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

[Marc Chevalier]

On Mon, 17 Mar 2025 06:41:37 GMT, Emanuel Peter <epeter 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.
>
> test/hotspot/jtreg/compiler/loopopts/Test8335708.java line 29:
> 
>> 27:  * @summary Crash Compile::verify_graph_edges
>> 28:  * @requires vm.debug == true & vm.flavor == "server"
>> 29:  * @library /test/lib
> 
> Suggestion:
> 
> 
> Can you test if you actually need this line?

I mach5'ed it. Without it, the `import jdk.test.lib.Utils;` seems to be failing, and the subsequent `Utils.adjustTimeout(500)` doesn't know about `Utils`. Re-introduced it and it works. So it seems, I need this line.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23977#discussion_r2001001385