RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v11]

[Emanuel Peter]

On Tue, 10 Sep 2024 19:11:30 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> This is the main body of the JEP 450: Compact Object Headers (Experimental).
>> 
>> It is also a follow-up to #20640, which now also includes (and supersedes) #20603 and #20605, plus the Tiny Class-Pointers parts that have been previously missing.
>> 
>> 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 add some changes to GC forwarding (see below) to protect the relevant (upper 22) 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.
>>  - Self-forwarding in GCs (which is used to deal with promotion failure) now uses a bit to indicate 'self-forwarding'. This is needed to preserve the crucial Klass* bits in the header. This also allows to get rid of preserved-header machinery in SerialGC and G1 (Parallel GC abuses preserved-marks to also find all other relevant oops).
>>  - Full GC forwarding now uses an encoding similar to compressed-oops. We have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the GC forwarding at all).
>>  - 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 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 _co...
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix FullGCForwarding initialization

@rkennke Can you please explain the changes in these tests:

test/hotspot/jtreg/compiler/c2/irTests/TestVectorizationMismatchedAccess.java
test/hotspot/jtreg/compiler/c2/irTests/TestVectorizationNotRun.java
test/hotspot/jtreg/compiler/loopopts/superword/TestAlignVector.java
test/hotspot/jtreg/compiler/loopopts/superword/TestIndependentPacksWithCyclicDependency.java
test/hotspot/jtreg/compiler/loopopts/superword/TestMulAddS2I.java


You added these IR rule restriction:
`@IR(applyIf = {"UseCompactObjectHeaders", "false"},`

This means that if `UseCompactObjectHeaders` is enabled, vectorization seems to be impacted - that could be concerning because it has a performance impact.

I have recently changed a few things in SuperWord, so maybe some of them can be removed, because they now vectorize anyway?

Of course some special tests may just rely on `UseCompactObjectHeaders == false` - but I would need some comments in the tests where you added it to justify why we add the restriction.

Please also test this patch with the cross combinations of `UseCompactObjectHeaders` and `AlignVector` enabled and disabled (and add `VerifyAlignVector` as well).

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2342983487