RFR: 8348411: C2: Remove the control input of LoadKlassNode and LoadNKlassNode [v2]

[Quan Anh Mai]

On Thu, 23 Jan 2025 18:28:56 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   remove always_see_exact_class
>
> src/hotspot/share/opto/parseHelper.cpp line 229:
> 
>> 227:   int element_klass_offset = in_bytes(ObjArrayKlass::element_klass_offset());
>> 228:   Node* p2 = basic_plus_adr(array_klass, array_klass, element_klass_offset);
>> 229:   Node* a_e_klass = _gvn.transform(LoadKlassNode::make(_gvn, immutable_memory(), p2, tak));
> 
> It looks like you are reverting the fix for JDK-8057622 here. How is it intended to work now?

Thanks for noticing, IIUC the issue with JDK-8057622 is that we see that the bottom type of the array is `java.lang.Object`, we then try to find the element type of `java.lang.Object` which is an invalid operation. The fix for the issue seems to do 2 things. It avoids the optimistic check if the bottom type is `java.lang.Object`, then it loads the element type from the constant only if the type check succeeds. The second part seems unnecessary, if the bottom type is not `java.lang.Object`, then it must be an array type, so the load must succeed. I added `is_aryklassptr` there to ensure that we are not having a bogus constant klass pointer.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23274#discussion_r1927557841