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

[Stefan Karlsson]

On Thu, 19 Sep 2024 05:36:41 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> src/hotspot/share/gc/parallel/psParallelCompact.cpp line 787:
>> 
>>> 785:     // The gap is always equal to min-fill-size, so nothing to do.
>>> 786:     return;
>>> 787:   }
>> 
>> Reading the comment, it is not obvious that this is correct if you set MinObjectAlignment to something larger than the default value:
>> 
>> void PSParallelCompact::fill_dense_prefix_end(SpaceId id) {
>>   // Comparing two sizes to decide if filling is required:
>>   //
>>   // 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.
>>   //
>>   // 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.
>> 
>>   // Note: If min-fill-size decreases to 1, this whole method becomes redundant.
>>   if (UseCompactObjectHeaders) {
>>     // The gap is always equal to min-fill-size, so nothing to do.
>>     return;
>>   }
>>   assert(CollectedHeap::min_fill_size() >= 2, "inv");
>
> 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`?

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1789797050