> 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 purpose of the flag is to > provide a fallback, in case that users unexpectedly observe problems with the > new implementation. The intention is that this flag will remain experimental > and opt-in for at least one release, then make it on-by-default and > diagnostic (?), and eventually deprecate and obsolete it. However, there are > a few unknowns in that plan, specifically, we may want to further improve > compact headers to 4 bytes, we are planning to enhance the Klass* encoding to > support virtually unlimited number of Klasses, at which point we could also > obsolete UseCompressedClassPointers. > - 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 #20603 and #20605 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. 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. > - The identity hash-code is temporarily narrowed to 25 bits. As soon as we > get Tiny Class-Pointers (planned before the JEP can be integrated, and to be > opened for review soon), we will widen the hash-bits back to 31 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. > - 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. Some build machinery is added so that > _coh variants of CDS archives are generated, next to the _nocoops variant. > - Note that oopDesc::klass_offset_in_bytes() is not used by +UCOH paths > anymore. The only exception is C2, which uses it as a placeholder/identifier > of the special memory slice that only LoadNKlass uses. The backend then > extracts the original oop and loads its mark-word and extracts the > narrow-Klass* from that. I played with other approaches to implement > LoadNKlass. Expanding it as a macro did not easily work, because C2 is > missing a way to cast a word-sized integral to a narrow-Klass* (o...
Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits: - Merge branch 'JDK-8305896-v2' into JDK-8305895-v3 - Merge branch 'JDK-8305896-v2' into JDK-8305895-v3 - Merge branch 'JDK-8305896-v2' into JDK-8305895-v3 - Explicitely make AVAILABLE=true - Export new hash constants to JVMCI - Improve asserts - 8305894: Implementation: JEP 450: Compact Object Headers (Experimental) ------------- Changes: https://git.openjdk.org/jdk/pull/20640/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20640&range=02 Stats: 1670 lines in 105 files changed: 1234 ins; 208 del; 228 mod Patch: https://git.openjdk.org/jdk/pull/20640.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/20640/head:pull/20640 PR: https://git.openjdk.org/jdk/pull/20640
