bit twiddling in java - decomposing long into long[] of bitmasks

Question

I am decomposing a single long into a long[] of single bit longs by

public static int decompose(long[] buffer, long base) {
  int count = Long.bitCount(base);
  for (int i=0; i<count; i++) {
    base ^= (buffer[i] = Long.lowestOneBit(base));
  }
  return count;
}

but I feel like there might be a faster way to do this, since it seems like there are some repeated steps. For example, counting the bits should already come pretty close to getting all the info needed to populate the result.

Any suggestions? I'm familiar with the premature optimization mantra, hence why I'm not pushing my solution any further on my time, but perhaps someone else has seen this before or is addicted to optimization...EDIT: please run any suggestions through the test harness Mark provided below. I am more than a bit surprised that my first inkling is actually holding up.

test code, which is inside a JUnit method:

Random rng = new Random();
long size = 0;
long[] hold = new long[Long.SIZE];
System.out.println("init:"+Long.toBinaryString(BitTwiddling.bitmask(rng.nextInt(Long.SIZE)))); //initialize BitTwiddling internals
long start = System.currentTimeMillis();
for (int i=0; i<compareIterations; i++) size+= BitTwiddling.decompose(hold,rng.nextInt(Integer.MAX_VALUE));
long delta = System.currentTimeMillis() - start;
double base = (double)size/delta;

size = 0;
start = System.currentTimeMillis();
for (int i=0; i<compareIterations; i++) size += BitTwiddling.decomposeAlt(hold, rng.nextInt(Integer.MAX_VALUE));
long delta2 = System.currentTimeMillis() - start;
double base2 = (double)size/delta2;

then compare base and base2

Answer 1

OK after trying long and hard to beat your original code, I found one that consistently beats yours (in my environment) by shamelessly stealing your code and tweaking it.

    protected int decompose(long[] buffer, long base) {
        int count = Long.bitCount(base);
        for (int i = 0; i < count; i++) {
            base -= (buffer[i] = Long.lowestOneBit(base));
        }
        return count;
    }

Only difference: uses subtraction instead of xor. I don't think there are any edge cases; I certainly didn't find any. No idea why those aren't optimized to the same thing (assuming there aren't edge cases), but it does make mine consistently faster (again, on my machine). I've added this version to the test answer.

Edit I guess the reason it can't be optimized is because the compiler doesn't know that the operand will always be less than base (since we're going from lower to higher bits).

Edit #2 With the -server flag set, Carl's is consistently marginally faster than mine again.

Edit #3 Yep sure enough, when run on a 64 bit JVM this is significantly faster:

    protected int decompose(long[] buffer, long base) {
        int count = 0;
        while ( 0 != (base -= (buffer[count++] = Long.lowestOneBit(base))));
        return count;
    }

(Or the equivalent xor version), because it can perform 64-bit comparisons using native processor operations.

Answer 2

Here's the harness I'm using at the moment. I consistently get the code in the question (Method 1) to perform significantly faster. Currently Mark Peters' answer is the fastest normally, but Carl's is faster with the -server flag set. Also, mikera's gets considerably more competitive in server mode (though still slower than Carl/Mark's).

import java.util.Random;

public abstract class Harness {
    public static void main(String[] args) {
        while (true) {
            long[] randomData = new long[4096];
            Random r = new Random();
            for (int i = 0; i < randomData.length; i++) {
                randomData[i] = r.nextLong();
            }
            long[] buffer = new long[64];

            Harness[] versions = new Harness[] {
                    new Control(randomData, buffer),
                    new Carl(randomData, buffer),
                    new MarkPeters(randomData, buffer),
                    new MarkPetersServer64Bit(randomData, buffer),
//                    new Rsp1(randomData, buffer), 
                    new Rsp2(randomData, buffer),
                    new Mikera(randomData, buffer)
                    };
            for (Harness v : versions) {
                v.doTest();
            }
        }
    }

    private final long[] buffer;
    private final long[] randomData;

    protected Harness(long[] randomData, long[] buffer) {
        this.randomData = randomData;
        this.buffer = buffer;
    }

    public void doTest() {
        long start = System.nanoTime();
        long count = 0;
        for (int times = 0; times < 1000; times++) {
            for (int i = 0; i < randomData.length; i++) {
                count = decompose(buffer, randomData[i]);
            }
        }
        long end = System.nanoTime();

        //verify decomposition of last item
        long l = 0;
        for ( int i = 0; i < count; i++ ) {
            l |= buffer[i];
        }

        System.out.println(getClass().getSimpleName() + " took " + (end - start)
                / 1000000.0 + " ms - last base: " + l);
    }

    protected abstract int decompose(long[] buffer, long base);

    private static class Control extends Harness {
        protected Control(long[] randomData, long[] buffer) {
            super(randomData, buffer);
        }

        @Override
        protected int decompose(long[] buffer, long base) {
            return 0;
        }
    }

    private static class Carl extends Harness {
        protected Carl(long[] randomData, long[] buffer) {
            super(randomData, buffer);
        }

