RFR: 8371065: C2 SuperWord: VTransformLoopPhiNode::apply set wrong type, led to wrong constant folding of phi [v2]

[Emanuel Peter]

On Thu, 6 Nov 2025 17:03:02 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   add diagnostic flag for product build
>
> LGTM, another approach may be to add a method `Type::local_bottom` that will return the local bottom of a type (e.g. `Type::INT` for an `Int`).

@merykitty Thanks for the approval! I just noticed that the attached test failed because of a missing `-XX:+UnlockDiagnosticVMOptions` for product.

Yes, there are a few alternatives. I don't really want to extend the type system in this bugfix... I also don't really want to mess with the types of all phis in the loop, because we could also lose information on some nodes that way. For example, the tripcount may have a restricted range, and I don't want to just widen that to `int`.

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PR Comment: https://git.openjdk.org/jdk/pull/28113#issuecomment-3500897330