RFR: 8365570: C2 fails assert(false) failed: Unexpected node in SuperWord truncation: CastII

[Emanuel Peter]

On Thu, 21 Aug 2025 06:18:12 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> My thinking was that since casts specifically change the type of a node, they may not be safe to truncate. In practice it might not matter because after the CastII pack is created, it's discarded because there is no backend implementation for vectorized CastII. I've opted to mark them as non-truncating to stay on the safer side.
>
> I see. Ok. Can you add a comment to the code for that?
> Because imagine we come along later and actually implement a backend vectorized version of CastII (no-op?). Maybe because we implement if-conversion. Then it would be nice to know if this was just a "to be on the safe side" check, or if it would run into issues when removed.

Because the current comment says "should not truncate". That sounds more strong than "to be on the safe side".

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PR Review Comment: https://git.openjdk.org/jdk/pull/26827#discussion_r2289966071