RFR: 8370666: VectorAPI: Add clear comments for vector relative code in c2 [v3]

[Emanuel Peter]

On Fri, 16 Jan 2026 08:50:02 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> src/hotspot/share/opto/vectornode.hpp line 1912:
>> 
>>> 1910: // Unsigned vector cast operations can only be used in Vector API unsigned
>>> 1911: // extensions between integral types so far. E.g., extending byte to float
>>> 1912: // is not supported now.
>> 
>> Actually, we could totally implement it for the Auto Vectorizer. It would be methods like `Byte.toUnsignedInt`.
>
> I filed it here with an example:
> [JDK-8375502](https://bugs.openjdk.org/browse/JDK-8375502) C2 SuperWord: implement unsigned casts
> 
> If anybody is interested in taking on this task, feel free to reassign it to yourself :)

I'd suggest to remove the comment about "Vector API only" in the code. Instead, I would explain that unsigned cast means zero-extension if dst is larger than src.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29130#discussion_r2697536340