RFR: 8360936: Test compiler/onSpinWait/TestOnSpinWaitAArch64.java fails after JDK-8359435 [v2]

[Aleksey Shipilev]

On Wed, 2 Jul 2025 08:49:45 GMT, Andrew Haley <aph at openjdk.org> wrote:

>>> OK, are you able to bisect which change? This fix to only do debug VM needs to be correctly linked to the actual cause, IMO.
>> 
>> 
>> 
>>> > It looks like `XX:+PrintAssembly` prints out debug info in release builds but `XX:CompileCommand=print` does not. I am switching back to `XX:+PrintAssembly`.
>>> 
>>> That's not great. What info do you need, exactly?
>> 
>> 
>>   # {method} {0x0000ffff50400378} 'test' '()V' in 'compiler/onSpinWait/TestOnSpinWaitAArch64$Launcher'
>>   #           [sp+0x20]  (sp of caller)
>>   0x0000ffff985731c0: ff83 00d1 | fd7b 01a9 | 2803 0018 | 8923 40b9 | 1f01 09eb
>> 
>>   0x0000ffff985731d4: ;*synchronization entry
>>                       ; - compiler.onSpinWait.TestOnSpinWaitAArch64$Launcher::test at -1 (line 224)
>>   0x0000ffff985731d4: 2102 0054 | 1f20 03d5 | 1f20 03d5 | 1f20 03d5 | 1f20 03d5 | 1f20 03d5 | 1f20 03d5
>> 
>>   0x0000ffff985731f0: ;*invokestatic onSpinWait {reexecute=0 rethrow=0 return_oop=0}
>>                       ; - compiler.onSpinWait.TestOnSpinWaitAArch64$Launcher::test at 0 (line 224)
>>   0x0000ffff985731f0: 1f20 03d5 | fd7b 41a9 | ff83 0091
>> 
>>   0x0000ffff985731fc: ;   {poll_return}
>>   0x0000ffff985731fc: 8817 40f9 | ff63 28eb | 4800 0054 | c003 5fd6
>> 
>>   0x0000ffff9857320c: ;   {internal_word}
>>   0x0000ffff9857320c: 88ff ff10 | 88a3 02f9
>> 
>>   0x0000ffff98573214: ;   {runtime_call SafepointBlob}
>>   0x0000ffff98573214: 5bc3 fe17
>> 
>>   0x0000ffff98573218: ;   {runtime_call Stub::method_entry_barrier}
>>   0x0000ffff98573218: 0850 96d2 | 480a b3f2 | e8ff dff2 | 0001 3fd6 | ecff ff17
>> 
>> 
>> The test searches for `- compiler.onSpinWait.TestOnSpinWaitAArch64$Launcher::test at 0` and `invokestatic onSpinWait`. They identify the place where to search instructions.
>> 
>> Assembly from all builds always has `{poll_return}`. I can use it as a search point.
>
>> ```
>> 
>> ```
>> 
>> 
>>     
>>   
>> 
>> The test searches for `- compiler.onSpinWait.TestOnSpinWaitAArch64$Launcher::test at 0` and `invokestatic onSpinWait`. They identify the place where to search instructions.
> 
> That's not great. C2 is free to move stuff around, so it's not certain this test will keep working. If you just want to make sure that the pattern is used, a block_comment() would be more reliable.

Then again, it does not address a central point: the test keeps relying on particular instruction scheduling, which is not reliable. Why do we even need to search for ldp+add anchor? Can we just blindly search for spin-wait-looking instructions? I would expect `block_comment` to be even more reliable, as @theRealAph suggested.

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PR Comment: https://git.openjdk.org/jdk/pull/26072#issuecomment-3045677596