How to find the first bit that is different in C?

Question

I want to compare to Integers in C and the problem is to find the least significant bit that is different. What is the fastest way in C to do this ?

Example:

               Bit
               3210
               ----
a = 13 (binary 1101)
b = 9  (binary 1001)
                ^

The result here should be 2, because bit 2 is the first bit that is different.

Answer 1

ffs() from <strings.h> returns the position of the first bit set, where the bits are numbered starting at 1 for the least significant bit (and ffs(0) returns zero):

unsigned a = 0x0D;
unsigned b = 0x09;

unsigned x = a ^ b;
int pos = ffs(x) - 1;
if (pos == -1) {
   // a and b are equal
} else {
   // pos is the position of the first difference 
}

Answer 2

Bit Twiddling Hacks offers an excellent collection of, er, bit twiddling hacks, with performance/optimisation discussion attached. For your problem (from that site), you can use multiply-and-lookup strategy:

unsigned int c = a ^ b;  
int r;           // result goes here
static const int MultiplyDeBruijnBitPosition[32] = 
{
  0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, 
  31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
};
r = MultiplyDeBruijnBitPosition[((uint32_t)((c & -c) * 0x077CB531U)) >> 27];

References:

• "Using de Bruijn Sequences to Index a 1 in a Computer Word" - Explanation about why the above code works.