possible code that can auto-vectorize. The one case that I can think of is that it might cause code that would be otherwise unprofitable to become vectorizable, but that would be because we don't have a cost model yet.

@jaskarth Let me know if there is anything we can help you with here :)

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PR Comment: https://git.openjdk.org/jdk/pull/23413#issuecomment-2742838142