RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v17]

[Dean Long]

On Tue, 29 Oct 2024 22:15:16 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> src/hotspot/share/code/nmethod.cpp line 1302:
>> 
>>> 1300:     _compiler_type           = type;
>>> 1301:     _orig_pc_offset          = 0;
>>> 1302:     _num_stack_arg_slots     = 0;
>> 
>> Was the old value wrong, unneeded, or is this set somewhere else?  If this field is not used, then we might want to set it to an illegal value in debug builds.
>
> We read this value from the freeze/thaw code in several places. Since the only compiled native frame we allow to freeze is Object.wait0 the old value would be zero too. But I think the correct thing is to just set it to zero always since a value > 0 is only meaningful for Java methods.

Isn't it possible that we might allow more compiled native frames in the future, and then we would have to undo this change?  I think this change should be reverted.  If continuations code wants to assert that this is 0, then that should be in continuations code, the nmethod code doesn't need to know how this field is used.  However, it looks like continuations code is the only client of this field, so I can see how it would be tempting to just set it to 0 here, but it doesn't feel right.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1823572138