RFR: 8347901: C2 should remove unused leaf / pure runtime calls [v3]

[Marc Chevalier]

On Tue, 8 Jul 2025 15:16:16 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> We could but it's not that direct. `ModFNode::Ideal` has 6 `return`s (without mine):
>> - 3 are `return replace_with_con(...);` which in their turn return `nullptr` but after making changes in the graph.
>> - 2 are `return nullptr;`
>> - 1 is actually returning a node.
>> And especially the final one is
>> 
>> return replace_with_con(igvn, TypeF::make(jfloat_cast(xr)));
>> 
>> If we change `replace_with_con` to actually return a `TupleNode` to do the job, we still have 2 places where to call the base class' `Ideal`. So I'm not sure how much better it would be to duplicate the call. It also adds a maintenance burden: if one adds another case where we don't want to make changes, one needs to add another call to `CallLeafPureNode::Ideal`. I think it's because of the structure of this function: rather than selecting cases where we want to do something and reaching the end with only the leftover cases, we select the cases we don't want to continue with, and we return early, making more cases where we should call the super method.
>> 
>> I'll try something.
>
> Ah, makes sense. Feel free to leave as-is then.

There, you can see the thing I've tried. It changes a bit more code, but overall, I think it makes it clearer and address your comment.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25760#discussion_r2194912203