RFR: 8365205: C2: Optimize popcount value computation using knownbits [v8]

[Jatin Bhateja]

On Tue, 16 Sep 2025 07:15:00 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Jatin Bhateja has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Extending the random ranges
>
> test/hotspot/jtreg/compiler/intrinsics/TestPopCountValueTransforms.java line 56:
> 
>> 54:     static final long rand_bndL2 = G.uniformLongs(-0xFFFFFFL, 0xFFFFFF).next();
>> 55:     static final long rand_popcL1 = G.uniformLongs(0, 4).next();
>> 56:     static final long rand_popcL2 = G.uniformLongs(0, 32).next();
> 
> Can you please give us some code comments why you are doing:
> - only uniform distribution. Is that needed? Generators generates special values more often for a good reason: it creates interesting edge cases, especially for bit operations like this here.
> - Why are you restricting the ranges? There could always be surprises outside the ranges you pick, and it would be a shame to not generate those. Unless you are absolutely sure they are not needed. Or if extending the range would mean we would generate interesting cases with a probability that is too small, that could be another reason to restrict the ranges.

Thanks @eme64!, comment addressed.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/27075#discussion_r2362138729