RFR: 8351392: C2 crash: failed: Expected Bool, but got OpaqueMultiversioning

[Christian Hagedorn]

On Fri, 7 Mar 2025 10:35:58 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> With https://github.com/openjdk/jdk/pull/23865 I mark the `OpaqueMultiversioning` useless once there are no loops for the multiversioning any more. The idea was that this way the `multiversion_if` would be folded away once the loops are gone, and then we should not encounter the `OpaqueMultiversioning` in `PhaseIdealLoop::conditional_move`.
> 
> But there is a case where we do not remove the `OpaqueMultiversioning` fast enough, see the attached regression test:
> - The loops disappear during IGVN.
> - At the beginning of the next loop-opts we mark the `OpaqueMultiversioning` as useless.
> - Later during loop-opts we encounter the useless `OpaqueMultiversioning` in `PhaseIdealLoop::conditional_move`, and assert.
> - But in the IGVN after this loop-opts phase we would have constant folded the `OpaqueMultiversioning` and `multiversion_if` anyway... we just did not get there fast enough ;)
> 
> Hence I propose to just create an explicit bailout for useless `OpaqueMultiversioning` nodes in `PhaseIdealLoop::conditional_move`.

src/hotspot/share/opto/loopopts.cpp line 794:

> 792:     return nullptr;
> 793:   }
> 794:   if (bol->is_OpaqueMultiversioning() && bol->as_OpaqueMultiversioning()->is_useless()) {

Isn't any `OpaqueMultiversioning` we find here supposed to be useless? Can we assert that instead?

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PR Review Comment: https://git.openjdk.org/jdk/pull/23943#discussion_r1984903770