On Wed, 21 May 2025 09:23:58 GMT, Radim Vansa <rva...@openjdk.org> wrote:
>> This problem should happen only with dynamic CDS archives. >> >> With the static CDS archive (and AOT caches as well; `-XX:AOTCache` >> basically are static CDS archives), Symbols are copied into the archive >> while preserving the order of their relative addresses. >> >> However, when creating a dynamic CDS archive, we first load a static >> archive. Let's say we load the static archive at address 0x800000000, a >> Symbol `A` in the static archive could have an address of `0x800001000`. >> Now, we load a class `K` who has a method named `B` but such a Symbol is not >> in the static archive, this Symbol will be dynamically allocated from the >> Metaspace. Since the address is decided by the OS, this symbol could have an >> address below the static archive. E.g., `0x700000000`; >> >> Let's say `K` defines two methods, `A()` and `B()`, they are stored in the >> `InstanceKlass::_methods` array for `K`. This array is sorted by ascending >> addresses of the method's names. So `K::B()` comes before `K::A()`. >> >> When `K` is copied into the dynamic archive, the symbol `B` will also be >> copied into the dynamic archive. The dynamic archive resides in a memory >> region above the static archive, so `B` will have an address like >> `0x800002000`. Therefore, we want the copy of `K` in the dynamic archive to >> have a `_methods` array where `K::A()` comes before `K::B()`. >> >> This is done in >> [`DynamicArchiveBuilder::sort_methods()`](https://github.com/openjdk/jdk/blob/cedd1a5343dceb5394b8ed5ea78bb717f05c8caf/src/hotspot/share/cds/dynamicArchive.cpp#L138) >> >> My recommendation is to sort your table using `fast_compare()`, and re-sort >> this table in the same fashion as `DynamicArchiveBuilder::sort_methods()`. > > Thanks for the pointers; in fact I had the problem with a (intermediate) > variant that encoded the symbol addresses right into the stream. Therefore > the issue was comparing value of symbol values before CDS persisting those > and after dehydration. Since we're comparing only symbols within single > class, I hope that the issue you describe above won't happen. I'll keep the > problem with dynamic archives in mind, though. > With the current code I can try using `fast_compare` again and get back if I > find any problems again. Even if the fields are only defined within the same class, the names of the fields are shared among different classes. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/24847#discussion_r2100530720
