RFR: 8348631: Crash in PredictedCallGenerator::generate after JDK-8347006

[Vladimir Kozlov]

On Tue, 28 Jan 2025 13:10:37 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> We crash / assert during C2 compilation of intrinsics like `_getLength` because the cast emitted by the array guard added by [JDK-8347006](https://bugs.openjdk.org/browse/JDK-8347006) is folded to top:
> https://github.com/openjdk/jdk/blob/c33c1cfe7349ac657cd7bf54861227709d3c8f1b/src/hotspot/share/opto/library_call.cpp#L4302-L4305
> 
> This happens when C2's type system determines that the type of the object that we cast implements an interface other than `Serializable` or `Cloneable` and therefore can't be an array. This is possible since [JDK-8297933](https://bugs.openjdk.org/browse/JDK-8297933). Now unfortunately, control via the layout helper check is not (yet) folded due to:
> https://github.com/openjdk/jdk/blob/c33c1cfe7349ac657cd7bf54861227709d3c8f1b/src/hotspot/share/opto/memnode.cpp#L2215-L2223
> 
> This is probably an oversight from [JDK-8297933](https://bugs.openjdk.org/browse/JDK-8297933). Given that this is a regression in JDK 24, I'm going with a conservative approach of simply checking the cast for top and not using it if that's the case. In addition, I made the code more robust and added a compilation bailout (assert in debug) if an intrinsic produces a `top` result.
> 
> We should then properly fix this by making sure that the layout helper check is folded. I filed [JDK-8348853](https://bugs.openjdk.org/browse/JDK-8348853) for this.
> 
> Big thanks to @cushon for reporting this just in time for fixing in JDK 24!
> 
> Best regards,
> Tobias

General question: how in other part of VM (runtime, gc) layout helper was changed for JDK-8297933?

src/hotspot/share/opto/library_call.cpp line 4308:

> 4306:     if (!cast->is_top()) {
> 4307:       *obj = cast;
> 4308:     }

Add comment why it could be TOP.

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PR Review: https://git.openjdk.org/jdk/pull/23331#pullrequestreview-2578931111
PR Review Comment: https://git.openjdk.org/jdk/pull/23331#discussion_r1932562269