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

[Stefan Karlsson]

On Tue, 8 Oct 2024 07:40:24 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> After re-reading this again I agree with what you're writing. If you make the change to use:
>> 
>> if (MinObjAlignment >= CollectedHeap::min_fill_size()) {
>>  return;
>>  }
>> 
>> 
>> do you even have to check for UCOH in this function?
>> 
>> I also wonder if you could tweak the comment now that this is not true when UCOH is on:
>> 
>> // The size of the filler (min-obj-size) is 2 heap words with the default
>> // MinObjAlignment, since both markword and klass take 1 heap word.
>
> I took UCOH into account when this code was written -- the current version of PR would fail the following assert. 
> 
> 
>   // Note: If min-fill-size decreases to 1, this whole method becomes redundant.
>   assert(CollectedHeap::min_fill_size() >= 2, "inv");
> 
> 
> The least intrusive way, IMO, is to put `if (UCOH) { return; }` right before `// Note: ...`, kind of like what Roman originally put it. I believe the advantage of this style is that when UCOH before always-true, it's obvious this whole method essentially becomes `return`and can be removed right away.

I was thinking that we should remove the entire:

  // Note: If min-fill-size decreases to 1, this whole method becomes redundant.
  assert(CollectedHeap::min_fill_size() >= 2, "inv");

block, since it is now incorrect, guarded by the proper check, and the comment is misleading since we now can have a min-fill-size that is 1.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1791409345