RFR: 8279856: Parallel: Use PreservedMarks to record promotion-failed objects

[Stefan Johansson]

On Tue, 11 Jan 2022 14:02:20 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple change of using `PreservedMarks` to skip young-gen walking in finding promotion-failed objs.
> 
> Test: tier1-6

Looks good. 

One think that would be interesting would be to do some measurements on the additional memory usage for the `PreservedMarks` stack with this change. To better understand what kind of memory trade-off we do for the quicker processing.

src/hotspot/share/gc/parallel/psPromotionManager.cpp line 363:

> 361:     push_contents(obj);
> 362: 
> 363:     _preserved_marks->push_always(obj, obj_mark);

I think a comment here explaining that we use the preserved marks no only to preserved "special" marks but also to restore and clear the forwardning pointer in "normal" marks.

src/hotspot/share/gc/shared/preservedMarks.hpp line 60:

> 58:   size_t size() const { return _stack.size(); }
> 59:   inline void push_if_necessary(oop obj, markWord m);
> 60:   inline void push_always(oop obj, markWord m);

Any reason not to just call this `push()` like we had in the past and maybe also re-use it in `push_if_necessary()`?

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Changes requested by sjohanss (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7028