RFR: 8349032: C2: Parse Predicate refactoring in Loop Unswitching broke fix for JDK-8290850 [v2]

[Vladimir Kozlov]

On Fri, 21 Feb 2025 22:31:15 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> In the refactoring for [JDK-8344035](https://bugs.openjdk.org/browse/JDK-8344035), the value passed for the `rewire_uncommon_proj_phi_inputs` parameter in `PhaseIdealLoop::create_new_if_for_predicate()` during Loop Unswitching was accidentally flipped. It should only be set to `true` when calling it for a false-path loop, which is the cloned loop. This is currently not the case and leads to a bad graph due to folding nodes wrongly:
>> https://github.com/openjdk/jdk/blob/735805d9259037ae594eb4f75e96860d43feea5d/src/hotspot/share/opto/predicates.cpp#L84-L88
>> 
>> I fixed this by just flipping the parameter from `is_true_path_loop` to `is_false_path_loop` to avoid a negation. I added an additional comment to `PhaseIdealLoop::create_new_if_for_predicate()` about `rewire_uncommon_proj_phi_inputs`.
>> 
>> More background about why we need `rewire_uncommon_proj_phi_inputs` in the first place can be found in the corresponding fix for [JDK-8290850](https://bugs.openjdk.org/browse/JDK-8290850): https://github.com/openjdk/jdk/pull/11452, and additionally in https://github.com/openjdk/jdk/pull/5185
>> 
>> Thanks,
>> Christian
>
> Christian Hagedorn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   restore comment line length

Good.

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Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23712#pullrequestreview-2634459527