RFR: 8373595: A new ObjectMonitorTable implementation [v4]

[Fredrik Bredberg]

On Tue, 27 Jan 2026 07:21:26 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Fredrik Bredberg has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Updated for Axel
>
> src/hotspot/share/runtime/objectMonitorTable.cpp line 47:
> 
>> 45:   const size_t _capacity_mask;       // One less than its power-of-two capacity
>> 46:   Table* volatile _prev;             // Set while rehashing
>> 47:   ObjectMonitor* volatile* _buckets; // The payload
> 
> I think this should have some opaque `enum class` type rather than `ObjectMonitor*` because we do not always store an `ObjectMonitor*` inside. We could repurpose `SpecialPointerValues` with another name.
> 
> So we avoid these reinterpret_casts from our special values to `ObjectMonitor*`. And only reinterpret_cast values we know to be `ObjectMonitor*` to `ObjectMonitor*`.

Fixed

> src/hotspot/share/runtime/objectMonitorTable.hpp line 56:
> 
>> 54:     removed = 2,
>> 55:     below_is_special = (removed + 1)
>> 56:   } SpecialPointerValues;
> 
> Could this be a C++ enum instead?
> 
> Suggestion:
> 
>   enum SpecialPointerValues {
>     empty = 0,
>     tombstone = 1,
>     removed = 2,
>     below_is_special = (removed + 1)
>   };
> 
> 
> I think I preferred it to be an strongly typed `enum class` as well. (But we do use it as an integral in the MacroAssembler so that would introduce some casts.

Fixed

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/29383#discussion_r2799762243
PR Review Comment: https://git.openjdk.org/jdk/pull/29383#discussion_r2799760957