[lworld] RFR: 8375441: [lworld] C2: assert(is_instance()) failed: bad cast

[Marc Chevalier]

On Tue, 20 Jan 2026 11:25:59 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Some code added by [JDK-8372700](https://bugs.openjdk.org/browse/JDK-8372700) can compute the constant value of a field of a (flatten) element in a flat array. We get a crash when the element of the array is known to be `null`, and so the field doesn't exist.
>> 
>> So, let's just check in `ciConstant ciFlatArray::field_value(int index, ciField* field)` whether we get a null constant before interpreting it as a `ciInstance` and trying to retrieve a field from there. This should be enough since a `ciObject` is (directly) derived by `ciNullObject`, `ciInstance` and `ciArray`. Since we are looking up a value of a flat array, an element cannot be a `ciArray` (arrays have identities and can't be contained in a flat array). After looking up whether the flat array element is null, the `obj->as_instance()` cast acts as an assert, should we ever add another derived class from `ciObject`.
>> 
>> In case of a null array element, `field_value` simply returns an invalid `ciConstant`.
>> 
>> Tested with tier1,tier2,tier3,hs-precheckin-comp,hs-comp-stress,valhalla-comp-stress. Looks good.
>> 
>> Thanks,
>> Marc
>
> src/hotspot/share/opto/type.cpp line 414:
> 
>> 412: 
>> 413:   if (con.is_valid() &&          // not a mismatched access
>> 414:       !con.is_null_or_zero()) {  // not a default value
> 
> This is incorrect, the value is not constant if the element of the array is `null`, not if the field we retrieved is `null`.

I think you're correct. While being sound, we might miss opportunities where the value is actually constant but we discard it. `con.is_value()` already  is enough to make sure the array element is not null (and constant).

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1923#discussion_r2708249153