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

[Quan Anh Mai]

On Tue, 20 Jan 2026 12:54:01 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

>> src/hotspot/share/ci/ciFlatArray.cpp line 136:
>> 
>>> 134: 
>>> 135: ciConstant ciFlatArray::field_value(int index, ciField* field) {
>>> 136:   auto get_field_from_object_constant = [field](const ciConstant& v) -> ciConstant {
>> 
>> I don't really agree with this fix, `ciFlatArray::field_value` should be dumber, it is the caller who knows that we do not fold the load if the element is `null`, the callee should just return the field as it is.
>
> 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`).

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PR Review Comment: https://git.openjdk.org/valhalla/pull/1923#discussion_r2708268808