RFR: 8365205: C2: Optimize popcount value computation using knownbits [v11]

[Quan Anh Mai]

On Fri, 19 Sep 2025 20:44:54 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> This patch optimizes PopCount value transforms using KnownBits information.
>> Following are the results of the micro-benchmark included with the patch
>> 
>> 
>> 
>> System: 13th Gen Intel(R) Core(TM) i3-1315U
>> 
>> Baseline:
>> Benchmark                                      Mode  Cnt       Score   Error  Units
>> PopCountValueTransform.LogicFoldingKerenLong  thrpt    2  215460.670          ops/s
>> PopCountValueTransform.LogicFoldingKerenlInt  thrpt    2  294014.826          ops/s
>> 
>> Withopt:
>> Benchmark                                      Mode  Cnt       Score   Error  Units
>> PopCountValueTransform.LogicFoldingKerenLong  thrpt    2  389978.082          ops/s
>> PopCountValueTransform.LogicFoldingKerenlInt  thrpt    2  417261.583          ops/s
>> 
>> 
>> Kindly review and share your feedback.
>> 
>> Best Regards,
>> Jatin
>
> Jatin Bhateja has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update countbitsnode.cpp

src/hotspot/share/opto/countbitsnode.cpp line 145:

> 143:   }
> 144:   KnownBits<juint> bits = t->isa_int()->_bits;
> 145:   return TypeInt::make(population_count(bits._ones), population_count(~bits._zeros), Type::WidenMax);

The `widen` of the output should be the same as the `widen` of the input, not `WidenMax` here.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27075#discussion_r2401703583