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