[External] : Re: A noob question about weird sequence of `*synchronization entry` in a C2 compiled code

[Tobias Hartmann]

Hi Jaroslav,

I finally got the time to investigate this properly and found the root cause. The fix is out for
review: https://github.com/openjdk/jdk/pull/12806

Thanks again for reporting this!

Best regards,
Tobias


On 07.02.22 23:20, dean.long at oracle.com wrote:
> It turns out the problem with DebugNonSafepoints is a known issue.  See
> https://bugs.openjdk.java.net/browse/JDK-8201516.
> 
> dl
> 
> On 2/7/22 7:58 AM, Jaroslav Bachorík wrote:
>> Hi Dean,
>>
>> The first thing I want to mention is that I isolated  this behaviour
>> to be triggered by `-XX:DebugNonSafepoints` JVM arg. When this option
>> is not specified I don't see the pattern at all.
>>
>> I have extracted a self-contained (almost) reproducer - it is not a
>> single class, unfortunately, but building and running it is as simple
>> as executing the attached `./run.sh`
>> The reproducer has a weak point, though - I am not able to get the
>> `*synchronization entry` pattern manifesting at the same locations.
>> Therefore it is more of PoC than a full test case - it requires going
>> to the assembly print out and searching for `*synchronization entry`
>> manually.
>>
>> The project can be found here -
>> https://urldefense.com/v3/__https://drive.google.com/file/d/1Z6rX4NpvNpctVA3AuYfaG0Qxjd4KjrED/view?usp=sharing__;!!ACWV5N9M2RV99hQ!f9PhKRCRHuSdH__32-XrLtTdu0RoQCQz8wJGjvsqF83SjHg4R2_vAcUJOJFYqd4$
>> It is a zipped gradle java project and the only thing it requires is a
>> working Java env.
>>
>> Thanks!
>>
>> -JB-
>>
>> On Sun, Feb 6, 2022 at 6:45 AM <dean.long at oracle.com> wrote:
>>>
>>> On 2/5/22 12:58 PM, Jaroslav Bachorík wrote:
>>>> Is this the hard limitation of what is possible to do with the debug
>>>> data at this level of optimization? Are some boundaries irreversibly
>>>> lost here?
>>>
>>> It's quite likely you've discovered a bug that can be fixed.  If you
>>> could narrow it down to a self-contained test case, that would really help.
>>>
>>> dl