Store (narrow) klass in object header

[rkennke]

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*.

Testing

• tier1
• tier2

---

Progress

• Change must not contain extraneous whitespace
• Change must be properly reviewed

Reviewers

• Aleksey Shipilev (@shipilev - Committer)

Reviewing

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Checkout this PR locally:
$ git fetch https://git.openjdk.java.net/lilliput pull/7/head:pull/7
$ git checkout pull/7

Update a local copy of the PR:
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$ git pr checkout 7

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

[bridgekeeper]

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[mlbridge]

Webrevs

• 01: Full - Incremental (75993587)
• 00: Full (b612a5ad)

[shipilev]

Cursory review follows.

[shipilev]

Looks fine for the experimental code.

[openjdk]

@rkennke This change now passes all automated pre-integration checks.

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Store (narrow) klass in object header

Reviewed-by: shade

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[rkennke]

/integrate

[openjdk]

@rkennke Pushed as commit 161e7c8.

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