RFR: 8365932: Implementation of JEP 516: Ahead-of-Time Object Caching with Any GC [v3]

[Erik Österlund]

On Thu, 16 Oct 2025 01:16:54 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Erik Österlund has updated the pull request incrementally with four additional commits since the last revision:
>> 
>>  - Dont print error message when there are no archived objects
>>  - Fix issue in new test
>>  - Change is_aot_thread implementation
>>  - update AotJdk problem lists
>
> src/hotspot/share/cds/cds_globals.hpp line 79:
> 
>> 77:           "the CDS archive, in the specified file")                         \
>> 78:                                                                             \
>> 79:   product(bool, AOTStreamableObjects, true,                                 \
> 
> The default should be `false`, as that will be the mode that the user will get with no GC options are specified.

Makes sense. Fixed.

> src/hotspot/share/cds/heapShared.cpp line 355:
> 
>> 353: }
>> 354: 
>> 355: bool HeapShared::is_loading_mapping_mode() {
> 
> I think `is_loading_mapping_mode()` should be removed and we should use only `is_loading_streaming_mode()`. This will make it easy to find all the places that checks between mapping/streaming. Same for `is_writing_xxx_mode()`.
> 
> Also, instead of having a tri-state of `_heap_load_mode`, it's better to have two boolean states:
> 
> - _is_loading_heap
> - _is_loading_heap_streaming_mode
> 
> The first boolean is the main switch, but most of the code will be checking only the second boolean.

Okay. @stefank is having a look at fixing this.

> src/hotspot/share/cds/heapShared.cpp line 410:
> 
>> 408:       FLAG_SET_ERGO(AOTStreamableObjects, true);
>> 409:       _heap_write_mode = HeapArchiveMode::_streaming;
>> 410:       return;
> 
> This should be changed to `if (!UseZGC) {`, as all other GCs support writing the "mapping" format.

Fixed.

> src/hotspot/share/cds/heapShared.cpp line 418:
> 
>> 416: 
>> 417:   // Select default mode
>> 418:   if (UseG1GC && UseCompressedOops) {
> 
> This is different than specified in JEP 516. The condition should be `if (UseCompressedOops)`.

Okay, I changed it and cleaned up some comments and code suggesting dumping must be done with G1.

> test/hotspot/jtreg/runtime/cds/appcds/TestSerialGCWithCDS.java line 117:
> 
>> 115:                               small2,
>> 116:                               coops,
>> 117:                               "-XX:-AOTStreamableObjects",
> 
> Is this change necessary?

No, I removed it. Nice catch.

> test/hotspot/jtreg/runtime/cds/appcds/TestSerialGCWithCDS.java line 147:
> 
>> 145:             // Regardless of which GC dumped the heap, there will be an object archive, either
>> 146:             // created with mapping if dumped with G1, or streaming if dumped with serial GC.
>> 147:             // At exec time, try to load them into a small SerialGC heap that may be too small.
> 
> Actually the original check of `dumpWithSerial == false` was obsolete, as SerialGC can also dump heap objects.  `dumpWithSerial == false` should be removed. The first two lines of comments should also be removed.

Okay, fixed.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27732#discussion_r2440306099
PR Review Comment: https://git.openjdk.org/jdk/pull/27732#discussion_r2440305408
PR Review Comment: https://git.openjdk.org/jdk/pull/27732#discussion_r2440309294
PR Review Comment: https://git.openjdk.org/jdk/pull/27732#discussion_r2440308642
PR Review Comment: https://git.openjdk.org/jdk/pull/27732#discussion_r2440303912
PR Review Comment: https://git.openjdk.org/jdk/pull/27732#discussion_r2440309778