RFR: 8275527: Separate/refactor forward pointer access [v4]

[Roman Kennke]

On Thu, 28 Oct 2021 12:35:37 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Accessing the forward pointer overlay of object headers is currently done in a somewhat crude way. oopDesc::forwardee() and oopeDesc::is_forwarded() both load the header-word. This seems kinda bad, because they are most often used in conjunction, e.g.:
>> fwd = obj->forwarded() ? obj->forwardee() : promote_obj();
>> 
>> or similar constructs.
>> 
>> Also, in some places, the forwardee is accessed in a more direct fashion using markWord::decode_pointer(), or as HeapWord*, sometimes is_forwarded() is determined by forwardee() == NULL checks (and crude overrides to prepare non-forwarded headers to return NULL in such checks, see g1Full* source files).
>> 
>> I propose to extract and refactor forward pointer access in a way to better support above pattern. In-fact I originally wrote this with performance-enhancement in mind (see oopForwarding.hpp header for more details), but I could not show any actual performance improvements, so let's consider this purely on cleanliness and separation-of-concerns basis.
>> 
>> Testing:
>>  - [x] tier
>>  - [x] tier2
>>  - [x] hotspot_gc
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Move forward impl into markWord and add assert

I've removed OopForwarding altogether, and only kept the actual refactorings and cleanups. Can you please re-review?

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PR: https://git.openjdk.java.net/jdk/pull/5955