RFR: 8350864: C2: verify structural invariants of the Ideal graph [v5]

[Marc Chevalier]

On Mon, 8 Sep 2025 15:08:54 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   One more ResourceMark
>
> src/hotspot/share/opto/graphInvariants.cpp line 45:
> 
>> 43:       }
>> 44:     }
>> 45:   }
> 
> It seems you are assuming that all CFG nodes are reachable "from below".
> That is true in most cases... but:
> Have we not had this pesky case where we have a "infinite loop", where there is really no reachability from below, but from above it is reachable.
> 
> See `_root_and_safepoints` in `PhaseCCP`. I'm not sure we need to worry about this, but I'd like to be sure that we have considered infinite loops here.
> 
> The risk is that otherwise you just call those nodes dead, and do not verify them, right? Or you would just ignore failures there.

I guess that is the risk, but I'm going from root, and follow the outputs, I'm checking reachability from above, no?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26362#discussion_r2332296280