Walking Set Bits

Sometimes you want to walk the set bits of an integer and do something for each bit. For example, consider printing a human readable version of a flag field (which could have up to say 32 unique flags set).

The naive way to accomplish this is of course to visit every bit and see if something is set. But we’re cooler than that.

A better approach is to repeatedly pick off the lowest set bit until there’s nothing left. Here’s what that looks like:

while (bits) {
  uint32_t lsb = bits & ~(bits - 1);
  // do something with lsb here..
  bits &= ~lsb; // mask off lsb and keep going
}

This gives you the value of each bit as you’re walking it (i.e. bit 7 will have the value 0x80 if we’re counting from 0). If you need the bit’s position as well, it’s convenient to use the Intel BSF (bit scan forward) instruction via compiler intrinsics:

  int bitpos;
  while (0 != (bitpos = __builtin_ffs(bits))) {
    uint32_t lsb = 1 << (bitpos - 1);
    // use bitpos, lsb as required..
    // bitpos is 1-based on GCC
    bits &= ~lsb;
  }

It sort of sucks that we still have to compute the LSB value so we can remove it and move on to the next bit. If your CPU has a fast CTZ instructions (count trailing zeroes) we can substitute that for the barrel shift:

while (bits) {
  uint32_t lsb = bits & ~(bits - 1);
  int bitpos = __builtin_ctz(lsb);
  // use bitpos, lsb as required..
  bits &= ~lsb; // mask off lsb and keep going
}

The intrinsics shown here work on GCC and Clang. There are similar ones for MSVC but I’m too lazy to write Windows examples!

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