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

[Marc Chevalier]

On Tue, 11 Mar 2025 09:12:44 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.

This pull request has now been integrated.

Changeset: 2bc4f64c
Author:    Marc Chevalier <marc.chevalier at oracle.com>
URL:       https://git.openjdk.org/jdk/commit/2bc4f64c56ebc844d494a4ce8ba72a25643d4075
Stats:     114 lines in 5 files changed: 98 ins; 0 del; 16 mod

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

Reviewed-by: chagedorn, epeter

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PR: https://git.openjdk.org/jdk/pull/23977