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]

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