RFR: 8275527: Separate/refactor forward pointer access

[Albert Mingkun Yang]

On Thu, 14 Oct 2021 16:37:02 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> 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).

Overriding the markword to return null for non-forwarded objs is indeed a bit hacky. However, from a caller's perspective, this is the most intuitive one.

Taking `G1FullGCCompactTask::G1CompactRegionClosure::apply` as an example:


  HeapWord* destination = cast_from_oop<HeapWord*>(obj->forwardee());
  if (destination == NULL) {
    // Object not moving
    return size;
  }
  ...
  Copy::aligned_conjoint_words(obj_addr, destination, size);


`destination` is either null (obj is not moved) or the new location after moving; rather smooth and straightforward control flow.

I wonder if we can use this pattern without overriding the markdown. Here's a straw man proposal.


oop oopDesc::forwardee() const {
  auto mark_word = mark();
  bool is_forwarded = mark_word.is_marked();
  if (!is_forwarded) {
    return nullptr;
  }
  return cast_to_oop(mark_word.decode_pointer());
}


The intention is to load the markword only once. Additionally, callers can continue using the `is_forwarded() / forwardee()` pair, or switch to the `forwardee() == nullptr` pattern on a case-by-case basis if desired.

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