RFR: 8361144: Strenghten the Ideal Verification in PhaseIterGVN::verify_Ideal_for by comparing the hash of a node before and after Ideal

[Galder Zamarreño]

On Tue, 1 Jul 2025 11:35:06 GMT, Benoît Maillard <bmaillard at openjdk.org> wrote:

> This PR adds a node hash comparison after calling `Ideal` in `PhaseIterGVN::verify_Ideal_for` to introduce an additional layer of verification for missed optimizations. Previously, we relied on the return value of `Ideal`, which is expected to be `nullptr` if no transformation was done.
> 
> By also checking the node's hash before and after `Ideal`, we could catch inconsistencies in the implementation or unintended modifications to the graph. Both of these may indicate missed or incomplete optimizations.
> 
> ### Testing
> - [x] [GitHub Actions](https://github.com/benoitmaillard/jdk/actions?query=branch%3AJDK-8361144)
> - [x] tier1-3, plus some internal testing
> 
> Thank you for reviewing!

Have you considered adding a test for this? Is that feasible?

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PR Review: https://git.openjdk.org/jdk/pull/26064#pullrequestreview-2975520753