RFR: 8342095: Add autovectorizer support for subword vector casts [v14]

[Jasmine Karthikeyan]

On Wed, 27 Aug 2025 10:20:37 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Jasmine Karthikeyan has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:
>> 
>>  - Update tests, cleanup logic
>>  - Merge branch 'master' into vectorize-subword
>>  - Check for AVX2 for byte/long conversions
>>  - Whitespace and benchmark tweak
>>  - Address more comments, make test and benchmark more exhaustive
>>  - Merge from master
>>  - Fix copyright after merge
>>  - Fix copyright
>>  - Merge
>>  - Implement patch with VectorCastNode::implemented
>>  - ... and 6 more: https://git.openjdk.org/jdk/compare/8fcbb110...aabaafba
>
> test/hotspot/jtreg/compiler/loopopts/superword/TestCompatibleUseDefTypeSize.java line 513:
> 
>> 511:     @Test
>> 512:     @IR(applyIfCPUFeature = { "avx", "true" },
>> 513:         applyIfOr = {"AlignVector", "false", "UseCompactObjectHeaders", "false"},
> 
> Do you think these would be supported with `asimd` as well?
> If you just cannot test with it feel free to file an RFE and then I can find someone to take care of it (e.g. as a starter bug).

I did a bit of testing on aarch64 hardware and have updated the tests to include `asimd` where supported.

> test/hotspot/jtreg/compiler/vectorization/TestSubwordTruncation.java line 76:
> 
>> 74: 
>> 75:     @Test
>> 76:     @IR(applyIfCPUFeature = { "avx2", "true" }, counts = { IRNode.VECTOR_CAST_I2S, IRNode.VECTOR_SIZE_ANY, ">0" })
> 
> Do you think we can make the vector size more precise here?

This is a good idea, I've replaced the `any` sizes with `min(int, <subword>)` which should continue to be portable across architectures.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23413#discussion_r2719788905
PR Review Comment: https://git.openjdk.org/jdk/pull/23413#discussion_r2719788217