RFR: 8361699: C2: assert(can_reduce_phi(n->as_Phi())) failed: Sanity: previous reducible Phi is no longer reducible before SUT

[Cesar Soares Lucas]

On Fri, 5 Sep 2025 09:39:54 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Please, review this patch to fix issue that may occur when reducing allocation merge.
>> 
>> As the assert message describe, the problem is a `Phi` considered reducible during one invocation of  `adjust_scalar_replaceable_state` turned out to be later non-reducible. This situation can happen if a subsequent invocation of the same method causes all inputs to the phi to be NSR; therefore there is no point in reducing the Phi. It can also happen during the propagation of NSR state done by `find_scalar_replaceable_allocs`. 
>> 
>> The change in `revisit_reducible_phi_status` is just a clean-up.
>> The real fix is in `find_scalar_replaceable_allocs`.
>> 
>> Tested on Linux x64/Aarch64 release/fastdebug with JTREG tier1-3.
>
> test/hotspot/jtreg/compiler/escapeAnalysis/TestReduceAllocationNotReducibleAnymore.java line 58:
> 
>> 56:             }
>> 57:         }
>> 58:     }
> 
> Could we make the catch exception matching more precise? I'd just like to avoid a case where we miscompile and throw the wrong exception and that gets caught silently.

I'll do a best effort attempt to minimize this test again.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27063#discussion_r2325647857