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

[Albert Mingkun Yang]

On Tue, 8 Oct 2024 08:12:31 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> 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.

It's still correct when UCOH is disabled -- therefore, the UCOH check can be placed at the start without changing any existing logic. (The "rest" of this method assumes min-fill-size is 2, `assert(CollectedHeap::min_fill_size() == 2, "inv")`.)

In this PR, since this method doesn't access UCOH, it can be easily forgotten to update this method when the UCOH flag is removed eventually -- it's not obvious to me that `MinObjAlignment >= checked_cast<int>(CollectedHeap::min_fill_size())` is related to (or can be affected by) `UCOH` at first glance.

(I slightly prefer having a `if (UCOH)` inside this method, but considering this method will be nuked in the long run, any short-time decision is fine by me, assuming the failing assert is fixed.)

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1791611304