RFR: 8349139: C2: Div looses dependency on condition that guarantees divisor not null in counted loop [v2]

[Roland Westrelin]

On Fri, 14 Feb 2025 18:24:25 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:
>> 
>>  - Merge branch 'master' into JDK-8349139
>>  - fix & test
>
> Hmmm, may be you are right. I think adding a comment at `PhiNode` saying that people must not rely on it being pinned at the `Region` for dependencies would be a wise move, I can't think of any reason for that besides value narrowing right now but being pinned is a property of `Phi` regardless and we should tell people not to rely on this behaviour.
> 
> For this bug, I think a more general fix is to try to compare the type of the `Phi` with that of the input it is going to be replaced with. If the former is not wider than the latter then we add a `CastNode`, since the cast is only about value range, not strict dependency, we can use `CarryDependency` instead of `UnconditionalDependency`. Am I right?

@merykitty see above my late reply to your comments if you missed it.

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PR Comment: https://git.openjdk.org/jdk/pull/23617#issuecomment-2743898063