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

[Emanuel Peter]

On Wed, 19 Nov 2025 10:31:53 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix bug number
>
> src/hotspot/share/opto/cfgnode.cpp line 1360:
> 
>> 1358:   // same (union of input types), but the new `_type` has now no speculative type, the result of `t->filter_speculative(_type)`
>> 1359:   // has the speculative type of `t` (if it's not removed because e.g. the resulting type is exact and non null) and not empty
>> 1360:   // (like the previously returned type). In such a case, doing the filtering one time more allows to reach a fixpoint.
> 
>>  From that `ft` has empty speculative type
> 
> I'm not very familiar with speculative types. Does "empty speculative" == TOP speculative type? Or rather "no speculative type", which essencially means it is BOTTOM type?
> 
> Because then if we filter x with TOP we should still get TOP, but if we filter with BOTTOM we get x. And that would fit better with your statement later on:
> 
>> but the new `_type` has now no speculative type, the result of `t->filter_speculative(_type)` has the speculative type of `t`
> 
> Can you clarify please for my understanding? :)

And does `cleanup_speculative` happen during `t->filter_speculative(_type)`, right?

> src/hotspot/share/opto/cfgnode.cpp line 1365:
> 
>> 1363:     ft = t->filter_speculative(first_ft);
>> 1364: #ifdef ASSERT
>> 1365:     // The following logic has been moved into TypeOopPtr::filter.
> 
> Why does this mean? What logic are you referring to? The one here? But then you say it was moved to TypeOopPtr::filter ...but it is still here? Can you clarify?

Or are you saying it is moved "from" rather than "into", i.e. this is some sort of code duplication?

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PR Review Comment: https://git.openjdk.org/jdk/pull/28331#discussion_r2541436857
PR Review Comment: https://git.openjdk.org/jdk/pull/28331#discussion_r2541500034