On Tue, 30 Sep 2025 06:22:52 GMT, Tobias Hartmann <[email protected]> wrote:
>> ## Act I: dead loops >> >> [JDK-8336003: [lworld] TestLWorld::test151 triggers "Should have been >> buffered" assert](https://bugs.openjdk.org/browse/JDK-8336003), allows to >> replace a Phi node by a single input recursively through phis. This change >> was only added to Valhalla. In mainline, a Phi can be simplified to its >> single direct input, not through other phis. The valhalla version will work >> on loops, while the mainline version will reduce all the phis into a single >> input only if the blob of Phis is acyclic. >> >> When a loop is dead, it is possible that the said single input is also an >> output of the phi. After applying the identity, the phi's unique input >> becomes its own output (or input). For most nodes, that is not allowed. This >> is probably relatively harmless as the whole code around is dead and will be >> removed, but it makes some verification fail. It is also possible that some >> other idealization are not protected against data loops without phis on the >> way, and would not terminate. It is interesting to notice that applying >> [JDK-8336003](https://bugs.openjdk.org/browse/JDK-8336003) to mainline is >> enough to reproduce the bug there too. >> >> We could detect when it's about to happen, and handle the situation >> differently if we are about to create a very small loop that would be caught >> by the dead loop detection. It would be possible to make big dead data loop. >> How annoying that is? Immediately, there is the non-termination problem >> mentioned above. But also, maybe some nodes can be optimized away by IGVN >> and end up with a small loop and then the assert would strike again. Is the >> dead loop check too weak then? It depends what we think is the purpose of >> this check. During IGVN, we cannot clean up eagerly every dead loop since it >> would be too expensive to traverse everything. Avoiding dead data loop would >> also need a lot of traversal. My understanding is that it's rather a sanity >> check, to make sure that one doesn't mess up the graph surgery and create >> dead loops accidentally, when something else was meant, and detecting as >> soon as possible is helpful. So I'm not sure it's worth strengthening the >> check. >> >> A way to avoid the creation of dead data loop is to simply limit the >> simplification allowed by >> [JDK-8336003](https://bugs.openjdk.org/browse/JDK-8336003) to constant >> nodes: since they don't have input, they can't make a cycle! And it seems >> enough for the bug initially reported. >> >> Yet, that makes `test151` in >> `compiler/valhalla/inlinetypes/TestLWorld.java... > > src/hotspot/share/opto/cfgnode.cpp line 1482: > >> 1480: } >> 1481: { >> 1482: Node* uin = unique_constant_input_recursive(phase); > > I think the original format was fine but I have no strong opinion. I wouldn't fight too hard for it, but when trying things, it was necessary (since I wanted different types for `uin`), but I also think it helps to see that it is not useful too long, and I can mess with it without having to look at what comes after. In this case, indeed, it feels a bit coming out of nowhere at the end... In C++17, I'd do: ```c++ if (Node* uin = unique_constant_input_recursive(phase); uin != nullptr) { return uin; } one could also do ```c++ if (Node* uin = unique_constant_input_recursive(phase)) { return uin; } if we are not afraid of implicit conditions (but we are, and I'm fine with it!). ------------- PR Review Comment: https://git.openjdk.org/valhalla/pull/1640#discussion_r2390390461
