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

[Stefan Karlsson]

On Thu, 19 Sep 2024 05:35:34 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   JVMCI support
>
> 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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1766186545