RFR: 8365205: C2: Optimize popcount value computation using knownbits [v3]

[Emanuel Peter]

On Wed, 10 Sep 2025 14:26:31 GMT, Hannes Greule <hgreule at openjdk.org> wrote:

>> testPopCountElisionInt1 and testPopCountElisionLong1 check for absence of PopCount IR nodes.
>
> I think Or and And nodes aren't updated to make use if KnownBits themselves (that generally makes testing based on KnownBits a bit difficult).

@SirYwell @jatin-bhateja I filed an RFE for And / Or. I think these would be really important to do soon, because any other KnownBits optimization relies on those working for verification (generating inputs and verifying outputs). https://bugs.openjdk.org/browse/JDK-8367341

@SirYwell @jatin-bhateja @merykitty I linked this issue here to the KnownBits RFE, to make sure we keep track of all KnownBits extensions. Can you please help me with linking any other RFEs that have already been filed or come up in the future? It would help track progress and avoid duplicated work.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27075#discussion_r2337091504
PR Review Comment: https://git.openjdk.org/jdk/pull/27075#discussion_r2337099580