RFR: 8342095: Add autovectorizer support for subword vector casts [v12]

[Emanuel Peter]

On Thu, 15 May 2025 02:30:46 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> @jaskarth Excellent! I'll run another round of testing, just to be sure :)
>> Please ping me again in 24h!
>
> @eme64 Pinging for test results :)

@jaskarth I do have one more question: I have seens patterns with `LShift` and `RShift` for sign extension in the past. For example, if we cast a signed `byte` to a signed `short`, we have to do sign extension, right? Do any of these patterns appear in your benchmarks here? Do you know under which circumstances they appear? Maybe the simple conversion examples we have here are too limited to cover all cases. Not saying that should stop integration, just wondering if it is worth finding an example and filing an RFE just to keep this as a direction to explore in the future ;)

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PR Comment: https://git.openjdk.org/jdk/pull/23413#issuecomment-2882768635