Editing Loop unswitching

{{Short description|Compiler optimization for conditionals in loops}}
'''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>{{Cite book|url=https://books.google.com/books?id=4yVQFVvsBNAC&q=%22Loop+unswitching%22&pg=PA520|title=Engineering a Compiler|isbn=9781558606982|last1=Cooper|first1=Keith|last2=Torczon|first2=Linda|year=2004|publisher=Elsevier }}</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 (programming language)|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 [[automatic parallelization|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 [[GNU Compiler Collection|gcc]] in version 3.4.<ref>{{Cite web|url=https://gcc.gnu.org/gcc-3.4/changes.html|title = GCC 3.4 Release Series — Changes, New Features, and Fixes - GNU Project}}</ref>

==References==
{{Reflist}}

{{Compiler optimizations}}

[[Category:Compiler optimizations]]