RFR: 8357781: Deep recursion in PhaseCFG::set_next_call leads to stack overflow

[Marc Chevalier]

On Tue, 27 May 2025 08:43:15 GMT, Manuel Hässig <mhaessig at openjdk.org> wrote:

>> There is nothing very wrong here:
>> - the graph is not broken
>> - the algorithm is correct
>> 
>> It just happens that the graph is very deep, it has a very long, narrow chain of nodes because of crazy unrolling, because of `LoopUnrollLimit=8192`. This depth simply makes the algorithm recurse deeper than the stack size allows. This kind of graph shape is not quite trivial to reproduce. The proposed reproducer is very easy to change into a non-reproducer, with many kinds of change, even that seem harmless to me.
>> 
>> The fix is also pretty direct: let's change the recursive traversal, with a worklist-based iterative one. It's not as elegant, but it doesn't overflow.
>> 
>> How sad stacks are still so bounded...
>
> src/hotspot/share/opto/lcm.cpp line 800:
> 
>> 798: void PhaseCFG::set_next_call(const Block* block, Node* init, VectorSet& next_call) const {
>> 799:   Node_List worklist;
>> 800:   worklist.push(init);
> 
> I guess the only reason `init` is not `const` is because of `Node_List::push()`?

Alas, yes. `Node_List` takes non-const and spit non-const. Unfortunate.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25448#discussion_r2108625514