RFR(xs): 8241581: Add BitMap::count_one_bits variant for arbitrary lengths

[Claes Redestad]

Hi,

On 2020-03-25 13:35, Thomas Stüfe wrote:
> Greetings,
> 
> could I have reviews for this small enhancement please:
> 
> JBS: https://bugs.openjdk.java.net/browse/JDK-8241581
> Webrev:
> http://cr.openjdk.java.net/~stuefe/webrevs/bitmap-arbitrary-range-popcount/webrev.00/webrev/

looks reasonable.

> 
> This adds the ability to call BitMap::count_one_bits on an arbitrary bit
> region, not just on the whole BitMap. I need this for the new Metaspace
> prototype.
> 
> Note that while it would be cool to use platform dependent popcnt
> instructions here, that is not the goal of this fix. This just expands on
> the existing implementation of BitMap::count_one_bits() using the algorithm
> in population:count.hpp.

Yeah, H/W-enabled popcnt would be great, and population_count.hpp
intends to be the place to add that similarly to what's been done for
other intrinsics such as count_leading_zeros.

However, on x86 we compile for a H/W target that doesn't guarantee
popcnt exists.. so intrinsics like gcc's __builtin_popcount are likely
to emit something similar to what's currently in population_count.hpp.
And last I checked that turned out to be a pessimization in the places
where we do 32-bit population counts (C2!).

> 
> It comes with quite thorough gtests. I ran the gtests on little and big
> endian machines and all is well. Submit tests also ran through.

Good!

/Claes