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

[Ioi Lam]

On Wed, 27 Aug 2025 09:23:38 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.
>
> src/hotspot/share/runtime/deoptimization.cpp line 1479:
> 
>> 1477: // Gets the fields of `klass` that are eliminated by escape analysis and need to be reassigned
>> 1478: static GrowableArray<ReassignedField>* get_reassigned_fields(InstanceKlass* klass, GrowableArray<ReassignedField>* fields, bool is_jvmci) {
>> 1479:   InstanceKlass* super = klass->java_super();
> 
> What is wrong with using `superklass()` here? We already know that `klass` is an `InstanceKlass*` so we don't need to make a virtual call to `java_super()`.

I got rid of `superklass()` because it was yet another way of looking for a "super" that has no documentation. 

In this particular case, the cost of a virtual code will be negligible compared to what happens in the following loop.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26908#discussion_r2304917105