RFR: 8340010: Fix vectorization tests with compact headers [v2]

[Christian Hagedorn]

On Thu, 21 Nov 2024 11:26:59 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> This is a followup to:
>> https://github.com/openjdk/jdk/pull/20677 / [JDK-8305895](https://bugs.openjdk.org/browse/JDK-8305895) Implement JEP 450: Compact Object Headers (Experimental)
>> 
>> @rkennke fixed the vectorization tests in a very rudamentary way. I now took some time to see what exactly is affected. At the time I reviewed the JEP, I thought only very minor cases were affected, like hand-unrolling etc. But it turns out that there are some more important cases that are affected, like **mixed-type loops**, such as **conversion** between types. Another class of affected loops is **hand-unrolled loops**.
>> 
>> **The problem is this:**
>> Since the offset from array-base to payload has changed (16 -> 12), some vector loads/stores can now no longer be aligned. This means vectorization under `+AlignVector` is not possible.
>> 
>> --------------------------------
>> 
>> First: only platforms that require strict-alignment are affected (i.e. `+AlignVector` or `Matcher::misaligned_vectors_ok=false`). I filed [JDK-8344424](https://bugs.openjdk.org/browse/JDK-8344424) for this, so the impact can be discussed. **The affected platforms seem to be exceptions**.
>> 
>> -------------------
>> 
>> Now on to fixing the tests, which we need to do now anyway. Some actually were currently failing.
>> 
>> I once ran over `tier1,tier2,tier3,tier4` plus our internal stress testing with `+-AlignVector` and `+-UseCompactObjectHeaders`, collected all failing tests. I also looked at all tests that were already guarding IR rules on `UseCompactObjectHeaders`.
>> 
>> To almost all tests I added runs with `+-AlignVector` and `+-UseCompactObjectHeaders`. We could leave this also to global runs with these flag combinations. But it is rare that we ever run this, so I thought I want to directly run the "interesting" tests with all combinations. This requires extra test runtime, but I think it is warranted.
>> 
>> In a few cases I also added stronger IR rules, in tests that were already affected - just to make sure we have the behavior we want. Some cases would not vectorize for other cases, and I put comments there for future reference.
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   make failOf comment explicit for Roberto

Still good!

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Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/22199#pullrequestreview-2457438122