RFR: 8335708: C2: Compile::verify_graph_edges must start at root and safepoints, just like CCP traversal [v3]

[duke]

On Thu, 20 Mar 2025 14:33:55 GMT, Marc Chevalier <duke at openjdk.org> wrote:

>> In CCP, we transform the nodes going up (toward inputs) starting from root and safepoints because infinite loops can be reachable from the root, but not co-reachable from the root, that is one can follow def-use from root to the loop, but not the use-def from root to loop. For more details, see:
>> https://github.com/openjdk/jdk/blob/4cf63160ad575d49dbe70f128cd36aba22b8f2ff/src/hotspot/share/opto/phaseX.cpp#L2063-L2070
>> 
>> Since we are specifically marking nodes as useful if they are above a safepoint, the check that no dead nodes must be there anymore must also consider nodes above a safepoint as alive: the same criterion must apply. We should nevertheless not start from a safepoint killed by CCP.
>> 
>> About the test, I use this trick found in `TestInfiniteLoopCCP` because I indeed need a really infinite loop, but I want a terminating test. The crash is not deterministic, as it needs StressIGVN, so I did a bit of stats. Using a little helper script, on 100 runs, 69 runs fail as in the JBS ticket and 31 are successful (so 0 fail in another way). After the fix, I find 100 successes.
>> 
>> And thanks to @eme64  who extracted such a concise reproducer.
>
> Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:
> 
>   mostly punctuation

@marc-chevalier 
Your change (at version 6a149e23f61fc61ec8175b8fef138b768b180015) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk/pull/23977#issuecomment-2740686065