RFR: 8342095: Add autovectorizer support for subword vector casts [v15]

[Emanuel Peter]

On Fri, 23 Jan 2026 06:19:20 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> Hi all,
>> This patch adds initial support for the autovectorizer to generate conversions between subword types. Currently, when superword sees two packs that have different basic types, it discards them and bails out of vectorization. This patch changes the behavior to ask the backend if a cast between the conflicting types is supported, and keeps the pack if it is. Later, when the `VTransform` graph is built, a synthetic cast is emitted when packs requiring casts are detected. Currently, only narrowing casts are supported as I wanted to re-use existing `VectorCastX2Y` logic for the initial version, but adding more conversions is simple and can be done with a subsequent RFE. I have attached a JMH benchmark and got these results on my Zen 3 machine:
>> 
>> 
>>                                                   Baseline                    Patch
>> Benchmark                  (SIZE)  Mode  Cnt    Score    Error  Units   Score    Error  Units    Improvement
>> VectorSubword.intToByte      1024  avgt   12  200.049 ± 19.787  ns/op   56.228 ± 3.535  ns/op  (3.56x)
>> VectorSubword.intToShort     1024  avgt   12  179.826 ±  1.539  ns/op   43.332 ± 1.166  ns/op  (4.15x)
>> VectorSubword.shortToByte    1024  avgt   12  245.580 ±  6.150  ns/op   29.757 ± 1.055  ns/op  (8.25x)
>> 
>> 
>> I've also added some IR tests and they pass on my linux x64 machine. Thoughts and reviews would be appreciated!
>
> Jasmine Karthikeyan has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 19 commits:
> 
>  - Fix whitespace
>  - Update tests after merge, apply changes from review
>  - Merge from master
>  - 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
>  - ... and 9 more: https://git.openjdk.org/jdk/compare/de6f35ef...13378368

Ok, I filed this as another follow-up:
[JDK-8376179](https://bugs.openjdk.org/browse/JDK-8376179): C2 SuperWord: improve subword vectorization, avoid cast to-and-from int

-------------

PR Comment: https://git.openjdk.org/jdk/pull/23413#issuecomment-3789240675