RFR: 8372634: C2: Materialize type information from instanceof checks [v2]

[Vladimir Ivanov]

On Thu, 27 Nov 2025 14:08:03 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Test fix
>
> src/hotspot/share/opto/parse2.cpp line 1739:
> 
>> 1737:   }
>> 1738: 
>> 1739:   // Match an instanceof check.
> 
> We seem to require that the input of `SubTypeCheck` is not `null`. What do you think about allowing `SubTypeCheck` to accept `null` and return `false`?

Yes, it's a good idea and the right direction to move. While experimenting with a different enhancement, I noticed that a subtype check leaves a null check behind irrespective of whether the check goes away or not.  

Unfortunately, there are some engineering considerations which complicates the change. `SubTypeCheck` is shared across all the places where subtype checks are performed, but `checkcast` and `instanceof` differ in the way `null` is handled. So, the proper way to fix it is to introduce a higher-level representation which implicitly handles nulls and then eventually lower it to `SubTypeCheck` and materialize null check if needed.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28517#discussion_r2578397399