[lworld] RFR: 8359345: [lworld] C2 crashes with -XX:ForceNonTearable=*

[Tobias Hartmann]

On Mon, 30 Jun 2025 12:20:47 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> For test method `testFieldLoad1` the graph looks like this after parsing:
> ![Screenshot from 2025-06-30 13-42-44](https://github.com/user-attachments/assets/94726f40-8260-47ab-86e1-a47b074a6929)
> 
> Now since [JDK-8354068](https://bugs.openjdk.org/browse/JDK-8354068) `MemNode::optimize_simple_memory_chain` rewires the memory input of strict field load `46 LoadL` to before the membar:
> ![Screenshot from 2025-06-30 15-06-13](https://github.com/user-attachments/assets/b8ac826e-1426-4212-8567-0aa1c8da5ad9)
> 
> Later, a precedence edge from the membar to the load (now named `15 LoadL`) is added, leading to an unschedulable graph:
> ![Screenshot from 2025-06-30 13-37-59](https://github.com/user-attachments/assets/ea1e5361-8863-4ebc-b733-155bc53bce72)
> 
> [JDK-8354981](https://bugs.openjdk.org/browse/JDK-8354981) added these `MemBarCPUOrders` around flat, atomic stores and then [JDK-8357474](https://bugs.openjdk.org/browse/JDK-8357474) also added them around loads. The same problem exists for `MemBarCPUOrders` emitted by stores. The memory input of following loads will be wired around them which is incorrect.
> 
> For now, I disabled skipping past `MemBarCPUOrders` in `MemNode::optimize_simple_memory_chain` and filed [JDK-8361145](https://bugs.openjdk.org/browse/JDK-8361145) for a follow-up investigation.
> 
> Thanks,
> Tobias

Thanks for your comments Quan-Anh, I missed them earlier. Let me double-check!

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PR Comment: https://git.openjdk.org/valhalla/pull/1494#issuecomment-3022626693