On Fri, 12 May 2023 18:03:13 GMT, Coleen Phillimore <cole...@openjdk.org> wrote:
>> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are building on #10907, #13582 and #13779 to >> protect the relevant (upper 32) bits of the mark-word. Significant parts of >> this PR deal with loading the compressed Klass* from the mark-word, and >> dealing with (monitor-)locked objects. When the object is monitor-locked, we >> load the displaced mark-word from the monitor, and load the compressed >> Klass* from there. This PR also changes some code paths (mostly in GCs) to >> be more careful when accessing Klass* (or mark-word or size) to be able to >> fetch it from the forwardee in case the object is forwarded, and/or reach >> through to the monitor when the object is locked by a monitor. >> - The identity hash-code is narrowed to 25 bits. >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will can now store their length at offset 8. Due to alignment >> restrictions, array elements will still start at offset 16. #11044 will >> resolve that restriction and allow array elements to start at offset 12 >> (except for long, double and uncompressed oops, which are still required to >> start at an element-aligned offset). >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. >> >> Testing: >> (+UseCompactObjectHeaders tests are run with the flag hard-patched into the >> build, to also catch @flagless tests, and to avoid mismatches with CDS - see >> above.) >> - [x] tier1 (x86_64) >> - [x] tier2 (x86_64) >> - [x] tier3 (x86_64) >> - [ ] tier4 (x86_64) >> - [x] tier1 (aarch64) >> - [x] tier2 (aarch64) >> - [x] tier3 (aarch64) >> - [ ] tier4 (aarch64) >> - [ ] tier1 (x86_64) +UseCompactObjectHeaders >> - [ ] tier2 (x86_64) +UseCompactObjectHeaders >> - [ ] tier3 (x86_64) +UseCompactObjectHeaders >> - [ ] tier4 (x86_64) +UseCompactObjectHeaders >> - [ ] tier1 (aarch64) +UseCompactObjectHeaders >> - [ ] tier2 (aarch64) +UseCompactObjectHeaders >> - [ ] tier3 (aarch64) +UseCompactObjectHeaders >> - [ ] tier4 (aarch64) +UseCompactObjectHeaders > > src/hotspot/share/cds/archiveBuilder.cpp line 726: > >> 724: >> k->set_prototype_header(markWord::prototype().set_narrow_klass(nk)); >> 725: } >> 726: #endif //_LP64 > > If CDS is turned off for UseCompactObjectHeaders, I don't understand this > change or the one to archiveHeapWriter. -Xshare:dump objects would be the > wrong size. If CDS is not supported, then there should be something in > arguments.cpp that gives an error for that. And write a test for that error > of mixing and matching. Yeah, we do have code in arguments.cpp that turns off CDS if the wrong setting is used (i.e. the opposite of the default setting). If you hard-code UseCompactObjectHeaders to be true, then the archives will be written in the compact layout, and can be read. That's what the changes in share/cds implement. (Note: I regularily hard-patch the UseCompactObjectHeaders flag to be true for testing, because that also catches all the @flagless tests, and I know that Daniel does that, too.) We disabled CDS jtreg tests when passing +UseCompactObjectHeaders via cmd line, because we would see a lot of test failures because of archive format mismatch. I am not sure if that's a good way to deal with that. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/13961#discussion_r1192698676
