RFR: 8376822: UseCompactObjectHeaders: fill Klass alignment gaps in AOT cache [v2]

[David Holmes]

On Mon, 23 Feb 2026 00:34:30 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> When  `UseCompactObjectHeaders` is enabled, `Klass` objects must be aligned by 1024 bytes. This leaves many gaps in the "rw" region of the AOT cache.
>> 
>> This PR manages the gaps using an `RBTree`. The gaps are sorted in ascending sizes. Gaps of the same size are sorted in ascending addresses. When allocating a small object, we try to find a suitable gap with the lowest address.
>> 
>> Because we have lots of small metaspace objects in the AOT cache, the gaps are always filled. We save abuot 5% of footprint with the JDK's default CDS archive (777KB out of 15.7MB):
>> 
>> 
>> $ java -Xshare:dump -XX:+UseCompactObjectHeaders -Xlog:aot,aot+alloc=trace | grep -i gap | tail -2
>> [0.407s][trace][aot,alloc] 8 bytes @ 0x716e804623f0 in a gap of 8 bytes (used gaps 10293 times, remain gap = 0 bytes in 0 blocks)
>> [0.443s][info ][aot      ] Allocated 10293 objects of 777264 bytes in gaps
>> $ ls -l images/jdk/lib/server/classes_coh.jsa 
>> -rw-rw-r-- 1 iklam iklam 15704064 Feb 22 16:17 images/jdk/lib/server/classes_coh.jsa
>> 
>> 
>> Related changes
>> 
>> - Address sorting added to AOTMapLogger
>> - The null assertion in `ArchivePtrMarker::mark_pointer()` was wrong for the dynamic archive (`ptr_base()` points to the bottom of the dynamic archive, not the bottom of the AOT metaspace).
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   moved offset != 0 check

Technically for enhancements it is not a "regression" test, but a functional test, but we only have noreg labels. :)

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PR Comment: https://git.openjdk.org/jdk/pull/29869#issuecomment-3949074038