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

BTW, I think determining whether the instruction after a `jsr` is reachable is not so easy.  Normally if the subroutine does a `ret` then we return to the instruction after the jsr, but I think the subroutine can also do things like throw an exception or loop forever, so it's probably better to treat `jsr` the same as `goto` and add it to `falls_through`.  

But now that I think about it, I'm wondering if this special logic is still needed by VerifyStack.  It might not be needed at all, or it might only be needed for specific situations, like an invoke instruction or an instruction with the reexecute flag set.

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PR Comment: https://git.openjdk.org/jdk/pull/25118#issuecomment-2868114758