RFR: 8316417: ObjectMonitorIterator does not return the most recent monitor and is incorrect if no monitors exists [v6]

[Chris Plummer]

On Thu, 21 Sep 2023 06:21:13 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> `inUseList` will end up with the same value as `inUseListHead`. The reason the old code worked is because `getAddressField` does not type check and `reinterpret_cast<addres>(&ObjectSynchronizer::_in_use_list) == reinterpret_cast<addres>(&ObjectSynchronizer::_in_use_list._head)`
>> 
>> Effectively I changed this to load it correctly (regardless of what `offset_of(MonitorList, _head)` ends up being) and name the variables more appropriately.
>> 
>> C++ interpretation of what the java change does:
>> ```C++
>> // Old code
>> //  Type type = db.lookupType("ObjectSynchronizer");
>> //  inUseList = type.getAddressField("_in_use_list").getValue();
>> address inUseList = *(reinterpret_cast<address*>(&ObjectSynchronizer::_in_use_list));
>> 
>> // New code
>> //  Type objectSynchronizerType = db.lookupType("ObjectSynchronizer");
>> //  Type monitorListType = db.lookupType("MonitorList");
>> //  Address monitorListAddr = objectSynchronizerType.getField("_in_use_list").getStaticFieldAddress();
>> //  inUseListHead = monitorListType.getAddressField("_head").getAddress(monitorListAddr); 
>> address monitorListAddr = reinterpret_cast<address>(&ObjectSynchronizer::_in_use_list);
>> address inUseList =  *(reinterpret_cast<address*>(monitorListAddr + offset_of(MonitorList, _head)));
>
> Just to clarify what might cause confusion (at least it is what confused me at first when I read this code) is that `getAddress()`/ `getAddressField(...).getValue()` does not return the address of the field. It returns the value of the field (loaded and) interpreted as an address.

I see now. _in_use_list is a MonitorList (not a pointer to one) and the first field of a MonitorList is the _head field. So that means the address of the _in_use_list field is also the address of the _head field.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15782#discussion_r1333390292