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Expertise: All
Language: C++

May 19, 1999

Unroll Loops to Optimize Your Code

A compiler can automatically optimize the code by unrolling loops. Consider this code:

 
  int *buff = new int[3];
  for (int i =0; i<3; i++)
 { 
   buff[i] = 0;
 }

On every iteration, the loop assigns a value to the next array element. However, precious CPU time is also wasted on testing and incrementing the loop counter, and performing a jump statement. To avoid this overhead, the compiler can unroll the loop into a sequence of three assignment statements:

 
  buff[0] = 0; 
  buff[1] = 0; 
  buff[2] = 0; 

This way, you avoid the unnecessary overhead of a loop. Note, however, that the compiler applies this optimization automatically; you shouldn't do it yourself.