What is the fastest way to return the positions of all set bits in a 64-bit integer?

Question

I need a fast way to get the position of all one bits in a 64-bit integer. For example, given x = 123703, I\'d like to fill an array idx[] = {0, 1, 2, 4,

Answer 1

Here's something very simple which might be faster - no way to know without testing. Much will depend on the number of bits set vs. the number unset. You could unroll this to remove branching altogether but with today's processors I don't know if it would speed up at all.

unsigned char idx[K+1]; // need one extra for overwrite protection
unsigned char *dst=idx;
for (unsigned char i = 0; i < 50; i++)
{
    *dst = i;
    dst += x & 1;
    x >>= 1;
}

P.S. your sample output in the question is wrong, see http://ideone.com/2o032E