RFR: 8343206: Final graph reshaping should not compress abstract or interface class pointers [v5]

[Tobias Hartmann]

> @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:

  Use is_encodable instead

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21784/files
  - new: https://git.openjdk.org/jdk/pull/21784/files/3da09500..bab7c5df

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21784&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21784&range=03-04

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21784.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21784/head:pull/21784

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