c++ - Portable branch prediction hints

Question

Is there any portable way of doing branch prediction hints? Consider the following example:

  if (unlikely_condition) {
    /* ..A.. */
  } else {
    /* ..B.. */
  }

Is this any different than doing:

  if (!unlikely_condition) {
    /* ..B.. */
  } else {
    /* ..A.. */
  }

Or is the only way to use compiler specific hints? (e.g. __builtin_expect on GCC)

Will compilers treat the if conditions any differently based on the ordering of the conditions?

Answer 1

The canonical way to do static branch prediction is that if is predicted not-branched (i.e. every if clause is executed, not else), and loops and backward-gotos are taken. So, don't put the common case in else if you expect static prediction to be significant. Getting around an untaken loop isn't as easy; I've never tried but I suppose putting it an an else clause should work pretty portably.

Many compilers support some form of #pragma unroll, but it will still be necessary to guard it with some kind of #if to protect other compilers.

Branch prediction hints can theoretically express a complete description of how to transform a program's flow-control graph and arrange the basic blocks in executable memory… so there are a variety of things to express, and most won't be very portable.

As GNU recommends in the documentation for __builtin_expect, profile-guided optimization is superior to hints, and with less effort.