Template:Short description 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.<ref>Template:Cite book</ref> 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:
<syntaxhighlight lang=c>
int i, w, x[1000], y[1000];
for (i = 0; i < 1000; i++) {
x[i] += y[i];
if (w)
y[i] = 0;
}
</syntaxhighlight>
The conditional inside this loop makes it difficult to safely parallelize this loop. When we unswitch the loop, this becomes:
<syntaxhighlight lang=c>
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];
}
}
</syntaxhighlight>
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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>