Re: Long.bitCount micro-optimization

[Isaac Levy]

Original message below:

The JDK impl of bitCount can be improved -- though most users will get
the hotspot intrinsic. The best source I could find for the suggestion
below is page 195: http://support.amd.com/techdocs/25112.pdf

Cheers,
Isaac Levy


Proposed Long.bitCount and Integer.bitCount:


public static int bitCount(long i)
{
    i -= (i >>> 1) & 0x5555555555555555L;
    i = (i & 0x3333333333333333L) + ((i >>> 2) & 0x3333333333333333L);
    i = (i + (i >>> 4)) & 0x0f0f0f0f0f0f0f0fL;
    return (i * 0x0101010101010101L) >>> 56;
}


public static int bitCount(int i) {
    i -= (i >>> 1) & 0x55555555;
    i = (i & 0x33333333) + ((i >>> 2) & 0x33333333);
    i = (i + (i >>> 4)) & 0x0f0f0f0f;
    return (i * 0x01010101) >>> 24;
}