RFR: 8370914: C2: Reimplement Type::join [v4]

[Quan Anh Mai]

On Sat, 22 Nov 2025 00:44:56 GMT, Dean Long <dlong at openjdk.org> wrote:

>> Quan Anh Mai has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:
>> 
>>  - Merge branch 'master' into typejoin
>>  - Move dual to ASSERT only
>>  - Keep old version for verification
>>  - whitespace
>>  - Reimplement Type::join
>
> Also, I think it would make sense to target this for jdk 27 after the fork.

@dean-long I think you are misunderstanding, the answer is incorrect because the result it gives, `java/nio/file/spi/FileSystemProvider:AnyNull *,iid=top`, is empty, while the correct answer is the set which contains the single value `null`, and is not empty. The reason for this inaccuracy is that there are 2 LCAs for the inputs on the lattice, and they do not subtype each other, we choose the wrong one out of those LCAs.

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PR Comment: https://git.openjdk.org/jdk/pull/28051#issuecomment-3566574122