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: 

bool w; 
int x[1000];
int y[1000];
for (int 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: 

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

References

  1. ^ Cooper, Keith; Torczon, Linda (2004). Engineering a Compiler. Elsevier. ISBN 9781558606982.
  2. ^ "GCC 3.4 Release Series — Changes, New Features, and Fixes - GNU Project".

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