RFR: 8336906: C2: assert(bb->is_reachable()) failed: getting result from unreachable basicblock [v2]

[Dean Long]

On Fri, 9 May 2025 15:22:56 GMT, Manuel Hässig <mhaessig at openjdk.org> wrote:

>> # Issue Summary
>> 
>> This PR addresses an `assert(bb->is_reachable())` that is triggered in the code for `-XX:+VerifyStack` after a deoptimization with reason `null_assert_or_unreached0` at a `getstatic` bytecode. Following the `getstatic` is an `areturn` and then an unreachable bytecode. When the code for `VerifyStack` tries to compute an oop map for the basic block of the unreachable bytecode, the assert triggers:
>> 
>> getstatic Field A.val:"LB"; // if class B is not loaded, C2 deopts with reason "null_assert_or_unreached0"
>> areturn;
>> // The following is unreachable
>> iconst_0;
>> 
>> 
>> This is a similar problem to [JDK-8271055](https://bugs.openjdk.org/browse/JDK-8271055) (#7331), but this particular deopt with reason `null_assert_or_unreached0` at `getstatic` of a field containing an object reference [deopts at the next bytecode](https://github.com/openjdk/jdk/blob/ad07426fab3396caefd7c08d924e085c1f6f61ba/src/hotspot/share/opto/parse3.cpp#L176-L199). The aforementioned issue introduced a check to skip stack verification of the next bytecode in the code if the execution after the deopted bytecode does not continue at the next bytecode in the code, i.e. falls through to the next bytecode.  Unfortunately, this check did not include `areturn` as a bytecode that does not fall-through:
>> https://github.com/openjdk/jdk/blob/ad07426fab3396caefd7c08d924e085c1f6f61ba/src/hotspot/share/runtime/deoptimization.cpp#L845-L856
>> 
>> # Change Summary
>> 
>> To fix the immediate issue described above, this PR adds `areturn` to the list of bytecodes that does not fall through. However, all return bytecodes exhibit the same behavior and might be susceptible to a similar issue. Even though I was not able to reproduce the same crash with `{d,f,i,l}return` because I could not get those or the preceding bytecode to deopt, I also added them to the `falls_through()` function. For the remaining bytecodes in `falls_through()` with the exception of `athrow` I wrote a regression test.
>> 
>> # Testing
>> 
>> - [x] [Github Actions](https://github.com/mhaessig/jdk/actions/runs/14595928439)
>> - [x] tier1 through tier3 on Oracle supported platforms and OSs plus Oracle internal testing
>> 
>> # Acknowledgements
>> Special thanks to @eme64 for his hard work on reducing a reproducer that works on all platforms.
>
> Manuel Hässig has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - Add lookupswitch and tableswitch
>  - Elaborate why we need that class file version
>  - Reorganized tests

I forgot that uncommon traps always use reexecute semantics, even if they don't explicitly set the reexecute flag on the debug info.  That's why they can update the JVM state and advance to the next bci.  It seems like it would be safe to skip the falls_through/try_next_mask logic for the top frame if we are reexecuting at the current bci, but that is a riskier change.

BTW, because some uncommon traps advance to the next bci, code like Deoptimization::gather_statistics() will not be gathering statistics for the bytecode that causes the trap, making the statistics less meaningful.  It seems better to allow uncommon traps to use the current bci instead and let the deoptimization code advance it if the reexecute flag is not set.

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PR Comment: https://git.openjdk.org/jdk/pull/25118#issuecomment-2869119219