RFR: 8343206: Final graph reshaping should not compress abstract or interface class pointers [v4]

[Erik Österlund]

On Thu, 31 Oct 2024 10:10:47 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> @fisk found this problematic optimization in final graph reshaping where we would convert a `CmpP` into a `CmpN` by converting a constant class pointer operand to a narrow class pointer. After  [JDK-8338526](https://bugs.openjdk.org/browse/JDK-8338526), this is not valid if the class pointer refers to an interface or abstract class.
>> 
>> I think it's not an issue in current code though. The only way we can get a dynamic narrow class is when loading from an object at `oopDesc::klass_offset_in_bytes()`:
>> https://github.com/openjdk/jdk/blob/7131f053b0d26b62cbf0d8376ec117d6e8d79f9e/src/hotspot/share/opto/type.cpp#L3521-L3522
>> 
>> Comparisons of such loads with a constant class pointer of interface or abstract class type are always folded during GVN:
>> https://github.com/openjdk/jdk/blob/7131f053b0d26b62cbf0d8376ec117d6e8d79f9e/src/hotspot/share/opto/subnode.cpp#L1164-L1171
>> 
>> And therefore, the code in `Compile::final_graph_reshaping_main_switch` will never trigger.
>> 
>> I added a corresponding assert and bailout in product to be on the safe side. The assert never triggered in my testing.
>> 
>> Thanks,
>> Tobias
>
> Tobias Hartmann has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Moved assert

This makes me feel less nervous. Thanks for adding this verification. Looks good.

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Marked as reviewed by eosterlund (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21784#pullrequestreview-2407650716