[master] RFR: Unconditionally preserve all marks during GC

[Aleksey Shipilev]

On Wed, 19 May 2021 10:27:20 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/g1FullGCCompactionPoint.cpp line 120:
>> 
>>> 118:       // fixed when restoring the preserved marks.
>>> 119:       assert(object->mark() == markWord::prototype_for_klass(object->klass()) || // Correct mark
>>> 120:              object->mark_must_be_preserved() || // Will be restored by PreservedMarksSet
>> 
>> It would seem this assert checks that mark is correct or it would be preserved. Since all marks are now preserved, this assert is basically `true` at all times? If you just drop `object->mark_must_be_preserved()`, it should be failing with forwarded objects, no?
>
> 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?

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