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

[Stefan Karlsson]

On Mon, 7 Oct 2024 10:49:39 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> 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.

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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1790074043