Loop unswitching

Loop unswitching is a compiler optimization. It moves a conditional inside a loop outside of it by duplicating the loop's body, and placing a version of it inside each of the if and else clauses of the conditional.^[1] This can improve the parallelization of the loop. Since modern processors can operate quickly on vectors, this improvement increases the speed of the program.

Here is a simple example. Suppose we want to add the two arrays x and y and also do something depending on the variable w. We have the following C code:

  int i, w, x[1000], y[1000];
  for (i = 0; i < 1000; i++) {
    x[i] += y[i];
    if (w)
      y[i] = 0;
  }

The conditional inside this loop makes it difficult to safely parallelize this loop. When we unswitch the loop, this becomes:

  int i, w, x[1000], y[1000];
  if (w) {
    for (i = 0; i < 1000; i++) {
      x[i] += y[i];
      y[i] = 0;
    }
  } else {
    for (i = 0; i < 1000; i++) {
      x[i] += y[i];
    }
  }

While the loop unswitching may double the amount of code written, each of these new loops may now be separately optimized.

Loop unswitching was introduced in gcc in version 3.4.^[2]