On Thu, 22 Aug 2024 11:05:18 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 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 t... > > 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) Superseding by #20677 ------------- PR Comment: https://git.openjdk.org/jdk/pull/20640#issuecomment-2304864714
