RFR: 8305898: Alternative self-forwarding mechanism

[Stefan Karlsson]

On Thu, 15 Aug 2024 20:52:13 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Currently, the Serial, Parallel and G1 GCs store a pointer to self into object headers, when promotion fails, to indicate that the object has been looked at, but failed promotion. 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 bit #3 (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.
> 
> A side-effect of this is that we can get rid of the machinery to preserve headers across promotion failures in Serial and G1. (Parallel GC [ab]uses the preserved-headers structure to also find all the forwarded objects for header restoration. This could be changed, I suppose, but it is not trivial.) If you prefer, I could break-out the removal of the preserved-headers stuff into a separate PR.
> 
> This is in preparation of upstreaming compact object headers, and I intend to push it only once all the parts have been approved.
> 
> Testing:
>  - [x] tier1
>  - [x] tier2
>  - [x] tier3
>  - [x] tier4
>  - [x] Running in production @ AWS since >1year without troubles

It might not be clear to reviewers, but the suggested change forces the usage of Lightweight locking on 32-bit JVMs. I think that is OK, especially given that Legacy locking is deprecated. However, before approving this PR it would be good to know if this has been communicated to the maintainers of the affected platforms?

And with that said, I couldn't find anything in this patch that prevented 32-bit JVMs from starting with Legacy. There's only these asserts:

NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)

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PR Review: https://git.openjdk.org/jdk/pull/20603#pullrequestreview-2251897087