RFR: 8369238: Allow virtual thread preemption on some common class initialization paths [v2]

[Dean Long]

On Thu, 16 Oct 2025 16:23:05 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> src/hotspot/cpu/aarch64/continuationFreezeThaw_aarch64.inline.hpp line 209:
>> 
>>> 207:     // the last_sp saved in the frame (remove possible alignment added while
>>> 208:     // thawing, see ThawBase::finish_thaw()). We also need to clear the last_sp
>>> 209:     // saved in the frame as it is not expected to be set in case we preempt again.
>> 
>> A bit stronger?
>> Suggestion:
>> 
>>     // saved in the frame because it must be clear if we freeze again.
>
> Just to add more context, not clearing last_sp will make this assert [1] fire if we freeze again. That assert is mostly a verification check, because we know the interpreter doesn’t set last_sp for the top frame when calling into the VM. But I don’t see a fundamental reason why it must be cleared (removing the assert and not clearing last_sp works). I don’t see any other code that checks last_sp needs to be cleared for the top frame (other than in the interpreter before calling into the VM).
> How about changing that last sentence with: `We also clear last_sp to match the behavior when calling the VM from the interpreter (we check for this in FreezeBase::prepare_freeze_interpreted_top_frame).`
> 
> [1] https://github.com/openjdk/jdk/blob/87092ef1d97e00ddb6674b0e309f2f904d307604/src/hotspot/cpu/aarch64/continuationFreezeThaw_aarch64.inline.hpp#L136

FWIW, interpreter_frame_tos_address() behaves differently depending on if last_sp() is cleared or not.  I know deoptimization sets last_sp temporarily but makes sure to clear it before giving control back to the interpreter.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27802#discussion_r2462366413