RFR: 8375038: C2: Enforce that Ideal() returns the root of the subgraph if any change was made by checking the node hash [v2]

[Benoît Maillard]

On Fri, 30 Jan 2026 09:19:32 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

>> Benoît Maillard has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Update src/hotspot/share/opto/node.cpp
>>   
>>   Co-authored-by: Roberto Castañeda Lozano <robcasloz at users.noreply.github.com>
>
> Is it going to be added in testing? Maybe we can at least add it to
> https://github.com/openjdk/jdk/blob/42370e22c5bc4ebd40fd500a2e6e9e07f0b8bcd8/test/hotspot/jtreg/compiler/c2/TestVerifyIterativeGVN.java#L24-L37
> 
> There starts to be a lot of sub-flags in this flag. Would is be meaningful to merge the new one with
> https://github.com/openjdk/jdk/blob/42370e22c5bc4ebd40fd500a2e6e9e07f0b8bcd8/src/hotspot/share/opto/c2_globals.hpp#L702
> Since it's also about verifying whether something changed? That would mean fixing everything before merging this, alas. And if I'm fine with merged flags that enable more things than maybe useful, I can also spam the `1`s for `VerifyIterativeGVN`.

@marc-chevalier I have added the bit flag to `TestVerifyIterativeGVN.java`, thanks for suggesting it.

The main reason for having a separate bit flag is that this assert is likely to cause quite a few failures in existing tests, so I would like to avoid the risk of breaking too many things. I plan to run some testing with this new assert (I already have pending changes waiting for this flag bit to be properly tested), and then we can gradually fix these failures and why not merging it with the bit for verifying def-use (`A`).

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PR Comment: https://git.openjdk.org/jdk/pull/29421#issuecomment-3859447637