[lworld] RFR: 8367245: [lworld] C2 compilation fails with "Missed optimization opportunity in PhaseIterGVN"

[Marc Chevalier]

On Thu, 6 Nov 2025 09:30:56 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> I disagree. The issue only surfaces in this particular occasion does not mean it will not appear in other circumstances, possibly in the future. Even if the code is there on purpose, it seems that the purpose is executed incorrectly.
>> 
>> Additionally, an empty speculative type is supposed to mean that the path is speculatively unreachable, which is not the case here. So, another approach may be to fix the injection of speculative and assert that we should not obtain an empty speculative type?
>
>> Additionally, an empty speculative type is supposed to mean that the path is speculatively unreachable,
> 
> Or that profile data for 2 different points in the execution is inconsistent which given how profile data is collected seems as likely to me.
> 
> Speculative types were added as a mechanism to fight profile pollution for scripting languages running on the JVM (specifically nashorn). They work really well in some cases. But they also have limited applicability. To me, it doesn't seem like a good use of developer time or our complexity budget to go with a complicated fix.

I'm trying to understand why `cleanup_speculative` is doing so. It seems to come from [JDK-8031755: Type speculation should be used to optimize explicit null checks](https://bugs.openjdk.org/browse/JDK-8031755). Maybe @rwestrel would remember something about it, but it has been quite a while since!

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PR Comment: https://git.openjdk.org/valhalla/pull/1717#issuecomment-3496451968