        @Override
        protected int decompose(long[] buffer, long base) {
            final int count = Long.bitCount(base);
            for (int i = 0; i < count; i++) {
                base ^= (buffer[i] = Long.lowestOneBit(base));
            }
            return count;
        }
    }

    private static class Mikera extends Harness {
        protected Mikera(long[] randomData, long[] buffer) {
            super(randomData, buffer);
        }

        @Override
        protected int decompose(long[] buffer, long base) {
            int count = 0;
            while (base != 0) {
                long mask = base & (-base);
                base &= ~mask;
                buffer[count++] = mask;
            }
            return count;
        }
    }

    private static class Rsp1 extends Harness {
        protected Rsp1(long[] randomData, long[] buffer) {
            super(randomData, buffer);
        }

        @Override
        protected int decompose(long[] buffer, long base) {
            int count = 0;

            if (0 != (base & 1)) {
                buffer[count++] = 1;
            }

            base >>>= 1;

            for (long bit = 1; 0 < bit && bit <= base; ) {
                if (0 < (base & bit)) {
                    buffer[count++] = (bit <<= 1);
                }
            }
            return count;
        }
    }

    private static class Rsp2 extends Harness {
        protected Rsp2(long[] randomData, long[] buffer) {
            super(randomData, buffer);
        }

        @Override
        protected int decompose(long[] buffer, long base) {
            int count = 0;

            for (long bit = 1; 0 != base; bit <<= 1, base >>>= 1) {
                if (0 < (base & 1)) {
                    buffer[count++] = bit;
                }
            }
            return count;
        }
    }

    private static class MarkPeters extends Harness {
        protected MarkPeters(long[] randomData, long[] buffer) {
            super(randomData, buffer);
        }

        @Override
        protected int decompose(long[] buffer, long base) {
            int count = Long.bitCount(base);
            for (int i = 0; i < count; i++) {
                base -= (buffer[i] = Long.lowestOneBit(base));
            }
            return count;

        }
    }

    private static class MarkPetersServer64Bit extends Harness {
        protected MarkPetersServer64Bit(long[] randomData, long[] buffer) {
            super(randomData, buffer);
        }

        @Override
        protected int decompose(long[] buffer, long base) {
            int count = 0;
            while ( 0 != (base ^= (buffer[count++] = Long.lowestOneBit(base))));
            return count;
        }
    }
}

Sample output

Which method is best depends on the situation.

Non-server, 32-bit JVM

Control took 41.175272 ms - last base: 0
Carl took 691.966919 ms - last base: 5852835112840111303
MarkPeters took 642.230253 ms - last base: 5852835112840111303
MarkPetersServer64Bit took 742.594626 ms - last base: 5852835112840111303
Rsp2 took 3886.203787 ms - last base: 5852835112840111303
Mikera took 1044.451494 ms - last base: 5852835112840111303

Winner: MarkPeters

Server, 32-bit JVM

Control took 2.354383 ms - last base: 0
Carl took 508.687401 ms - last base: 338317437500027646
MarkPeters took 521.831297 ms - last base: 338317437500027646
MarkPetersServer64Bit took 727.052206 ms - last base: 338317437500027646
Rsp2 took 3811.75662 ms - last base: 338317437500027646
Mikera took 665.252599 ms - last base: 338317437500027646

Winner: Carl

Server (implicit or explicit), 64-bit JVM

Control took 0.007186 ms - last base: 0
Carl took 543.701859 ms - last base: -8898262206218882664
MarkPeters took 439.706079 ms - last base: -8898262206218882664
MarkPetersServer64Bit took 391.831055 ms - last base: -8898262206218882664
Rsp2 took 1861.40449 ms - last base: -8898262206218882664
Mikera took 435.964319 ms - last base: -8898262206218882664

Winner: MarkPetersServer64Bit

Note: there is no comparable 64-bit JVM that can run in non-server mode to my knowledge. See here:

Are both -client and -server VM modes available in 64-bit Java?

Currently only the Java HotSpot Server VM supports 64-bit operation, and the -server option is implicit with the use of -d64. This is subject to change in a future release.

Answer 3

Since I love optimization*, here's a version you might try instead:

public static int decompose(long[] buffer, long base) {
    int count = 0;
    while (base != 0) {
        long mask = base & (-base);
        base &= ~mask;
        buffer[count++] = mask;
    }
    return count;
}

Main things I did were:

• Inline the lowest one bit calculation to avoid a method call overhead. This can be a win in the (rare but possible) cases where the compiler/JVM isn't clever enough to do it for you....
• Pass an array of longs as an input parameter to avoid a memory allocation and the need to count the bits to determine the array size. Function now returns the number of bitmasks found.
• Zero the bits as you go along, so that you can bail out as early as possible from the loop

*because it's fun, regardless of whether it is premature or not

Answer 4

I'd use a mask, calculated at each loop with the shift operator:

for (int i= 0; i < result.length; i++)
    result[i]= base & (1<<i);

Should be both clear and fast.