Invalid code generated by C2 compiler in OpenJDK 21

[David Holmes]

Hi,

<snip>
> On 15/12/23 3:42 pm, Antoine DESSAIGNE wrote:
>> Hello everyone,
>>
>> I've found an issue while migrating to OpenJDK 21. A valued local 
>> variable (effectively final) has its value removed and it throws a 
>> NullPointerException. Unfortunately, I cannot provide the source code 
>> and the data to reproduce the issue, and I couldn't create a smaller 
>> code snippet to show the issue. That said, I'll happily show the code 
>> and perform many tests during calls.
>>
>> Here's what I did so far to diagnose the issue.
>>
>> I bisected the repository to find where the regression comes from. I 
>> found this commit 3696711efa5 [1] but it's a merge so I bisected the 
>> branch and found 10737e168c9 [2]. Looking at this commit, I have no 
>> idea how it could introduce this kind of regression.

It is likely because that commit has a bug in it, fixed recently by

https://bugs.openjdk.org/browse/JDK-8322282

The fix is in 21.0.3

Cheers,
David
-----

>>
>> Then, thanks to the guidance from Aleksey Shipilëv, I tested many things
>> * Issue does *not* happen with the following 
>> flags: -Xint, -XX:-TieredCompilation, -XX:TieredStopAtLevel=1, -XX:TieredStopAtLevel=2, -XX:TieredStopAtLevel=3
>> * Issue also happens with fastdebug builds of OpenJDK, without 
>> crashing due to assertions
>> * Issue still happens in the latest version of the code 
>> (commit b31454e3623)
>> * Issue happens no matter which GC is used, I tried SerialGC, 
>> ParallelGC, G1GC, and ShenandoahGC
>>
>> The tests were performed in Docker containers running on 4 different 
>> hosts.
>>
>> Therefore it looks like C2 is generating an invalid assembly code. 
>> Unfortunately, I'm not great with assembly and the generated assembly 
>> is quite big (main code is around 20k).
>>
>> Do you have an idea of why this is happening? Do you know what test I 
>> can run? If one of you is available, we can schedule calls for me to 
>> show you the code and my tests. Thank you very much for your assistance.
>>
>> Have a nice day,
>>
>> Antoine DESSAIGNE
>>
>> [1] 
>> https://github.com/openjdk/jdk/commit/3696711efa566fb776d6923da86e17b0e1e22964
>> [2] 
>> https://github.com/openjdk/jdk/commit/10737e168c967a08e257927251861bf2c14795ab
>>
>>