Question

How to find number of trailing 0s in a binary number?Based on K&R bitcount example of finding 1s in a binary number i modified it a bit to find the trailing 0s.

int bitcount(unsigned x)
{
  int b;
  for(b=0;x!=0;x>>=1)
      {
        if(x&01)
          break;
        else
          b++;
      }

I would like to review this method.

Answer 1

Here’s a way to compute the count in parallel for better efficiency:

unsigned int v;      // 32-bit word input to count zero bits on right
unsigned int c = 32; // c will be the number of zero bits on the right
v &= -signed(v);
if (v) c--;
if (v & 0x0000FFFF) c -= 16;
if (v & 0x00FF00FF) c -= 8;
if (v & 0x0F0F0F0F) c -= 4;
if (v & 0x33333333) c -= 2;
if (v & 0x55555555) c -= 1;