RFR: 8342095: Add autovectorizer support for subword vector casts [v15]

[Emanuel Peter]

On Tue, 27 Jan 2026 03:11:05 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> Ok, I filed this as another follow-up:
>> [JDK-8376179](https://bugs.openjdk.org/browse/JDK-8376179): C2 SuperWord: improve subword vectorization, avoid cast to-and-from int
>
> Hi @eme64, thanks a lot for the review! I've pushed an update that should address the review comments and update the bug annotations and copyright years. About the cast to and from int, the only places where that should be required is when the node doesn't support truncation. Right now it looks like reductions also cast to int even when they're not required, such as with `AndReduction`. I can do some further investigation in a followup patch to see where the int vectors are generated.

@jaskarth Something is going a bit strange with my testing script. Could you merge with latest master, maybe that helps?

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PR Comment: https://git.openjdk.org/jdk/pull/23413#issuecomment-3806048519