[master] RFR: Unconditionally preserve all marks during GC

[Roman Kennke]

On Wed, 19 May 2021 13:09:33 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> This block concerns about non-forwarded objects because forwarded objects are handled above.
>
> OK, but I am still confused what `mark_must_be_preserved` case does in the original assert. Faithfully, you would need to replace that case with `true`, and then the whole assert should fold to always passing? Maybe it handles identityHashCode-bearing or displaced-header objects?

Right. Originally, only relatively few headers need to be preserved. Specifically, all headers that are involved in locking or which carry a hash-code. The assert checks that we either have a header that gets preserved, or a default/prototype header. Now that all headers are preserved, the asserts fold-up to be always true and can go away. I'll fix that.

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PR: https://git.openjdk.java.net/lilliput/pull/6