Withdrawn: 8305898: Alternative self-forwarding mechanism

[Roman Kennke]

On Wed, 7 Feb 2024 18:33:00 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

This pull request has been closed without being integrated.

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PR: https://git.openjdk.org/jdk/pull/17755