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

[Marc Chevalier]

On Tue, 20 Jan 2026 13:04:15 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> I fear I don't understand. Let's say, I have a flat array `MyValue[] arr` where `MyValue` is a value class with a single field `f`. Let's also assume `arr[0] == null`, `arr.field_value(0, f)` (assuming the `ci...` versions of it with matching names) tries to get the constant value of the field `f` of `arr[0]`, and `arr[0].f` is not null, it's rather undefined. It's not about stability and folding. On the other hand, if `arr[0]` is not null, but `arr[0].f` is null, `arr.field_value(0, f)` already returns `null` (the `ciConstant` that means that).
>> 
>> Am I missing something?
>
> That is only true from the Java perspective. From the VM perspective, a flat array would be something like (C++ pseudocode):
> 
>     class MyValuePayload {
>       oop* f;
>       bool null_marker;
>     };
> 
>     MyValuePayload* arr = new MyValuePayload[n];
> 
> Then, it is clear that even if `arr[0].null_marker == false`, `arr[0].f` is still defined and has a value (which should be `nullptr`).

As discussed offline, I'm trying change that.

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1923#discussion_r2708846928