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

[Roman Kennke]

On Wed, 11 Sep 2024 08:24:16 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> @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).

IIRC (it has been a while), the problem is that with Lilliput (and also without compact headers, but disabling compressed class-pointers -UseCompressedClassPointers, but nobody ever does that), byte[] and long[] start at different offsets (12 and 16, respectively). That is because with compact headers, we are using the 4 bytes after the arraylength, but long-arrays cannot do that because of alignment constraints. The impact is that these tests don't work as expected, because vectorization triggers differently. I don't remember the details, TBH, but I believe they would now generate pre-loops, or some might even not vectorize at all. Those seemed to be use-cases that did not look very important, but I may be wrong. It would be nice to properly fix those tests, or make corresponding tests for compact headers, instead, or improve vectorization to better deal with the offset mismatch, if necessary/possible.

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2343693629