RFR: 8305898: Alternative self-forwarding mechanism [v4]

[Thomas Schatzl]

On Thu, 8 Feb 2024 14:37:20 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Currently, the Serial, Parallel and G1 GCs store a pointer to self into object headers to indicate promotion failure. This is problematic for compact object headers ([JDK-8294992](https://bugs.openjdk.org/browse/JDK-8294992)) because it would (temporarily) over-write the crucial class information, which we need for heap parsing. I would like to propose an alternative: use the currently unused 3rd header bit (previously biased-locking bit) to indicate that an object is 'self-forwarded'. That preserves the crucial class information in the upper bits of the header until the full header gets restored.
>> 
>> This is a trimmed-down/simplified version of the original proposal #13779:
>>  - It doesn't use/introduce any flags and avoids the associated branching.
>>  - It doesn't (need to) deal with displaced headers. (Current code would preserve header if necessary, Lilliput code would not use displaced headers and set the 3rd bit directly in existing header.)
>> 
>> Testing:
>>  - [x] hotspot_gc
>>  - [x] tier1
>>  - [x] tier2
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More consistent use of markWord::is_forwarded()

We often do preparatory changes separately from the main improvement for several reasons, one of them to reduce complexity of the follow-up.

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

PR Comment: https://git.openjdk.org/jdk/pull/17755#issuecomment-1966241228