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

[Albert Mingkun Yang]

On Mon, 7 Oct 2024 11:49:21 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> 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.
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1791362310