RFR: 8342095: Add autovectorizer support for subword vector casts [v4]

[Jasmine Karthikeyan]

On Thu, 13 Feb 2025 09:57:16 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Jasmine Karthikeyan has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add new conversions to benchmark
>
> src/hotspot/cpu/x86/matcher_x86.hpp line 273:
> 
>> 271:     if (to_bt == from_bt) {
>> 272:       return false;
>> 273:     }
> 
> Hmm, do we expect that this ever gets triggered? Or would that be a bug? Maybe not, but could be worth adding a defensice assert here, what do you think?

This is a good point, I think it shouldn't be possible to run into the case where `to_bt` and `from_bt` are the same here so a defensive assert would be better here.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23413#discussion_r1954844820