RFR: 8366024: Remove unnecessary InstanceKlass::cast()

[Ioi Lam]

On Wed, 27 Aug 2025 09:17:31 GMT, Andrew Dinn <adinn at openjdk.org> wrote:

>> We have a lot of `InstanceKlass::cast(k)` calls where `k` is statically known to be an `InstanceKlass`. I fixed many instances of this pattern:
>> 
>> 
>> InstanceKlass* x = ....;
>> Klass* s = x->super(); // should call java_super()
>> InstanceKlass::cast(s)->xyz();
>> 
>> 
>> The `super()` method has a very confusing API. It has the return type of `Klass*` because for for an `ObjArrayKlass` like `[Ljava/lang/String;`:
>> 
>> - `super()` returns `[Ljava/lang/Object;`
>> - `java_super()` returns `Ljava/lang/Object;`
>> 
>> However, for `[Ljava/lang/Object;`, all `TypeArrayKlasses` and all `InstanceKlasses`, `super()` and `java_super()` return an identical value of that always have the actual type of `InstanceKlass*`.
>> 
>> See here about the difference between `super()` and `java_super()`: https://github.com/openjdk/jdk/blob/7b9969dc8f20989497ff617abb45543d182b684d/src/hotspot/share/oops/klass.hpp#L218-L221
>> 
>> Unfortunately, we have a lot of code that incorrectly uses `super()` instead of `java_super()`, which leads to ` InstanceKlass::cast()` calls. I tried to fixed a bunch of easy ones in this PR, although there are a few more to go.
>> 
>> I also fixed some calls to `local_interafaces()->at()` that widens the return type for `InstanceKlass*` to `Klass*`, which may lead to unnecessary  ` InstanceKlass::cast()` calls.
>> 
>> I also removed the `Klass::superklass()` API. This was used only in a few places and all of them can be safely replaced with `Klass::java_super()`.
>> 
>> To avoid confusion, I think we should rename `super()` to something more obvious, but let's do that in a future PR.
>
> I found two more occurrences of casting super() to InstanceKlass:
> 
>     src/hotspot/share/jfr/leakprofiler/chains/edgeUtils.cpp:79
> 
>         ik = (const InstanceKlass*)ik->super();
> 
>     src/hotspot/share/prims/jni.cpp:209
> 
>         Klass* field_klass = k;
>         Klass* super_klass = field_klass->super();
>         // With compressed oops the most super class with nonstatic fields would
>         // be the owner of fields embedded in the header.
>         while (InstanceKlass::cast(super_klass)->has_nonstatic_fields() &&
>                InstanceKlass::cast(super_klass)->contains_field_offset(offset)) {
>           field_klass = super_klass;   // super contains the field also
>           super_klass = field_klass->super();
>         }
> 
> The first one ought perhaps to be using InstanceKlass::superklass()?

Thanks @adinn @dholmes-ora @coleenp for the review

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PR Comment: https://git.openjdk.org/jdk/pull/26908#issuecomment-3240791556