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

Has this been found to be too slow?
Small and crude, but it's all in the cache and CPU registers;

void mybits(uint64_t x, unsigned char *idx)
{
  unsigned char n = 0;
  do {
    if (x & 1) *(idx++) = n;
    n++;
  } while (x >>= 1);          // If x is signed this will never end
  *idx = (unsigned char) 255; // List Terminator
}

It's still 3 times faster to unroll the loop and produce an array of 64 true/false values (which isn't quite what's wanted)

void mybits_3_2(uint64_t x, idx_type idx[])
{
#define SET(i) (idx[i] = (x & (1UL<