Integrated: 8350578: Refactor useless Parse and Template Assertion Predicate elimination code by using a PredicateVisitor

[Christian Hagedorn]

On Wed, 12 Mar 2025 16:18:53 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

> This patch cleans the Parse and Template Assertion Predicate elimination code up. We now use a single `PredicateVisitor` and share code in a new `EliminateUselessPredicates` class which contains the code previously found in `PhaseIdealLoop::eliminate_useless_predicates()`.
> 
> ### Unified Logic to Clean Up Parse and Template Assertion Predicates
> We now use the following algorithm:
> https://github.com/openjdk/jdk/blob/5e4b6ca0ddafa80eee60690caacd257b74305d4e/src/hotspot/share/opto/predicates.cpp#L1174-L1179
> 
> This is different from the old algorithm where we used a single boolean state `_useless`. But that does no longer work because when we first mark Template Assertion Predicates useless, we are no longer visiting them when iterating through predicates:
> 
> https://github.com/openjdk/jdk/blob/a21fa463c4f8d067c18c09a072f3cdfa772aea5e/src/hotspot/share/opto/predicates.hpp#L704-L708
> 
> We therefore require a third state. Thus, I introduced a new tri-state `PredicateState` that provides a special `MaybeUseful` value which we can set each Predicate to.
> 
> #### Ignoring Useless Parse Predicates
> While working on this patch, I've noticed that we are always visiting Parse Predicates - even when they useless. We should change that to align it with what we have for the other Predicates (changed in JDK-8351280). To make this work, we also replace the `_useless` state in `ParsePredicateNode` with a new `PredicateState`.
> 
> #### Sharing Code for Parse and Template Assertion Predicates
> With all the mentioned changes in place, I could nicely share code for the elimination of Parse and Template Assertion Predicates in `EliminateUselessPredicates` by using templates. The following additional changes were required:
> 
> - Changing the template parameter of `_template_assertion_predicate_opaques` to the more specific `OpaqueTemplateAssertionPredicateNode` type.
> - Adding accessor methods to get the Predicate lists from `Compile`.
> - Updating `ParsePredicate::mark_useless()` to pass in `PhaseIterGVN`, as done for Assertion Predicates
> 
> Note that we still do not directly replace the useless Predicates but rather mark them useless as initiated by JDK-8351280.
> 
> ### Other Included Changes
> - During the various refactoring steps, I somehow dropped the code to add newly cloned Template Assertion Predicate to the `_template_assertion_predicate_opaques` list. It was done directly in the old cloning methods. This is not relevant for correctness but could hinder some optimizations. I've added the code now i...

This pull request has now been integrated.

Changeset: e57b2725
Author:    Christian Hagedorn <chagedorn at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/e57b2725065eaa79db7f9132f4152cbae9279f8e
Stats:     460 lines in 12 files changed: 263 ins; 143 del; 54 mod

8350578: Refactor useless Parse and Template Assertion Predicate elimination code by using a PredicateVisitor

Reviewed-by: epeter, kvn, roland

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PR: https://git.openjdk.org/jdk/pull/24013