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

[Damon Fenacci]

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!

Thanks @benoitmaillard! Definitely an additional check worth doing. I left a couple of inline comments.

src/hotspot/share/opto/phaseX.cpp line 1821:

> 1819:   // The number of nodes shoud not increase.
> 1820:   uint old_unique = C->unique();
> 1821:   uint old_hash = n->hash();

Just to be consistent with `old_unique` we could add a small comment (here or below for both). What do you think?

src/hotspot/share/opto/phaseX.cpp line 1838:

> 1836:       stringStream ss; // Print as a block without tty lock.
> 1837:       ss.cr();
> 1838:       ss.print_cr("Ideal optimization did not make progress but hash node changed.");

Suggestion:

      ss.print_cr("Ideal optimization did not make progress but node hash changed.");

-------------

PR Review: https://git.openjdk.org/jdk/pull/26064#pullrequestreview-2977964471
PR Review Comment: https://git.openjdk.org/jdk/pull/26064#discussion_r2179270798
PR Review Comment: https://git.openjdk.org/jdk/pull/26064#discussion_r2179279429