RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v21]

[Roman Kennke]

On Mon, 7 Oct 2024 08:49:58 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Style note: The added code is inserted between a comment and the code that the comment refers to. It would be nice to tidy this up.
>
> Did you figure out if the code above is correct w.r.t. `MinObjectAlignment=16`?

When MinObjectAlignment=16, then that method does nothing anyway:


  if (MinObjAlignment > 1) {
    return;
  }



I think what it really means to say is

  if (MinObjAlignment >=  CollectedHeap::min_fill_size()) {
    return;
  }



That's also what the comment says: "The size of the gap (if any) right before dense-prefix-end is MinObjAlignment. Need to fill in the gap only if it's smaller than min-obj-size, and the filler obj will extend to next region."

If I interpret that correctly, we need to deal with the situation only when MinObjAlignment < min_fill_size, because the filler object would extend to the next region, and we need to adjust the next region and mark-bitmap for that extra word. @albertnetymk might want to confirm.

I'll move the if (UCOH) block down a little bit to right before if (MinObjAlignment) block.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1789983561