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

[Albert Mingkun Yang]

On Thu, 15 Feb 2024 11:00:52 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> test/hotspot/gtest/gc/shared/test_preservedMarks.cpp line 35:
>> 
>>> 33:   // could be interpreted as 'self-forwarded' when encoded as
>>> 34:   // forwardee in the mark-word.
>>> 35:   alignas(BytesPerLong) oopDesc _oop;
>> 
>> Could you explain why this is required? (I'd expect `oopDesc` to be properly aligned already, since it's >= 8 bytes.)
>
> This is a problem with x86_32 builds, where oopDesc is only 4 byte. It is only a problem in this test, in real world, objects in the heap are always at least 8-byte-aligned.

Even on 32bits system, shouldn't `_mark` = 4 bytes and `_metadata` = 4 bytes, so the total size of `oopDesc` is 8 bytes?

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PR Review Comment: https://git.openjdk.org/jdk/pull/17755#discussion_r1491039970