Integrated: 8343206: Final graph reshaping should not compress abstract or interface class pointers

[Tobias Hartmann]

On Wed, 30 Oct 2024 11:38:53 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

This pull request has now been integrated.

Changeset: 2432c4f8
Author:    Tobias Hartmann <thartmann at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/2432c4f862e66e91c60e75ccc43b376020d80a1f
Stats:     8 lines in 1 file changed: 8 ins; 0 del; 0 mod

8343206: Final graph reshaping should not compress abstract or interface class pointers

Reviewed-by: coleenp, eosterlund, kvn

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PR: https://git.openjdk.org/jdk/pull/21784