RFR: 8371716: C2: Phi node fails Value()'s verification when speculative types clash [v2]

[Emanuel Peter]

On Wed, 19 Nov 2025 08:55:07 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

>> This bug was originally found and reported as a Valhalla problem. It quickly became apparent it has no reason to be Valhalla-specific, while I couldn't prove so. Roland managed to make a mainline reproducer. The explanation details my Valhalla investigation, but it has nothing to do with value classes anyway.
>> 
>> # Analysis
>> ## Obervationally
>> ### IGVN
>> During IGVN, in `PhiNode::Value`, a `PhiNode` has 2 inputs. Their types are:
>> 
>> in(1): java/lang/Object * (speculative=TestSpeculativeTypes$C2:NotNull:exact * (inline_depth=3))
>> in(2): null
>> 
>> We compute the join (HS' meet):
>> https://github.com/openjdk/jdk/blob/09b25cd0a24a4eaddce49917d958adc667ab5465/src/hotspot/share/opto/cfgnode.cpp#L1299-L1306
>> 
>> t=java/lang/Object * (speculative=compiler/igvn/ClashingSpeculativeTypePhiNode$C2:exact *)
>> 
>> But the current `_type` (of the `PhiNode` as a `TypeNode`) is
>> 
>> _type=java/lang/Object * (speculative=compiler/igvn/ClashingSpeculativeTypePhiNode$C1:exact *)
>> 
>> We filter `t` by `_type`
>> https://github.com/openjdk/jdk/blob/09b25cd0a24a4eaddce49917d958adc667ab5465/src/hotspot/share/opto/cfgnode.cpp#L1321
>> and we get
>> 
>> ft=java/lang/Object *
>> 
>> which is what we return. After the end of `Value`, the returned becomes the new `PhiNode`'s `_type`.
>> https://github.com/openjdk/jdk/blob/09b25cd0a24a4eaddce49917d958adc667ab5465/src/hotspot/share/opto/phaseX.cpp#L2150-L2164
>> and
>> https://github.com/openjdk/jdk/blob/09b25cd0a24a4eaddce49917d958adc667ab5465/src/hotspot/share/opto/node.cpp#L1117-L1123
>> 
>> 
>> ### Verification
>> On verification, `in(1)`, `in(2)` have the same value, so does `t`. But this time
>> 
>> _type=java/lang/Object *
>> 
>> and so after filtering `t` by (new) `_type` and we get
>> 
>> ft=java/lang/Object * (speculative=compiler/igvn/ClashingSpeculativeTypePhiNode$C2:exact *)
>> 
>> which is retuned. Verification gets angry because the new `ft` is not the same as the previous one.
>> 
>> ## But why?!
>> ### Details on type computation
>> In short, we are doing
>> 
>> t = typeof(in(1)) / typeof(in(2))
>> ft  = t /\ _type (* IGVN *)
>> ft' = t /\ ft    (* Verification *)
>> 
>> and observing that `ft != ft'`. It seems our lattice doesn't ensure `(a /\ b) /\ b = a /\ b` which is problematic for this kind of verfication that will just "try again and see if something change".
>> 
>> To me, the surprising fact was that the intersection
>> 
>> java/lang/Object * (speculative=compiler/igvn/ClashingSpeculativeTypePhiNode$C2:exact *)
>> /\
>> _type=java/lang/Objec...
>
> Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix bug number

test/hotspot/jtreg/compiler/igvn/ClashingSpeculativeTypePhiNode.java line 36:

> 34:  *                   -XX:CompileCommand=quiet
> 35:  *                   -XX:TypeProfileLevel=222
> 36:  *                   -XX:+AlwaysIncrementalInline

Suggestion:

 *                   -XX:+AlwaysIncrementalInline


GitHub Actions is giving us this:


Error: VM option 'AlwaysIncrementalInline' is develop and is available only in debug version of VM.
Improperly specified VM option 'AlwaysIncrementalInline'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.


Probably this would help: `-XX:+IgnoreUnrecognizedVMOptions`

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PR Review Comment: https://git.openjdk.org/jdk/pull/28331#discussion_r2541516825