RFR: 8305895: Implementation: JEP 450: Compact Object Headers (Experimental) [v7]

[Aleksey Shipilev]

On Thu, 11 May 2023 10:00:00 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 28 commits:
>> 
>>  - Merge branch 'JDK-8305898' into JDK-8305895
>>  - @shipilev review, round 2
>>  - Fix build
>>  - @shipilev comments, round 1
>>  - Allow to resolve mark with LW locking
>>  - Use new lightweight locking with compact headers
>>  - Merge branch 'JDK-8305898' into JDK-8305895
>>  - Imporve GetObjectSizeIntrinsicsTest
>>  - Some GC fixes
>>  - Add BaseOffsets test
>>  - ... and 18 more: https://git.openjdk.org/jdk/compare/39c33727...58046e58
>
> src/hotspot/share/gc/parallel/psPromotionManager.inline.hpp line 250:
> 
>> 248:   Copy::aligned_disjoint_words(cast_from_oop<HeapWord*>(o), cast_from_oop<HeapWord*>(new_obj), new_obj_size);
>> 249: 
>> 250:   if (!new_obj->mark().is_marked()) {
> 
> For this check to work correctly, we are assuming that Copy::aligned_disjoint_words respects word level atomicity, even though we are using one of the non-atomic copying functions. That doesn't feel safe.

True, it is not exactly safe. I wonder if we can plug this particular leak by doing the following:


  // Copy obj
  Copy::aligned_disjoint_words(cast_from_oop<HeapWord*>(o), cast_from_oop<HeapWord*>(new_obj), new_obj_size);

  if (UseCompactObjectHeaders) {
    // The copy above is not atomic. Make sure we have seen the proper mark
    // and re-install it into the copy, so that Klass* is guaranteed to be correct.
    markWord mark = o->mark_acquire();
    if (!mark.is_marked()) {
      new_obj->set_mark(mark);
      ContinuationGCSupport::transform_stack_chunk(new_obj);
    } else {
      // If we copied a mark-word that indicates 'forwarded' state, the object
      // installation would not succeed. We cannot access Klass* anymore either.
      // Skip the transformation.
    }
  } else {
    ContinuationGCSupport::transform_stack_chunk(new_obj);
  }

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/13844#discussion_r1190970182