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

[Quan Anh Mai]

On Mon, 26 May 2025 07:15:31 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!

src/hotspot/share/opto/superword.cpp line 2496:

> 2494:   int opc = in->Opcode();
> 2495:   return opc == Op_AddI || opc == Op_SubI || opc == Op_MulI || opc == Op_AndI || opc == Op_OrI || opc == Op_XorI
> 2496:     || opc == Op_ReverseBytesS || opc == Op_ReverseBytesUS;

Are you sure? I don't think you can truncate a `ReverseByteS` to a `byte`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25440#discussion_r2106775202