[master] RFR: Store (narrow) klass in object header

Roman Kennke rkennke at openjdk.java.net
Fri May 21 14:28:12 UTC 2021

This change stores compressed Klass* in the upper 32bits of the object header. It doesn't really use it, yet. But I added some code in heap-object-stats to verify that all objects receive it.

It is implemented by storing the compressed Klass* into the prototype-header for each Klass, and use that to initialize objects, pretty much like we used to do for +BiasedLocking.

It will take some serious effort to change all code to use this new location to find the Klass*:
- We need to change all generated code (intepreter, c1, c2) to load Klass* from the header, and call to runtime when it encounters a displaced header word.
- We need to change (sliding-compacting) GCs to be able to iterate heap while objects are forwarded. Most likely by using a forwarding table, but I'm not quite sure yet.
- We need to change rest of runtime to load Klass* from header.

In this change, I modified some code in G1 that deals with checking consistent header, the same asserts that we have seen in #6. This is rather ugly, but I actually expect those asserts to go away or be rewritten in a more useful way.

CDS support is noteworthy because we need to ensure that we propagate the actual narrowKlass whenever we (re-)initialize object headers, because that may be encoded with a different base address for archived Klass instances than other Klass instances, and would create a mess if we would re-encode the Klass*.

 - [x] tier1
 - [x] tier2


Commit messages:
 - Store (narrow) klass in object header

Changes: https://git.openjdk.java.net/lilliput/pull/7/files
 Webrev: https://webrevs.openjdk.java.net/?repo=lilliput&pr=7&range=00
  Stats: 128 lines in 19 files changed: 64 ins; 35 del; 29 mod
  Patch: https://git.openjdk.java.net/lilliput/pull/7.diff
  Fetch: git fetch https://git.openjdk.java.net/lilliput pull/7/head:pull/7

PR: https://git.openjdk.java.net/lilliput/pull/7

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