What techniques to avoid conditional branching do you know?

Question

Sometimes a loop where the CPU spends most of the time has some branch prediction miss (misprediction) very often (near .5 probability.) I\'ve seen a few techniques on very isol

Answer 1

An extension of the technique demonstrated in the original question applies when you have to do several nested tests to get an answer. You can build a small bitmask from the results of all the tests, and the "look up" the answer in a table.

if (a) {
  if (b) {
    result = q;
  } else {
    result = r;
  }
} else {
  if (b) {
    result = s;
  } else {
    result = t;
  }
}

If a and b are nearly random (e.g., from arbitrary data), and this is in a tight loop, then branch prediction failures can really slow this down. Can be written as:

// assuming a and b are bools and thus exactly 0 or 1 ...
static const table[] = { t, s, r, q };
unsigned index = (a << 1) | b;
result = table[index];

You can generalize this to several conditionals. I've seen it done for 4. If the nesting gets that deep, though, you want to make sure that testing all of them is really faster than doing just the minimal tests suggested by short-circuit evaluation.