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

Assuming sparsity in number of set bits,

int count = 0;
unsigned int tmp_bitmap = x;        
while (tmp_bitmap > 0) {
    int next_psn = __builtin_ffs(tmp_bitmap) - 1;
    tmp_bitmap &= (tmp_bitmap-1);
    id[count++] = next_psn;
}