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 some tight code, written for 1-byte (8-bits), but it should easily, obviously expand to 64-bits.

int main(void)
{
    int x = 187;

    int ans[8] = {-1,-1,-1,-1,-1,-1,-1,-1};
    int idx = 0;

    while (x)
    {
        switch (x & ~(x-1))
        {
        case 0x01: ans[idx++] = 0; break;
        case 0x02: ans[idx++] = 1; break;
        case 0x04: ans[idx++] = 2; break;
        case 0x08: ans[idx++] = 3; break;
        case 0x10: ans[idx++] = 4; break;
        case 0x20: ans[idx++] = 5; break;
        case 0x40: ans[idx++] = 6; break;
        case 0x80: ans[idx++] = 7; break;
        }

        x &= x-1;
    }

   getchar();
   return 0;
}

Output array should be:

ans = {0,1,3,4,5,7,-1,-1};