RFR: 8306042: C2: failed: Missed optimization opportunity in PhaseCCP (adding LShift->Cast->Add notification) [v4]

[Vladimir Kozlov]

On Tue, 2 May 2023 06:12:45 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> src/hotspot/share/opto/phaseX.cpp line 1961:
>> 
>>> 1959:   // Push root onto worklist
>>> 1960:   worklist.push(C->root());
>>> 1961:   DEBUG_ONLY(Unique_Node_List worklist_verify;)
>> 
>> Should you put `worklist_verify` to `local_arena` too?
>
> @vnkozlov I could do that, but it is not required. The CCP `worklist` gets passed downward much farther, including the  graph walks in `push_child_nodes_to_worklist`. So there it is nice to be able to have `ResourceMarks`. But `worklist_verify` is only modified directly in `PhaseCCP::analyze` and one layer deeper in `PhaseCCP::verify_analyze`. So I would never expect a `ResourceMark` to mess with re-allocation.
> 
> So at this point the only reason to add `worklist_verify` to `local_arena` is to ensure it is de-allocated afterward. We could also use a `ResourceMark` and leave it on `Thread::current()->resource_area()`.
> 
> Anyway, I'll just move it to `local_arena`, after all it is `DEBUG_ONLY`.

ok

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PR Review Comment: https://git.openjdk.org/jdk/pull/13611#discussion_r1182752605