Loop unswitching

Loop unswitching is a compiler optimization. It moves a conditional statement inside a loop outside 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 enhances loop's parallelization. As modern processors can efficiently handle vectors, this optimization increases program speed.

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:

boolw;
intx[1000];
inty[1000];
for(inti=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:

boolw;
intx[1000];
inty[1000];
if(w){
for(inti=0;i<1000;i++){
x[i]+=y[i];
y[i]=0;
}
}else{
for(inti=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]

• Compiler optimizations

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