On Fri, 12 May 2023 17:27:25 GMT, Roman Kennke <rken...@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 This pull request has been closed without being integrated. ------------- PR: https://git.openjdk.org/jdk/pull/13961
