RFR: 8350177: C2 SuperWord: Integer.numberOfLeadingZeros, numberOfTrailingZeros, reverse and bitCount have input types wrongly turncated for byte and short [v7]

[Emanuel Peter]

On Mon, 30 Jun 2025 03:55:41 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

>> Hi all,
>> This patch fixes cases in SuperWord when compiling subword types where vectorized code would be given a narrower type than expected, leading to miscompilation due to truncation. This fix is a generalization of the same fix applied for `Integer.reverseBytes` in [JDK-8305324](https://bugs.openjdk.org/browse/JDK-8305324). The patch introduces a check for nodes that are known to tolerate truncation, so that any future cases of subword truncation will avoid creating miscompiled code.
>> 
>> The patch reuses the existing logic to set the type of the vectors to int, which currently disables vectorization for the affected patterns entirely. Once [JDK-8342095](https://bugs.openjdk.org/browse/JDK-8342095) is merged and automatic casting support is added the autovectorizer should automatically insert casts to and from int, maintaining correctness.
>> 
>> I've added an IR test that checks for correctly compiled outputs. Thoughts and reviews would be appreciated!
>
> Jasmine Karthikeyan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add case for bytes and prevent vector ops from truncating

src/hotspot/share/opto/superword.cpp line 2612:

> 2610:   case Op_XorReductionV:
> 2611:   case Op_MaxReductionV:
> 2612:   case Op_MinReductionV:

Can we now remove those, since you are already handling all vectors explicitly above?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25440#discussion_r2174277639