RFR: 8340454: C2 EA asserts with "previous reducible Phi is no longer reducible before SUT" [v3]

[Tobias Hartmann]

On Thu, 31 Oct 2024 21:53:45 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

>> Please, consider this patch to fix an issue that happens when a Phi previously considered reducible become later irreducible. The overall situation that causes the problem is like so:
>> 
>> - Consider that there are at least 2 scalar replaceable objects (Obj1 and Obj2; Obj2 is stored in a field of Obj1) when we start iterating the loop at escape.cpp:301
>> 
>> - In the first iteration of the loop the call chain starting with `adjust_scalar_replaceable_state` ends up calling `can_reduce_phi` and considering Phi1 as reducible. This Phi has only Obj1 as *SR* input.
>> 
>> - In another iteration of the loop Obj2 is flagged as NSR. For instance, because we are storing Obj2 in an unknown position of an array. This will cause `found_nsr_alloc` to be set to `true`.
>> 
>> After the loop finishes, the execution will go to `find_scalar_replaceable_allocs`. The code will process Obj1, because it's still scalar replaceable, but will find that this object is stored in a field of a - **now** - NSR object. Therefore, correctly, Obj1 will also be marked as NSR. When Obj1 is marked as NSR Phi1 becomes irreducible because it doesn't have any more scalar replaceable input.
>> 
>> The solution I'm proposing is simply revisit the "reducibility" of the Phis when an object is marked as NSR.
>> 
>> ---------
>> 
>> ### Tests
>> 
>> Win, Mac & Linux tier1-4 on x64 & Aarch64.
>
> Cesar Soares Lucas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address PR feedback: include test execution options.

Marked as reviewed by thartmann (Reviewer).

Thanks Cesar, that looks good to me. I'll run a final round of testing and report back once it passed.

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PR Review: https://git.openjdk.org/jdk/pull/21778#pullrequestreview-2412246708
PR Comment: https://git.openjdk.org/jdk/pull/21778#issuecomment-2453917034