RFR: 8372845: Fold identity hash code if object is constant

[Vladimir Kozlov]

On Tue, 2 Dec 2025 20:53:56 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>>> would it be safer for us to move the constant detection after generate_virtual_guard in the is_virtual if block? 
>> 
>> Good catch. I missed that the intrinsic is shared between `System::identityHashCode()` and `Object::hashCode`.
>> 
>> I'm not sure it makes sense to support `Object::hashCode` unless C2 can eliminate `generate_virtual_guard` for a constant receiver. I'd just limit constant folding to `!is_virtual` case for now.
>
>> I'm not sure it makes sense to support Object::hashCode unless C2 can eliminate generate_virtual_guard for a constant receiver. I'd just limit constant folding to !is_virtual case for now.
> 
> Or, alternatively, inspect constant object's v-table during compilation and ensure that corresponding slot points at `Object::hashCode`.

@iwanowww please fix title to match JBS.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/28589#issuecomment-3603933760