Vectorizing POPCNT (Long.bitCount)

[Stefan Reich]

Hi Paul,

that's excellent.

Thanks,
Stefan

On Tue, 6 Sept 2022 at 20:47, Paul Sandoz <paul.sandoz at oracle.com> wrote:

> Hi Stefan,
>
> We added, on supporting hardware, vectorized bit count to the soon to be
> released API in JDK 19.
>
>
> https://download.java.net/java/early_access/jdk19/docs/api/jdk.incubator.vector/jdk/incubator/vector/VectorOperators.html#BIT_COUNT
>
> You should be able to experiment with the EA build if you don’t want to
> wait:
>
>   https://jdk.java.net/19/
>
> See here for an overview in the history section:
>
>   https://openjdk.org/jeps/426
>
> Paul.
>
> > On Sep 2, 2022, at 5:59 PM, Stefan Reich <
> stefan.reich.maker.of.eye at googlemail.com> wrote:
> >
> > Hey tthere,
> >
> > I'd like to parallelize this function:
> >
> > static int countDifferingBits(long[] array1, long[] array2) {
> >   int n = array1.length;
> >   int diff = 0;
> >   for (int i = 0; i < n; i++)
> >     diff += Long.bitCount(array1[i]^array2[i]);
> >   return diff;
> > }
> >
> > The function calculates the difference between two shapes (pixel by
> pixel) and is the central piece of an image recognition I am making.
> >
> > Example for a bunch of shapes (these are all the shapes found in the
> extended latin alphabet, in fact): https://botcompany.de/images/1103149
> > Example for a shape comparison: https://botcompany.de/images/1103151
> >
> > The routine above is already mega-fast as is (thanks to HotSpot!), I can
> compare two 256 byte arrays in 29 CPU cycles. That's less than one cycle
> per 64 bit!
> >
> > But I'd still like to try to vectorize it.
> >
> > However, I think the bitCount function will prevent this because it
> doesn't seem to exist in a vectorized form. Or does it? (That would be my
> main question here.)
> >
> > Many greetings,
> > Stefan
> >
> > --
> > == Gaz.AI ==
>
>

-- 
== Gaz.AI ==
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